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<div class="subTitle"><span class="packageLabelInType">Package</span>&nbsp;<a href="package-summary.html">org.apache.kafka.streams.kstream</a></div>
<h2 title="Interface SessionWindowedKStream" class="title">Interface SessionWindowedKStream&lt;K,&#8203;V&gt;</h2>
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<dt><span class="paramLabel">Type Parameters:</span></dt>
<dd><code>K</code> - Type of keys</dd>
<dd><code>V</code> - Type of values</dd>
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<hr>
<pre>public interface <span class="typeNameLabel">SessionWindowedKStream&lt;K,&#8203;V&gt;</span></pre>
<div class="block"><code>SessionWindowedKStream</code> is an abstraction of a <i>windowed</i> record stream of <a href="../KeyValue.html" title="class in org.apache.kafka.streams"><code>KeyValue</code></a> pairs.
It is an intermediate representation after a grouping and windowing of a <a href="KStream.html" title="interface in org.apache.kafka.streams.kstream"><code>KStream</code></a> before an aggregation is
applied to the new (partitioned) windows resulting in a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> (a <emph>windowed</emph>
<code>KTable</code> is a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> with key type <a href="Windowed.html" title="class in org.apache.kafka.streams.kstream"><code>Windowed<K></code></a>).
<p>
<a href="SessionWindows.html" title="class in org.apache.kafka.streams.kstream"><code>SessionWindows</code></a> are dynamic data driven windows.
They have no fixed time boundaries, rather the size of the window is determined by the records.
<p>
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating
materialized view) that can be queried using the name provided in the <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> instance.
Furthermore, updates to the store are sent downstream into a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream, where
"windowed" implies that the <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> key is a combined key of the original record key and a window ID.
New events are added to sessions until their grace period ends (see <a href="SessionWindows.html#grace(java.time.Duration)"><code>SessionWindows.grace(Duration)</code></a>).
<p>
A <code>SessionWindowedKStream</code> must be obtained from a <a href="KGroupedStream.html" title="interface in org.apache.kafka.streams.kstream"><code>KGroupedStream</code></a> via
<a href="KGroupedStream.html#windowedBy(org.apache.kafka.streams.kstream.SessionWindows)"><code>KGroupedStream.windowedBy(SessionWindows)</code></a>.</div>
<dl>
<dt><span class="seeLabel">See Also:</span></dt>
<dd><a href="KStream.html" title="interface in org.apache.kafka.streams.kstream"><code>KStream</code></a>,
<a href="KGroupedStream.html" title="interface in org.apache.kafka.streams.kstream"><code>KGroupedStream</code></a>,
<a href="SessionWindows.html" title="class in org.apache.kafka.streams.kstream"><code>SessionWindows</code></a></dd>
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<tr>
<th class="colFirst" scope="col">Modifier and Type</th>
<th class="colSecond" scope="col">Method</th>
<th class="colLast" scope="col">Description</th>
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<tr id="i0" class="altColor">
<td class="colFirst"><code>&lt;VR&gt;&nbsp;<a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;VR&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger)">aggregate</a></span>&#8203;(<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream">Initializer</a>&lt;VR&gt;&nbsp;initializer,
<a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream">Aggregator</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;VR&gt;&nbsp;aggregator,
<a href="Merger.html" title="interface in org.apache.kafka.streams.kstream">Merger</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR&gt;&nbsp;sessionMerger)</code></th>
<td class="colLast">
<div class="block">Aggregate the values of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i1" class="rowColor">
<td class="colFirst"><code>&lt;VR&gt;&nbsp;<a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;VR&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger,org.apache.kafka.streams.kstream.Materialized)">aggregate</a></span>&#8203;(<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream">Initializer</a>&lt;VR&gt;&nbsp;initializer,
<a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream">Aggregator</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;VR&gt;&nbsp;aggregator,
<a href="Merger.html" title="interface in org.apache.kafka.streams.kstream">Merger</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR&gt;&nbsp;sessionMerger,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</code></th>
<td class="colLast">
<div class="block">Aggregate the values of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i2" class="altColor">
<td class="colFirst"><code>&lt;VR&gt;&nbsp;<a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;VR&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger,org.apache.kafka.streams.kstream.Named)">aggregate</a></span>&#8203;(<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream">Initializer</a>&lt;VR&gt;&nbsp;initializer,
<a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream">Aggregator</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;VR&gt;&nbsp;aggregator,
<a href="Merger.html" title="interface in org.apache.kafka.streams.kstream">Merger</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR&gt;&nbsp;sessionMerger,
<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named)</code></th>
<td class="colLast">
<div class="block">Aggregate the values of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i3" class="rowColor">
<td class="colFirst"><code>&lt;VR&gt;&nbsp;<a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;VR&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger,org.apache.kafka.streams.kstream.Named,org.apache.kafka.streams.kstream.Materialized)">aggregate</a></span>&#8203;(<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream">Initializer</a>&lt;VR&gt;&nbsp;initializer,
<a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream">Aggregator</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;VR&gt;&nbsp;aggregator,
<a href="Merger.html" title="interface in org.apache.kafka.streams.kstream">Merger</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR&gt;&nbsp;sessionMerger,
<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</code></th>
<td class="colLast">
<div class="block">Aggregate the values of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i4" class="altColor">
<td class="colFirst"><code><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#count()">count</a></span>()</code></th>
<td class="colLast">
<div class="block">Count the number of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i5" class="rowColor">
<td class="colFirst"><code><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#count(org.apache.kafka.streams.kstream.Materialized)">count</a></span>&#8203;(<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</code></th>
<td class="colLast">
<div class="block">Count the number of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i6" class="altColor">
