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<article data-swiftype-index='true'>
<a class='edit-link' href="https://github.com/apache/ignite/tree/IGNITE-7595/docs/_docs/key-value-api/continuous-queries.adoc" target="_blank">Edit</a>
<h1>Using Continuous Queries</h1>
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>A continuous query is a query that monitors data modifications occurring in a cache.
Once a continuous query is started, you get notified of all the data changes that fall into your query filter.</p>
</div>
<div class="paragraph">
<p>All update events are propagated to the <a href="#local-listener">local listener</a> that must be registered in the query.
Continuous query implementation guarantees exactly once delivery of an event to the local listener.</p>
</div>
<div class="paragraph">
<p>You can also specify a remote filter to narrow down the range of entries that are monitored for updates.</p>
</div>
<div class="admonitionblock caution">
<table>
<tr>
<td class="icon">
<div class="title">Caution</div>
</td>
<td class="content">
<h3 id="continuous-queries-and-mvcc" class="discrete">Continuous Queries and MVCC</h3>
<div class="paragraph">
<p>Continuous queries have a number of <a href="/docs/latest/transactions/mvcc">functional limitations</a> when used with MVCC-enabled caches.</p>
</div>
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="local-listener">Local Listener</h2>
<div class="sectionbody">
<div class="paragraph">
<p>When a cache gets modified (an entry is inserted, updated, or deleted), an event is sent to the continuous query&#8217;s local listener so that your application can react accordingly.
The local listener is executed on the node that initiated the query.</p>
</div>
<div class="paragraph">
<p>Note that the continuous query throws an exception if started without a local listener.</p>
</div>
<code-tabs><code-tab data-tab='Java'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="java"><span class="nc">IgniteCache</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;</span> <span class="n">cache</span> <span class="o">=</span> <span class="n">ignite</span><span class="o">.</span><span class="na">getOrCreateCache</span><span class="o">(</span><span class="s">"myCache"</span><span class="o">);</span>
<span class="nc">ContinuousQuery</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;</span> <span class="n">query</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ContinuousQuery</span><span class="o">&lt;&gt;();</span>
<span class="n">query</span><span class="o">.</span><span class="na">setLocalListener</span><span class="o">(</span><span class="k">new</span> <span class="nc">CacheEntryUpdatedListener</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;()</span> <span class="o">{</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="kt">void</span> <span class="nf">onUpdated</span><span class="o">(</span><span class="nc">Iterable</span><span class="o">&lt;</span><span class="nc">CacheEntryEvent</span><span class="o">&lt;?</span> <span class="kd">extends</span> <span class="nc">Integer</span><span class="o">,</span> <span class="o">?</span> <span class="kd">extends</span> <span class="nc">String</span><span class="o">&gt;&gt;</span> <span class="n">events</span><span class="o">)</span>
<span class="kd">throws</span> <span class="nc">CacheEntryListenerException</span> <span class="o">{</span>
<span class="c1">// react to the update events here</span>
<span class="o">}</span>
<span class="o">});</span>
<span class="n">cache</span><span class="o">.</span><span class="na">query</span><span class="o">(</span><span class="n">query</span><span class="o">);</span></code></pre>
</div>
</div></code-tab><code-tab data-tab='C#/.NET'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="csharp"><span class="k">class</span> <span class="nc">LocalListener</span> <span class="p">:</span> <span class="n">ICacheEntryEventListener</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;</span>
<span class="p">{</span>