<td class="colFirst"><code><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#count(org.apache.kafka.streams.kstream.Named)">count</a></span>&#8203;(<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named)</code></th>
<td class="colLast">
<div class="block">Count the number of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i7" class="rowColor">
<td class="colFirst"><code><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#count(org.apache.kafka.streams.kstream.Named,org.apache.kafka.streams.kstream.Materialized)">count</a></span>&#8203;(<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</code></th>
<td class="colLast">
<div class="block">Count the number of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i8" class="altColor">
<td class="colFirst"><code><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#reduce(org.apache.kafka.streams.kstream.Reducer)">reduce</a></span>&#8203;(<a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream">Reducer</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reducer)</code></th>
<td class="colLast">
<div class="block">Combine the values of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i9" class="rowColor">
<td class="colFirst"><code><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#reduce(org.apache.kafka.streams.kstream.Reducer,org.apache.kafka.streams.kstream.Materialized)">reduce</a></span>&#8203;(<a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream">Reducer</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reducer,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</code></th>
<td class="colLast">
<div class="block">Combine the values of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i10" class="altColor">
<td class="colFirst"><code><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#reduce(org.apache.kafka.streams.kstream.Reducer,org.apache.kafka.streams.kstream.Named)">reduce</a></span>&#8203;(<a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream">Reducer</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reducer,
<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named)</code></th>
<td class="colLast">
<div class="block">Combine the values of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
<tr id="i11" class="rowColor">
<td class="colFirst"><code><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#reduce(org.apache.kafka.streams.kstream.Reducer,org.apache.kafka.streams.kstream.Named,org.apache.kafka.streams.kstream.Materialized)">reduce</a></span>&#8203;(<a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream">Reducer</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reducer,
<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</code></th>
<td class="colLast">
<div class="block">Combine the values of records in this stream by the grouped key and defined sessions.</div>
</td>
</tr>
</table>
</li>
</ul>
</section>
</li>
</ul>
</div>
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<h3>Method Detail</h3>
<a id="count()">
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<h4>count</h4>
<pre class="methodSignature"><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>&gt;&nbsp;count()</pre>
<div class="block">Count the number of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
<p>
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view).
The default key serde from the config will be used for serializing the result.
If a different serde is required then you should use <a href="#count(org.apache.kafka.streams.kstream.Materialized)"><code>count(Materialized)</code></a>.
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
Not all updates might get sent downstream, as an internal cache is used to deduplicate consecutive updates to
the same session and key.
The rate of propagated updates depends on your input data rate, the number of distinct keys, the number of
parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a> parameters for
<a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
The changelog topic will be named "${applicationId}-${internalStoreName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "internalStoreName" is an internal name
and "-changelog" is a fixed suffix.
Note that the internal store name may not be queryable through Interactive Queries.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys and <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink"><code>Long</code></a> values
that represent the latest (rolling) count (i.e., number of records) for each key per session</dd>
</dl>
</li>
</ul>
<a id="count(org.apache.kafka.streams.kstream.Named)">
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<li class="blockList">
<h4>count</h4>
<pre class="methodSignature"><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>&gt;&nbsp;count&#8203;(<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named)</pre>
<div class="block">Count the number of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
<p>
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view).
The default key serde from the config will be used for serializing the result.
If a different serde is required then you should use <a href="#count(org.apache.kafka.streams.kstream.Named,org.apache.kafka.streams.kstream.Materialized)"><code>count(Named, Materialized)</code></a>.
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
Not all updates might get sent downstream, as an internal cache is used to deduplicate consecutive updates to
the same session and key.
The rate of propagated updates depends on your input data rate, the number of distinct keys, the number of
parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a> parameters for
<a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
The changelog topic will be named "${applicationId}-${internalStoreName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "internalStoreName" is an internal name
and "-changelog" is a fixed suffix.
Note that the internal store name may not be queryable through Interactive Queries.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>named</code> - a <a href="Named.html" title="class in org.apache.kafka.streams.kstream"><code>Named</code></a> config used to name the processor in the topology. Cannot be <code>null</code>.</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys and <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink"><code>Long</code></a> values
that represent the latest (rolling) count (i.e., number of records) for each key per session</dd>
</dl>
</li>
</ul>
<a id="count(org.apache.kafka.streams.kstream.Materialized)">
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</a>
<ul class="blockList">
<li class="blockList">
<h4>count</h4>
<pre class="methodSignature"><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>&gt;&nbsp;count&#8203;(<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</pre>
<div class="block">Count the number of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
<p>
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view)
that can be queried using the name provided with <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a>.
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
<p>
Not all updates might get sent downstream, as an internal cache will be used to deduplicate consecutive updates
to the same window and key if caching is enabled on the <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> instance.
When caching is enabled the rate of propagated updates depends on your input data rate, the number of distinct
keys, the number of parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a>
parameters for <a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
To query the local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> it must be obtained via
<a href="../KafkaStreams.html#store(org.apache.kafka.streams.StoreQueryParameters)"><code>KafkaStreams#store(...)</code></a>:
<pre><code>
KafkaStreams streams = ... // compute sum
Sting queryableStoreName = ... // the queryableStoreName should be the name of the store as defined by the Materialized instance
ReadOnlySessionStore&lt;String,Long&gt; localWindowStore = streams.store(queryableStoreName, QueryableStoreTypes.&lt;String, Long&gt;ReadOnlySessionStore&lt;String, Long&gt;);
String key = "some-key";
KeyValueIterator&lt;Windowed&lt;String&gt;, Long&gt; sumForKeyForWindows = localWindowStore.fetch(key); // key must be local (application state is shared over all running Kafka Streams instances)
</code></pre>
For non-local keys, a custom RPC mechanism must be implemented using <a href="../KafkaStreams.html#allMetadata()"><code>KafkaStreams.allMetadata()</code></a> to
query the value of the key on a parallel running instance of your Kafka Streams application.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
Therefore, the store name defined by the Materialized instance must be a valid Kafka topic name and cannot
contain characters other than ASCII alphanumerics, '.', '_' and '-'.
The changelog topic will be named "${applicationId}-${storeName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "storeName" is the provide store name defined
in <code>Materialized</code>, and "-changelog" is a fixed suffix.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>materialized</code> - an instance of <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> used to materialize a state store. Cannot be <code>null</code>.
Note: the valueSerde will be automatically set to <a href="../../common/serialization/Serdes.html#Long()"><code>Serdes#Long()</code></a>
if there is no valueSerde provided</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys and <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink"><code>Long</code></a> values
that represent the latest (rolling) count (i.e., number of records) for each key per session</dd>
</dl>
</li>
</ul>
<a id="count(org.apache.kafka.streams.kstream.Named,org.apache.kafka.streams.kstream.Materialized)">
<!-- -->
</a>
<ul class="blockList">
<li class="blockList">
<h4>count</h4>
<pre class="methodSignature"><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>&gt;&nbsp;count&#8203;(<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink">Long</a>,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</pre>
<div class="block">Count the number of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
<p>
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view)
that can be queried using the name provided with <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a>.
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
<p>
Not all updates might get sent downstream, as an internal cache will be used to deduplicate consecutive updates
to the same window and key if caching is enabled on the <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> instance.
When caching is enabled the rate of propagated updates depends on your input data rate, the number of distinct
keys, the number of parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a>
parameters for <a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
To query the local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> it must be obtained via
<a href="../KafkaStreams.html#store(org.apache.kafka.streams.StoreQueryParameters)"><code>KafkaStreams#store(...)</code></a>:
<pre><code>
KafkaStreams streams = ... // compute sum
Sting queryableStoreName = ... // the queryableStoreName should be the name of the store as defined by the Materialized instance
ReadOnlySessionStore&lt;String,Long&gt; localWindowStore = streams.store(queryableStoreName, QueryableStoreTypes.&lt;String, Long&gt;ReadOnlySessionStore&lt;String, Long&gt;);
String key = "some-key";
KeyValueIterator&lt;Windowed&lt;String&gt;, Long&gt; sumForKeyForWindows = localWindowStore.fetch(key); // key must be local (application state is shared over all running Kafka Streams instances)
</code></pre>
For non-local keys, a custom RPC mechanism must be implemented using <a href="../KafkaStreams.html#allMetadata()"><code>KafkaStreams.allMetadata()</code></a> to
query the value of the key on a parallel running instance of your Kafka Streams application.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
Therefore, the store name defined by the Materialized instance must be a valid Kafka topic name and cannot
contain characters other than ASCII alphanumerics, '.', '_' and '-'.
The changelog topic will be named "${applicationId}-${storeName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "storeName" is the provide store name defined
in <code>Materialized</code>, and "-changelog" is a fixed suffix.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>named</code> - a <a href="Named.html" title="class in org.apache.kafka.streams.kstream"><code>Named</code></a> config used to name the processor in the topology. Cannot be <code>null</code>.</dd>
<dd><code>materialized</code> - an instance of <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> used to materialize a state store. Cannot be <code>null</code>.
Note: the valueSerde will be automatically set to <a href="../../common/serialization/Serdes.html#Long()"><code>Serdes#Long()</code></a>
if there is no valueSerde provided</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys and <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Long.html?is-external=true" title="class or interface in java.lang" class="externalLink"><code>Long</code></a> values
that represent the latest (rolling) count (i.e., number of records) for each key per session</dd>
</dl>
</li>
</ul>
<a id="aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger)">
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<h4>aggregate</h4>
<pre class="methodSignature">&lt;VR&gt;&nbsp;<a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;VR&gt;&nbsp;aggregate&#8203;(<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream">Initializer</a>&lt;VR&gt;&nbsp;initializer,
<a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream">Aggregator</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;VR&gt;&nbsp;aggregator,
<a href="Merger.html" title="interface in org.apache.kafka.streams.kstream">Merger</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR&gt;&nbsp;sessionMerger)</pre>
<div class="block">Aggregate the values of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
Aggregating is a generalization of <a href="#reduce(org.apache.kafka.streams.kstream.Reducer)"><code>combining via reduce(...)</code></a> as it, for example,
allows the result to have a different type than the input values.
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view).
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
<p>
The specified <a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a> is applied directly before the first input record per session is processed to
provide an initial intermediate aggregation result that is used to process the first record per session.