<span class="k">public</span> <span class="k">void</span> <span class="nf">OnEvent</span><span class="p">(</span><span class="n">IEnumerable</span><span class="p">&lt;</span><span class="n">ICacheEntryEvent</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;&gt;</span> <span class="n">evts</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">foreach</span> <span class="p">(</span><span class="kt">var</span> <span class="n">cacheEntryEvent</span> <span class="k">in</span> <span class="n">evts</span><span class="p">)</span>
<span class="p">{</span>
<span class="c1">//react to update events here</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">public</span> <span class="k">static</span> <span class="k">void</span> <span class="nf">ContinuousQueryListenerDemo</span><span class="p">()</span>
<span class="p">{</span>
<span class="kt">var</span> <span class="n">ignite</span> <span class="p">=</span> <span class="n">Ignition</span><span class="p">.</span><span class="nf">Start</span><span class="p">(</span><span class="k">new</span> <span class="n">IgniteConfiguration</span>
<span class="p">{</span>
<span class="n">DiscoverySpi</span> <span class="p">=</span> <span class="k">new</span> <span class="n">TcpDiscoverySpi</span>
<span class="p">{</span>
<span class="n">LocalPort</span> <span class="p">=</span> <span class="m">48500</span><span class="p">,</span>
<span class="n">LocalPortRange</span> <span class="p">=</span> <span class="m">20</span><span class="p">,</span>
<span class="n">IpFinder</span> <span class="p">=</span> <span class="k">new</span> <span class="n">TcpDiscoveryStaticIpFinder</span>
<span class="p">{</span>
<span class="n">Endpoints</span> <span class="p">=</span> <span class="k">new</span><span class="p">[]</span>
<span class="p">{</span>
<span class="s">"127.0.0.1:48500..48520"</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">});</span>
<span class="kt">var</span> <span class="n">cache</span> <span class="p">=</span> <span class="n">ignite</span><span class="p">.</span><span class="n">GetOrCreateCache</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;(</span><span class="s">"myCache"</span><span class="p">);</span>
<span class="kt">var</span> <span class="n">query</span> <span class="p">=</span> <span class="k">new</span> <span class="n">ContinuousQuery</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;(</span><span class="k">new</span> <span class="nf">LocalListener</span><span class="p">());</span>
<span class="kt">var</span> <span class="n">handle</span> <span class="p">=</span> <span class="n">cache</span><span class="p">.</span><span class="nf">QueryContinuous</span><span class="p">(</span><span class="n">query</span><span class="p">);</span>
<span class="n">cache</span><span class="p">.</span><span class="nf">Put</span><span class="p">(</span><span class="m">1</span><span class="p">,</span> <span class="s">"1"</span><span class="p">);</span>
<span class="n">cache</span><span class="p">.</span><span class="nf">Put</span><span class="p">(</span><span class="m">2</span><span class="p">,</span> <span class="s">"2"</span><span class="p">);</span>
<span class="p">}</span></code></pre>
</div>
</div></code-tab><code-tab data-tab='C++'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cpp"><span class="cm">/**
* Listener class.
*/</span>
<span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="nc">K</span><span class="p">,</span> <span class="k">typename</span> <span class="nc">V</span><span class="p">&gt;</span>
<span class="k">class</span> <span class="nc">Listener</span> <span class="o">:</span> <span class="k">public</span> <span class="n">event</span><span class="o">::</span><span class="n">CacheEntryEventListener</span><span class="o">&lt;</span><span class="n">K</span><span class="p">,</span> <span class="n">V</span><span class="o">&gt;</span>
<span class="p">{</span>
<span class="nl">public:</span>
<span class="cm">/**
* Default constructor.
*/</span>
<span class="n">Listener</span><span class="p">()</span>
<span class="p">{</span>
<span class="c1">// No-op.</span>
<span class="p">}</span>
<span class="cm">/**
* Event callback.
*
* @param evts Events.
* @param num Events number.