The specified <a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream"><code>Aggregator</code></a> is applied for each input record and computes a new aggregate using the current
aggregate (or for the very first record using the intermediate aggregation result provided via the
<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a>) and the record's value.
The specified <a href="Merger.html" title="interface in org.apache.kafka.streams.kstream"><code>Merger</code></a> is used to merge two existing sessions into one, i.e., when the windows overlap,
they are merged into a single session and the old sessions are discarded.
Thus, <code>aggregate()</code> can be used to compute aggregate functions like count (c.f. <a href="#count()"><code>count()</code></a>).
<p>
The default key and value serde from the config will be used for serializing the result.
If a different serde is required then you should use
<a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger,org.apache.kafka.streams.kstream.Materialized)"><code>aggregate(Initializer, Aggregator, Merger, Materialized)</code></a>.
<p>
Not all updates might get sent downstream, as an internal cache is used to deduplicate consecutive updates to
the same window and key.
The rate of propagated updates depends on your input data rate, the number of distinct keys, the number of
parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a> parameters for
<a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
The changelog topic will be named "${applicationId}-${internalStoreName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "internalStoreName" is an internal name
and "-changelog" is a fixed suffix.
Note that the internal store name may not be queryable through Interactive Queries.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Type Parameters:</span></dt>
<dd><code>VR</code> - the value type of the resulting <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>initializer</code> - an <a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a> that computes an initial intermediate aggregation result. Cannot be <code>null</code>.</dd>
<dd><code>aggregator</code> - an <a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream"><code>Aggregator</code></a> that computes a new aggregate result. Cannot be <code>null</code>.</dd>
<dd><code>sessionMerger</code> - a <a href="Merger.html" title="interface in org.apache.kafka.streams.kstream"><code>Merger</code></a> that combines two aggregation results. Cannot be <code>null</code>.</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys, and values that represent
the latest (rolling) aggregate for each key per session</dd>
</dl>
</li>
</ul>
<a id="aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger,org.apache.kafka.streams.kstream.Named)">
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<pre class="methodSignature">&lt;VR&gt;&nbsp;<a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;VR&gt;&nbsp;aggregate&#8203;(<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream">Initializer</a>&lt;VR&gt;&nbsp;initializer,
<a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream">Aggregator</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;VR&gt;&nbsp;aggregator,
<a href="Merger.html" title="interface in org.apache.kafka.streams.kstream">Merger</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR&gt;&nbsp;sessionMerger,
<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named)</pre>
<div class="block">Aggregate the values of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
Aggregating is a generalization of <a href="#reduce(org.apache.kafka.streams.kstream.Reducer)"><code>combining via reduce(...)</code></a> as it, for example,
allows the result to have a different type than the input values.
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view).
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
<p>
The specified <a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a> is applied directly before the first input record per session is processed to
provide an initial intermediate aggregation result that is used to process the first record per session.
The specified <a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream"><code>Aggregator</code></a> is applied for each input record and computes a new aggregate using the current
aggregate (or for the very first record using the intermediate aggregation result provided via the
<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a>) and the record's value.
The specified <a href="Merger.html" title="interface in org.apache.kafka.streams.kstream"><code>Merger</code></a> is used to merge two existing sessions into one, i.e., when the windows overlap,
they are merged into a single session and the old sessions are discarded.
Thus, <code>aggregate()</code> can be used to compute aggregate functions like count (c.f. <a href="#count()"><code>count()</code></a>).
<p>
The default key and value serde from the config will be used for serializing the result.
If a different serde is required then you should use
<a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger,org.apache.kafka.streams.kstream.Named,org.apache.kafka.streams.kstream.Materialized)"><code>aggregate(Initializer, Aggregator, Merger, Named, Materialized)</code></a>.
<p>
Not all updates might get sent downstream, as an internal cache is used to deduplicate consecutive updates to
the same window and key.
The rate of propagated updates depends on your input data rate, the number of distinct
keys, the number of parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a>
parameters for <a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
The changelog topic will be named "${applicationId}-${internalStoreName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "internalStoreName" is an internal name
and "-changelog" is a fixed suffix.
Note that the internal store name may not be queryable through Interactive Queries.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Type Parameters:</span></dt>
<dd><code>VR</code> - the value type of the resulting <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>initializer</code> - an <a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a> that computes an initial intermediate aggregation result. Cannot be <code>null</code>.</dd>
<dd><code>aggregator</code> - an <a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream"><code>Aggregator</code></a> that computes a new aggregate result. Cannot be <code>null</code>.</dd>
<dd><code>sessionMerger</code> - a <a href="Merger.html" title="interface in org.apache.kafka.streams.kstream"><code>Merger</code></a> that combines two aggregation results. Cannot be <code>null</code>.</dd>
<dd><code>named</code> - a <a href="Named.html" title="class in org.apache.kafka.streams.kstream"><code>Named</code></a> config used to name the processor in the topology. Cannot be <code>null</code>.</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys, and values that represent
the latest (rolling) aggregate for each key per session</dd>
</dl>
</li>
</ul>
<a id="aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger,org.apache.kafka.streams.kstream.Materialized)">
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<pre class="methodSignature">&lt;VR&gt;&nbsp;<a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;VR&gt;&nbsp;aggregate&#8203;(<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream">Initializer</a>&lt;VR&gt;&nbsp;initializer,
<a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream">Aggregator</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;VR&gt;&nbsp;aggregator,
<a href="Merger.html" title="interface in org.apache.kafka.streams.kstream">Merger</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR&gt;&nbsp;sessionMerger,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</pre>
<div class="block">Aggregate the values of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
Aggregating is a generalization of <a href="#reduce(org.apache.kafka.streams.kstream.Reducer)"><code>combining via reduce(...)</code></a> as it, for example,
allows the result to have a different type than the input values.
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view)
that can be queried using the store name as provided with <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a>.
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
<p>
The specified <a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a> is applied directly before the first input record per session is processed to
provide an initial intermediate aggregation result that is used to process the first record per session.
The specified <a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream"><code>Aggregator</code></a> is applied for each input record and computes a new aggregate using the current
aggregate (or for the very first record using the intermediate aggregation result provided via the
<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a>) and the record's value.
The specified <a href="Merger.html" title="interface in org.apache.kafka.streams.kstream"><code>Merger</code></a> is used to merge two existing sessions into one, i.e., when the windows overlap,
they are merged into a single session and the old sessions are discarded.
Thus, <code>aggregate()</code> can be used to compute aggregate functions like count (c.f. <a href="#count()"><code>count()</code></a>).
<p>
Not all updates might get sent downstream, as an internal cache is used to deduplicate consecutive updates to
the same window and key if caching is enabled on the <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> instance.
When caching is enabled the rate of propagated updates depends on your input data rate, the number of distinct keys, the number of
parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a> parameters for
<a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
To query the local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> it must be obtained via
<a href="../KafkaStreams.html#store(org.apache.kafka.streams.StoreQueryParameters)"><code>KafkaStreams#store(...)</code></a>:
<pre><code>
KafkaStreams streams = ... // some windowed aggregation on value type double
Sting queryableStoreName = ... // the queryableStoreName should be the name of the store as defined by the Materialized instance
ReadOnlySessionStore&lt;String, Long&gt; sessionStore = streams.store(queryableStoreName, QueryableStoreTypes.&lt;String, Long&gt;sessionStore());
String key = "some-key";
KeyValueIterator&lt;Windowed&lt;String&gt;, Long&gt; aggForKeyForSession = localWindowStore.fetch(key); // key must be local (application state is shared over all running Kafka Streams instances)
</code></pre>
For non-local keys, a custom RPC mechanism must be implemented using <a href="../KafkaStreams.html#allMetadata()"><code>KafkaStreams.allMetadata()</code></a> to
query the value of the key on a parallel running instance of your Kafka Streams application.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
Therefore, the store name defined by the <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> instance must be a valid Kafka topic name and
cannot contain characters other than ASCII alphanumerics, '.', '_' and '-'.
The changelog topic will be named "${applicationId}-${storeName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "storeName" is the
provide store name defined in <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a>, and "-changelog" is a fixed suffix.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Type Parameters:</span></dt>
<dd><code>VR</code> - the value type of the resulting <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>initializer</code> - an <a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a> that computes an initial intermediate aggregation result. Cannot be <code>null</code>.</dd>
<dd><code>aggregator</code> - an <a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream"><code>Aggregator</code></a> that computes a new aggregate result. Cannot be <code>null</code>.</dd>
<dd><code>sessionMerger</code> - a <a href="Merger.html" title="interface in org.apache.kafka.streams.kstream"><code>Merger</code></a> that combines two aggregation results. Cannot be <code>null</code>.</dd>
<dd><code>materialized</code> - a <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> config used to materialize a state store. Cannot be <code>null</code>.</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys, and values that represent
the latest (rolling) aggregate for each key per session</dd>
</dl>
</li>
</ul>
<a id="aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger,org.apache.kafka.streams.kstream.Named,org.apache.kafka.streams.kstream.Materialized)">
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<h4>aggregate</h4>
<pre class="methodSignature">&lt;VR&gt;&nbsp;<a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;VR&gt;&nbsp;aggregate&#8203;(<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream">Initializer</a>&lt;VR&gt;&nbsp;initializer,
<a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream">Aggregator</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;VR&gt;&nbsp;aggregator,
<a href="Merger.html" title="interface in org.apache.kafka.streams.kstream">Merger</a>&lt;? super <a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR&gt;&nbsp;sessionMerger,
<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;VR,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</pre>
<div class="block">Aggregate the values of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
Aggregating is a generalization of <a href="#reduce(org.apache.kafka.streams.kstream.Reducer)"><code>combining via reduce(...)</code></a> as it, for example,
allows the result to have a different type than the input values.
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view)
that can be queried using the store name as provided with <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a>.
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
<p>
The specified <a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a> is applied directly before the first input record per session is processed to
provide an initial intermediate aggregation result that is used to process the first record per session.
The specified <a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream"><code>Aggregator</code></a> is applied for each input record and computes a new aggregate using the current
aggregate (or for the very first record using the intermediate aggregation result provided via the
<a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a>) and the record's value.
The specified <a href="Merger.html" title="interface in org.apache.kafka.streams.kstream"><code>Merger</code></a> is used to merge two existing sessions into one, i.e., when the windows overlap,
they are merged into a single session and the old sessions are discarded.
Thus, <code>aggregate()</code> can be used to compute aggregate functions like count (c.f. <a href="#count()"><code>count()</code></a>).