*/</span>
<span class="k">virtual</span> <span class="kt">void</span> <span class="n">OnEvent</span><span class="p">(</span><span class="k">const</span> <span class="n">CacheEntryEvent</span><span class="o">&lt;</span><span class="n">K</span><span class="p">,</span> <span class="n">V</span><span class="o">&gt;*</span> <span class="n">evts</span><span class="p">,</span> <span class="kt">uint32_t</span> <span class="n">num</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">for</span> <span class="p">(</span><span class="kt">uint32_t</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o">&lt;</span> <span class="n">num</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"Queried entry [key="</span> <span class="o">&lt;&lt;</span> <span class="p">(</span><span class="n">evts</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">HasValue</span><span class="p">()</span> <span class="o">?</span> <span class="n">evts</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">GetKey</span><span class="p">()</span> <span class="o">:</span> <span class="n">K</span><span class="p">())</span>
<span class="o">&lt;&lt;</span> <span class="s">", val="</span> <span class="o">&lt;&lt;</span> <span class="p">(</span><span class="n">evts</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">HasValue</span><span class="p">()</span> <span class="o">?</span> <span class="n">evts</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">GetValue</span><span class="p">()</span> <span class="o">:</span> <span class="n">V</span><span class="p">())</span> <span class="o">&lt;&lt;</span> <span class="sc">']'</span>
<span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">};</span>
<span class="kt">int</span> <span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="n">IgniteConfiguration</span> <span class="n">cfg</span><span class="p">;</span>
<span class="n">cfg</span><span class="p">.</span><span class="n">springCfgPath</span> <span class="o">=</span> <span class="s">"/path/to/configuration.xml"</span><span class="p">;</span>
<span class="n">Ignite</span> <span class="n">ignite</span> <span class="o">=</span> <span class="n">Ignition</span><span class="o">::</span><span class="n">Start</span><span class="p">(</span><span class="n">cfg</span><span class="p">);</span>
<span class="n">Cache</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">cache</span> <span class="o">=</span> <span class="n">ignite</span><span class="p">.</span><span class="n">GetOrCreateCache</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span><span class="p">(</span><span class="s">"myCache"</span><span class="p">);</span>
<span class="c1">// Declaring custom listener.</span>
<span class="n">Listener</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">listener</span><span class="p">;</span>
<span class="c1">// Declaring continuous query.</span>
<span class="n">continuous</span><span class="o">::</span><span class="n">ContinuousQuery</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">query</span><span class="p">(</span><span class="n">MakeReference</span><span class="p">(</span><span class="n">listener</span><span class="p">));</span>
<span class="n">continuous</span><span class="o">::</span><span class="n">ContinuousQueryHandle</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">handle</span> <span class="o">=</span> <span class="n">cache</span><span class="p">.</span><span class="n">QueryContinuous</span><span class="p">(</span><span class="n">query</span><span class="p">);</span>
<span class="p">}</span></code></pre>
</div>
</div></code-tab></code-tabs>
</div>
</div>
<div class="sect1">
<h2 id="initial-query">Initial Query</h2>
<div class="sectionbody">
<div class="paragraph">
<p>You can specify an initial query that is executed before the continuous query gets registered in the cluster and before you start to receive updates.
To specify an initial query, use the <code>ContinuousQuery.setInitialQuery(&#8230;&#8203;)</code> method.</p>
</div>
<div class="paragraph">
<p>Just like scan queries, a continuous query is executed via the <code>query()</code> method that returns a cursor. When an initial query is set, you can use that cursor to iterate over the results of the initial query.</p>
</div>
<code-tabs><code-tab data-tab='Java'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="java"><span class="nc">IgniteCache</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;</span> <span class="n">cache</span> <span class="o">=</span> <span class="n">ignite</span><span class="o">.</span><span class="na">getOrCreateCache</span><span class="o">(</span><span class="s">"myCache"</span><span class="o">);</span>
<span class="nc">ContinuousQuery</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;</span> <span class="n">query</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ContinuousQuery</span><span class="o">&lt;&gt;();</span>
<span class="c1">// Setting an optional initial query.</span>
<span class="c1">// The query will return entries for the keys greater than 10.</span>