<p>
Not all updates might get sent downstream, as an internal cache will be used to deduplicate consecutive updates
to the same window and key if caching is enabled on the <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> instance.
When caching is enabled the rate of propagated updates depends on your input data rate, the number of distinct
keys, the number of parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a>
parameters for <a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
To query the local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> it must be obtained via
<a href="../KafkaStreams.html#store(org.apache.kafka.streams.StoreQueryParameters)"><code>KafkaStreams#store(...)</code></a>:
<pre><code>
KafkaStreams streams = ... // some windowed aggregation on value type double
Sting queryableStoreName = ... // the queryableStoreName should be the name of the store as defined by the Materialized instance
ReadOnlySessionStore&lt;String, Long&gt; sessionStore = streams.store(queryableStoreName, QueryableStoreTypes.&lt;String, Long&gt;sessionStore());
String key = "some-key";
KeyValueIterator&lt;Windowed&lt;String&gt;, Long&gt; aggForKeyForSession = localWindowStore.fetch(key); // key must be local (application state is shared over all running Kafka Streams instances)
</code></pre>
For non-local keys, a custom RPC mechanism must be implemented using <a href="../KafkaStreams.html#allMetadata()"><code>KafkaStreams.allMetadata()</code></a> to
query the value of the key on a parallel running instance of your Kafka Streams application.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
Therefore, the store name defined by the <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> instance must be a valid Kafka topic name and
cannot contain characters other than ASCII alphanumerics, '.', '_' and '-'.
The changelog topic will be named "${applicationId}-${storeName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "storeName" is the
provide store name defined in <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a>, and "-changelog" is a fixed suffix.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Type Parameters:</span></dt>
<dd><code>VR</code> - the value type of the resulting <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>initializer</code> - an <a href="Initializer.html" title="interface in org.apache.kafka.streams.kstream"><code>Initializer</code></a> that computes an initial intermediate aggregation result. Cannot be <code>null</code>.</dd>
<dd><code>aggregator</code> - an <a href="Aggregator.html" title="interface in org.apache.kafka.streams.kstream"><code>Aggregator</code></a> that computes a new aggregate result. Cannot be <code>null</code>.</dd>
<dd><code>sessionMerger</code> - a <a href="Merger.html" title="interface in org.apache.kafka.streams.kstream"><code>Merger</code></a> that combines two aggregation results. Cannot be <code>null</code>.</dd>
<dd><code>named</code> - a <a href="Named.html" title="class in org.apache.kafka.streams.kstream"><code>Named</code></a> config used to name the processor in the topology. Cannot be <code>null</code>.</dd>
<dd><code>materialized</code> - a <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> config used to materialize a state store. Cannot be <code>null</code>.</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys, and values that represent
the latest (rolling) aggregate for each key per session</dd>
</dl>
</li>
</ul>
<a id="reduce(org.apache.kafka.streams.kstream.Reducer)">
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<h4>reduce</h4>
<pre class="methodSignature"><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reduce&#8203;(<a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream">Reducer</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reducer)</pre>
<div class="block">Combine the values of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
Combining implies that the type of the aggregate result is the same as the type of the input value
(c.f. <a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger)"><code>aggregate(Initializer, Aggregator, Merger)</code></a>).
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view).
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
The default key and value serde from the config will be used for serializing the result.
If a different serde is required then you should use <a href="#reduce(org.apache.kafka.streams.kstream.Reducer,org.apache.kafka.streams.kstream.Materialized)"><code>reduce(Reducer, Materialized)</code></a> .
<p>
The value of the first record per session initialized the session result.
The specified <a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream"><code>Reducer</code></a> is applied for each additional input record per session and computes a new
aggregate using the current aggregate (first argument) and the record's value (second argument):
<pre><code>
// At the example of a Reducer&lt;Long&gt;
new Reducer&lt;Long&gt;() {
public Long apply(Long aggValue, Long currValue) {
return aggValue + currValue;
}
}
</code></pre>
Thus, <code>reduce()</code> can be used to compute aggregate functions like sum, min, or max.
<p>
Not all updates might get sent downstream, as an internal cache is used to deduplicate consecutive updates to
the same window and key.
The rate of propagated updates depends on your input data rate, the number of distinct keys, the number of
parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a> parameters for
<a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
The changelog topic will be named "${applicationId}-${internalStoreName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "internalStoreName" is an internal name
and "-changelog" is a fixed suffix.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>reducer</code> - a <a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream"><code>Reducer</code></a> that computes a new aggregate result. Cannot be <code>null</code>.</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys, and values that represent
the latest (rolling) aggregate for each key per session</dd>
</dl>
</li>
</ul>
<a id="reduce(org.apache.kafka.streams.kstream.Reducer,org.apache.kafka.streams.kstream.Named)">
<!-- -->
</a>
<ul class="blockList">
<li class="blockList">
<h4>reduce</h4>
<pre class="methodSignature"><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reduce&#8203;(<a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream">Reducer</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reducer,
<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named)</pre>
<div class="block">Combine the values of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
Combining implies that the type of the aggregate result is the same as the type of the input value
(c.f. <a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger)"><code>aggregate(Initializer, Aggregator, Merger)</code></a>).
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view).
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
The default key and value serde from the config will be used for serializing the result.
If a different serde is required then you should use <a href="#reduce(org.apache.kafka.streams.kstream.Reducer,org.apache.kafka.streams.kstream.Named,org.apache.kafka.streams.kstream.Materialized)"><code>reduce(Reducer, Named, Materialized)</code></a> .
<p>
The value of the first record per session initialized the session result.
The specified <a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream"><code>Reducer</code></a> is applied for each additional input record per session and computes a new
aggregate using the current aggregate (first argument) and the record's value (second argument):
<pre><code>
// At the example of a Reducer&lt;Long&gt;
new Reducer&lt;Long&gt;() {
public Long apply(Long aggValue, Long currValue) {
return aggValue + currValue;
}
}
</code></pre>
Thus, <code>reduce()</code> can be used to compute aggregate functions like sum, min, or max.
<p>
Not all updates might get sent downstream, as an internal cache is used to deduplicate consecutive updates to
the same window and key.
The rate of propagated updates depends on your input data rate, the number of distinct keys, the number of
parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a> parameters for
<a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
The changelog topic will be named "${applicationId}-${internalStoreName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "internalStoreName" is an internal name
and "-changelog" is a fixed suffix.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>reducer</code> - a <a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream"><code>Reducer</code></a> that computes a new aggregate result. Cannot be <code>null</code>.</dd>
<dd><code>named</code> - a <a href="Named.html" title="class in org.apache.kafka.streams.kstream"><code>Named</code></a> config used to name the processor in the topology. Cannot be <code>null</code>.</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys, and values that represent
the latest (rolling) aggregate for each key per session</dd>
</dl>
</li>
</ul>
<a id="reduce(org.apache.kafka.streams.kstream.Reducer,org.apache.kafka.streams.kstream.Materialized)">
<!-- -->
</a>
<ul class="blockList">
<li class="blockList">
<h4>reduce</h4>
<pre class="methodSignature"><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reduce&#8203;(<a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream">Reducer</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reducer,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</pre>
<div class="block">Combine the values of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
Combining implies that the type of the aggregate result is the same as the type of the input value
(c.f. <a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger)"><code>aggregate(Initializer, Aggregator, Merger)</code></a>).
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view)
that can be queried using the store name as provided with <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a>.
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
<p>
The value of the first record per session initialized the session result.
The specified <a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream"><code>Reducer</code></a> is applied for each additional input record per session and computes a new
aggregate using the current aggregate (first argument) and the record's value (second argument):
<pre><code>
// At the example of a Reducer&lt;Long&gt;
new Reducer&lt;Long&gt;() {
public Long apply(Long aggValue, Long currValue) {
return aggValue + currValue;
}
}
</code></pre>
Thus, <code>reduce()</code> can be used to compute aggregate functions like sum, min, or max.
<p>
Not all updates might get sent downstream, as an internal cache will be used to deduplicate consecutive updates
to the same window and key if caching is enabled on the <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> instance.
When caching is enabled the rate of propagated updates depends on your input data rate, the number of distinct
keys, the number of parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a>
parameters for <a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
To query the local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> it must be obtained via
<a href="../KafkaStreams.html#store(org.apache.kafka.streams.StoreQueryParameters)"><code>KafkaStreams#store(...)</code></a>:
<pre><code>
KafkaStreams streams = ... // compute sum
Sting queryableStoreName = ... // the queryableStoreName should be the name of the store as defined by the Materialized instance
ReadOnlySessionStore&lt;String,Long&gt; localWindowStore = streams.store(queryableStoreName, QueryableStoreTypes.&lt;String, Long&gt;ReadOnlySessionStore&lt;String, Long&gt;);
String key = "some-key";
KeyValueIterator&lt;Windowed&lt;String&gt;, Long&gt; sumForKeyForWindows = localWindowStore.fetch(key); // key must be local (application state is shared over all running Kafka Streams instances)
</code></pre>
For non-local keys, a custom RPC mechanism must be implemented using <a href="../KafkaStreams.html#allMetadata()"><code>KafkaStreams.allMetadata()</code></a> to
query the value of the key on a parallel running instance of your Kafka Streams application.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
Therefore, the store name defined by the Materialized instance must be a valid Kafka topic name and cannot
contain characters other than ASCII alphanumerics, '.', '_' and '-'.
The changelog topic will be named "${applicationId}-${storeName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "storeName" is the provide store name defined
in <code>Materialized</code>, and "-changelog" is a fixed suffix.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>reducer</code> - a <a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream"><code>Reducer</code></a> that computes a new aggregate result. Cannot be <code>null</code>.</dd>
<dd><code>materialized</code> - a <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> config used to materialize a state store. Cannot be <code>null</code>.</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys, and values that represent
the latest (rolling) aggregate for each key per session</dd>
</dl>
</li>
</ul>
<a id="reduce(org.apache.kafka.streams.kstream.Reducer,org.apache.kafka.streams.kstream.Named,org.apache.kafka.streams.kstream.Materialized)">
<!-- -->
</a>
<ul class="blockListLast">
<li class="blockList">
<h4>reduce</h4>
<pre class="methodSignature"><a href="KTable.html" title="interface in org.apache.kafka.streams.kstream">KTable</a>&lt;<a href="Windowed.html" title="class in org.apache.kafka.streams.kstream">Windowed</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>&gt;,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reduce&#8203;(<a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream">Reducer</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>&gt;&nbsp;reducer,
<a href="Named.html" title="class in org.apache.kafka.streams.kstream">Named</a>&nbsp;named,
<a href="Materialized.html" title="class in org.apache.kafka.streams.kstream">Materialized</a>&lt;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">K</a>,&#8203;<a href="SessionWindowedKStream.html" title="type parameter in SessionWindowedKStream">V</a>,&#8203;<a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state">SessionStore</a>&lt;org.apache.kafka.common.utils.Bytes,&#8203;byte[]&gt;&gt;&nbsp;materialized)</pre>
<div class="block">Combine the values of records in this stream by the grouped key and defined sessions.