<span class="n">query</span><span class="o">.</span><span class="na">setInitialQuery</span><span class="o">(</span><span class="k">new</span> <span class="nc">ScanQuery</span><span class="o">&lt;&gt;((</span><span class="n">k</span><span class="o">,</span> <span class="n">v</span><span class="o">)</span> <span class="o">-&gt;</span> <span class="n">k</span> <span class="o">&gt;</span> <span class="mi">10</span><span class="o">));</span>
<span class="c1">//mandatory local listener</span>
<span class="n">query</span><span class="o">.</span><span class="na">setLocalListener</span><span class="o">(</span><span class="n">events</span> <span class="o">-&gt;</span> <span class="o">{</span>
<span class="o">});</span>
<span class="k">try</span> <span class="o">(</span><span class="nc">QueryCursor</span><span class="o">&lt;</span><span class="nc">Cache</span><span class="o">.</span><span class="na">Entry</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;&gt;</span> <span class="n">cursor</span> <span class="o">=</span> <span class="n">cache</span><span class="o">.</span><span class="na">query</span><span class="o">(</span><span class="n">query</span><span class="o">))</span> <span class="o">{</span>
<span class="c1">// Iterating over the entries returned by the initial query</span>
<span class="k">for</span> <span class="o">(</span><span class="nc">Cache</span><span class="o">.</span><span class="na">Entry</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;</span> <span class="n">e</span> <span class="o">:</span> <span class="n">cursor</span><span class="o">)</span>
<span class="nc">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">"key="</span> <span class="o">+</span> <span class="n">e</span><span class="o">.</span><span class="na">getKey</span><span class="o">()</span> <span class="o">+</span> <span class="s">", val="</span> <span class="o">+</span> <span class="n">e</span><span class="o">.</span><span class="na">getValue</span><span class="o">());</span>
<span class="o">}</span></code></pre>
</div>
</div></code-tab><code-tab data-tab='C#/.NET' data-unavailable='true'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code>This API is not presently available for C#/.NET.</code></pre>
</div>
</div></code-tab><code-tab data-tab='C++'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cpp"><span class="n">Cache</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">cache</span> <span class="o">=</span> <span class="n">ignite</span><span class="p">.</span><span class="n">GetOrCreateCache</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span><span class="p">(</span><span class="s">"myCache"</span><span class="p">);</span>
<span class="c1">// Custom listener</span>
<span class="n">Listener</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">listener</span><span class="p">;</span>
<span class="c1">// Declaring continuous query.</span>
<span class="n">continuous</span><span class="o">::</span><span class="n">ContinuousQuery</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">query</span><span class="p">(</span><span class="n">MakeReference</span><span class="p">(</span><span class="n">listener</span><span class="p">));</span>
<span class="c1">// Declaring optional initial query</span>
<span class="n">ScanQuery</span> <span class="n">initialQuery</span> <span class="o">=</span> <span class="n">ScanQuery</span><span class="p">();</span>
<span class="n">continuous</span><span class="o">::</span><span class="n">ContinuousQueryHandle</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">handle</span> <span class="o">=</span> <span class="n">cache</span><span class="p">.</span><span class="n">QueryContinuous</span><span class="p">(</span><span class="n">query</span><span class="p">,</span> <span class="n">initialQuery</span><span class="p">);</span>
<span class="c1">// Iterating over existing data stored in the cache.</span>
<span class="n">QueryCursor</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">cursor</span> <span class="o">=</span> <span class="n">handle</span><span class="p">.</span><span class="n">GetInitialQueryCursor</span><span class="p">();</span>
<span class="k">while</span> <span class="p">(</span><span class="n">cursor</span><span class="p">.</span><span class="n">HasNext</span><span class="p">())</span>
<span class="p">{</span>
<span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="n">cursor</span><span class="p">.</span><span class="n">GetNext</span><span class="p">().</span><span class="n">GetKey</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">}</span></code></pre>
</div>
</div></code-tab></code-tabs>
</div>
</div>
<div class="sect1">
<h2 id="remote-filter">Remote Filter</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This filter is executed for each updated key and evaluates whether the update should be propagated to the query&#8217;s local listener.
If the filter returns <code>true</code>, then the local listener is notified about the update.</p>
</div>
<div class="paragraph">
<p>For redundancy reasons, the filter is executed for both primary and backup versions (if backups are configured) of the key.