Note that sessions are generated on a per-key basis and records with different keys create independent sessions.
Records with <code>null</code> key or value are ignored.
Combining implies that the type of the aggregate result is the same as the type of the input value
(c.f. <a href="#aggregate(org.apache.kafka.streams.kstream.Initializer,org.apache.kafka.streams.kstream.Aggregator,org.apache.kafka.streams.kstream.Merger)"><code>aggregate(Initializer, Aggregator, Merger)</code></a>).
The result is written into a local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> (which is basically an ever-updating materialized view)
that can be queried using the store name as provided with <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a>.
Furthermore, updates to the store are sent downstream into a <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> changelog stream.
<p>
The value of the first record per session initialized the session result.
The specified <a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream"><code>Reducer</code></a> is applied for each additional input record per session and computes a new
aggregate using the current aggregate (first argument) and the record's value (second argument):
<pre><code>
// At the example of a Reducer&lt;Long&gt;
new Reducer&lt;Long&gt;() {
public Long apply(Long aggValue, Long currValue) {
return aggValue + currValue;
}
}
</code></pre>
Thus, <code>reduce()</code> can be used to compute aggregate functions like sum, min, or max.
<p>
Not all updates might get sent downstream, as an internal cache will be used to deduplicate consecutive updates
to the same window and key if caching is enabled on the <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> instance.
When caching is enabled the rate of propagated updates depends on your input data rate, the number of distinct
keys, the number of parallel running Kafka Streams instances, and the <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>configuration</code></a>
parameters for <a href="../StreamsConfig.html#CACHE_MAX_BYTES_BUFFERING_CONFIG"><code>cache size</code></a>, and
<a href="../StreamsConfig.html#COMMIT_INTERVAL_MS_CONFIG"><code>commit interval</code></a>.
<p>
To query the local <a href="../state/SessionStore.html" title="interface in org.apache.kafka.streams.state"><code>SessionStore</code></a> it must be obtained via
<a href="../KafkaStreams.html#store(org.apache.kafka.streams.StoreQueryParameters)"><code>KafkaStreams.store(StoreQueryParameters)</code></a> KafkaStreams#store(...)}:
<pre><code>
KafkaStreams streams = ... // compute sum
Sting queryableStoreName = ... // the queryableStoreName should be the name of the store as defined by the Materialized instance
ReadOnlySessionStore&lt;String,Long&gt; localWindowStore = streams.store(queryableStoreName, QueryableStoreTypes.&lt;String, Long&gt;ReadOnlySessionStore&lt;String, Long&gt;);
String key = "some-key";
KeyValueIterator&lt;Windowed&lt;String&gt;, Long&gt; sumForKeyForWindows = localWindowStore.fetch(key); // key must be local (application state is shared over all running Kafka Streams instances)
</code></pre>
For non-local keys, a custom RPC mechanism must be implemented using <a href="../KafkaStreams.html#allMetadata()"><code>KafkaStreams.allMetadata()</code></a> to
query the value of the key on a parallel running instance of your Kafka Streams application.
<p>
For failure and recovery the store will be backed by an internal changelog topic that will be created in Kafka.
Therefore, the store name defined by the Materialized instance must be a valid Kafka topic name and cannot
contain characters other than ASCII alphanumerics, '.', '_' and '-'.
The changelog topic will be named "${applicationId}-${storeName}-changelog", where "applicationId" is
user-specified in <a href="../StreamsConfig.html" title="class in org.apache.kafka.streams"><code>StreamsConfig</code></a> via parameter
<a href="../StreamsConfig.html#APPLICATION_ID_CONFIG"><code>APPLICATION_ID_CONFIG</code></a>, "storeName" is the provide store name defined
in <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a>, and "-changelog" is a fixed suffix.
<p>
You can retrieve all generated internal topic names via <a href="../Topology.html#describe()"><code>Topology.describe()</code></a>.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>reducer</code> - a <a href="Reducer.html" title="interface in org.apache.kafka.streams.kstream"><code>Reducer</code></a> that computes a new aggregate result. Cannot be <code>null</code>.</dd>
<dd><code>named</code> - a <a href="Named.html" title="class in org.apache.kafka.streams.kstream"><code>Named</code></a> config used to name the processor in the topology. Cannot be <code>null</code>.</dd>
<dd><code>materialized</code> - a <a href="Materialized.html" title="class in org.apache.kafka.streams.kstream"><code>Materialized</code></a> config used to materialize a state store. Cannot be <code>null</code>.</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a windowed <a href="KTable.html" title="interface in org.apache.kafka.streams.kstream"><code>KTable</code></a> that contains "update" records with unmodified keys, and values that represent
the latest (rolling) aggregate for each key per session</dd>
</dl>
</li>
</ul>
</li>
</ul>
</section>
</li>
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