Because of this, a remote filter can be used as a remote listener for update events.</p>
</div>
<code-tabs><code-tab data-tab='Java'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="java"><span class="nc">ContinuousQuery</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;</span> <span class="n">qry</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ContinuousQuery</span><span class="o">&lt;&gt;();</span>
<span class="n">qry</span><span class="o">.</span><span class="na">setLocalListener</span><span class="o">(</span><span class="n">events</span> <span class="o">-&gt;</span>
<span class="n">events</span><span class="o">.</span><span class="na">forEach</span><span class="o">(</span><span class="n">event</span> <span class="o">-&gt;</span> <span class="nc">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">format</span><span class="o">(</span><span class="s">"Entry: key=[%s] value=[%s]\n"</span><span class="o">,</span> <span class="n">event</span><span class="o">.</span><span class="na">getKey</span><span class="o">(),</span> <span class="n">event</span><span class="o">.</span><span class="na">getValue</span><span class="o">()))</span>
<span class="o">);</span>
<span class="n">qry</span><span class="o">.</span><span class="na">setRemoteFilterFactory</span><span class="o">(</span><span class="k">new</span> <span class="nc">Factory</span><span class="o">&lt;</span><span class="nc">CacheEntryEventFilter</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;&gt;()</span> <span class="o">{</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="nc">CacheEntryEventFilter</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;</span> <span class="nf">create</span><span class="o">()</span> <span class="o">{</span>
<span class="k">return</span> <span class="k">new</span> <span class="nc">CacheEntryEventFilter</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;()</span> <span class="o">{</span>
<span class="nd">@Override</span>
<span class="kd">public</span> <span class="kt">boolean</span> <span class="nf">evaluate</span><span class="o">(</span><span class="nc">CacheEntryEvent</span><span class="o">&lt;?</span> <span class="kd">extends</span> <span class="nc">Integer</span><span class="o">,</span> <span class="o">?</span> <span class="kd">extends</span> <span class="nc">String</span><span class="o">&gt;</span> <span class="n">e</span><span class="o">)</span> <span class="o">{</span>
<span class="nc">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">format</span><span class="o">(</span><span class="s">"the value for key [%s] was updated from [%s] to [%s]\n"</span><span class="o">,</span> <span class="n">e</span><span class="o">.</span><span class="na">getKey</span><span class="o">(),</span> <span class="n">e</span><span class="o">.</span><span class="na">getOldValue</span><span class="o">(),</span> <span class="n">e</span><span class="o">.</span><span class="na">getValue</span><span class="o">());</span>
<span class="k">return</span> <span class="kc">true</span><span class="o">;</span>
<span class="o">}</span>
<span class="o">};</span>
<span class="o">}</span>
<span class="o">});</span></code></pre>
</div>
</div></code-tab><code-tab data-tab='C#/.NET'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="csharp"><span class="k">class</span> <span class="nc">LocalListener</span> <span class="p">:</span> <span class="n">ICacheEntryEventListener</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;</span>
<span class="p">{</span>
<span class="k">public</span> <span class="k">void</span> <span class="nf">OnEvent</span><span class="p">(</span><span class="n">IEnumerable</span><span class="p">&lt;</span><span class="n">ICacheEntryEvent</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;&gt;</span> <span class="n">evts</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">foreach</span> <span class="p">(</span><span class="kt">var</span> <span class="n">cacheEntryEvent</span> <span class="k">in</span> <span class="n">evts</span><span class="p">)</span>
<span class="p">{</span>
<span class="c1">//react to update events here</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">class</span> <span class="nc">RemoteFilter</span> <span class="p">:</span> <span class="n">ICacheEntryEventFilter</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;</span>
<span class="p">{</span>
<span class="k">public</span> <span class="kt">bool</span> <span class="nf">Evaluate</span><span class="p">(</span><span class="n">ICacheEntryEvent</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;</span> <span class="n">e</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="n">e</span><span class="p">.</span><span class="n">Key</span> <span class="p">==</span> <span class="m">1</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">return</span> <span class="k">false</span><span class="p">;</span>
<span class="p">}</span>
<span class="n">Console</span><span class="p">.</span><span class="nf">WriteLine</span><span class="p">(</span><span class="s">"the value for key {0} was updated from {1} to {2}"</span><span class="p">,</span> <span class="n">e</span><span class="p">.</span><span class="n">Key</span><span class="p">,</span> <span class="n">e</span><span class="p">.</span><span class="n">OldValue</span><span class="p">,</span> <span class="n">e</span><span class="p">.</span><span class="n">Value</span><span class="p">);</span>
<span class="k">return</span> <span class="k">true</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">public</span> <span class="k">static</span> <span class="k">void</span> <span class="nf">ContinuousQueryFilterDemo</span><span class="p">()</span>
<span class="p">{</span>
<span class="kt">var</span> <span class="n">ignite</span> <span class="p">=</span> <span class="n">Ignition</span><span class="p">.</span><span class="nf">Start</span><span class="p">(</span><span class="k">new</span> <span class="n">IgniteConfiguration</span>
<span class="p">{</span>
<span class="n">DiscoverySpi</span> <span class="p">=</span> <span class="k">new</span> <span class="n">TcpDiscoverySpi</span>
<span class="p">{</span>
<span class="n">LocalPort</span> <span class="p">=</span> <span class="m">48500</span><span class="p">,</span>
<span class="n">LocalPortRange</span> <span class="p">=</span> <span class="m">20</span><span class="p">,</span>
<span class="n">IpFinder</span> <span class="p">=</span> <span class="k">new</span> <span class="n">TcpDiscoveryStaticIpFinder</span>
<span class="p">{</span>
<span class="n">Endpoints</span> <span class="p">=</span> <span class="k">new</span><span class="p">[]</span>
<span class="p">{</span>
<span class="s">"127.0.0.1:48500..48520"</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">});</span>
<span class="kt">var</span> <span class="n">cache</span> <span class="p">=</span> <span class="n">ignite</span><span class="p">.</span><span class="n">GetOrCreateCache</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;(</span><span class="s">"myCache"</span><span class="p">);</span>
<span class="kt">var</span> <span class="n">query</span> <span class="p">=</span> <span class="k">new</span> <span class="n">ContinuousQuery</span><span class="p">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">&gt;(</span><span class="k">new</span> <span class="nf">LocalListener</span><span class="p">(),</span> <span class="k">new</span> <span class="nf">RemoteFilter</span><span class="p">());</span>
<span class="kt">var</span> <span class="n">handle</span> <span class="p">=</span> <span class="n">cache</span><span class="p">.</span><span class="nf">QueryContinuous</span><span class="p">(</span><span class="n">query</span><span class="p">);</span>
<span class="n">cache</span><span class="p">.</span><span class="nf">Put</span><span class="p">(</span><span class="m">1</span><span class="p">,</span> <span class="s">"1"</span><span class="p">);</span>
<span class="n">cache</span><span class="p">.</span><span class="nf">Put</span><span class="p">(</span><span class="m">2</span><span class="p">,</span> <span class="s">"2"</span><span class="p">);</span>
<span class="p">}</span></code></pre>
</div>
</div></code-tab><code-tab data-tab='C++'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="cpp"><span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span> <span class="nc">K</span><span class="p">,</span> <span class="k">typename</span> <span class="nc">V</span><span class="p">&gt;</span>
<span class="k">struct</span> <span class="nc">RemoteFilter</span> <span class="o">:</span> <span class="n">event</span><span class="o">::</span><span class="n">CacheEntryEventFilter</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span>
<span class="p">{</span>
<span class="cm">/**
* Default constructor.
*/</span>
<span class="n">RemoteFilter</span><span class="p">()</span>
<span class="p">{</span>
<span class="c1">// No-op.</span>
<span class="p">}</span>
<span class="cm">/**
* Destructor.
*/</span>
<span class="k">virtual</span> <span class="o">~</span><span class="n">RemoteFilter</span><span class="p">()</span>
<span class="p">{</span>
<span class="c1">// No-op.</span>
<span class="p">}</span>
<span class="cm">/**
* Event callback.
*
* @param event Event.
* @return True if the event passes filter.
*/</span>
<span class="k">virtual</span> <span class="kt">bool</span> <span class="n">Process</span><span class="p">(</span><span class="k">const</span> <span class="n">CacheEntryEvent</span><span class="o">&lt;</span><span class="n">K</span><span class="p">,</span> <span class="n">V</span><span class="o">&gt;&amp;</span> <span class="n">event</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">"The value for key "</span> <span class="o">&lt;&lt;</span> <span class="n">event</span><span class="p">.</span><span class="n">GetKey</span><span class="p">()</span> <span class="o">&lt;&lt;</span>
<span class="s">" was updated from "</span> <span class="o">&lt;&lt;</span> <span class="n">event</span><span class="p">.</span><span class="n">GetOldValue</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="s">" to "</span> <span class="o">&lt;&lt;</span> <span class="n">event</span><span class="p">.</span><span class="n">GetValue</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="k">return</span> <span class="nb">true</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">};</span>
<span class="k">namespace</span> <span class="n">ignite</span>
<span class="p">{</span>
<span class="k">namespace</span> <span class="n">binary</span>
<span class="p">{</span>
<span class="k">template</span><span class="o">&lt;</span><span class="p">&gt;</span>
<span class="k">struct</span> <span class="nc">BinaryType</span><span class="o">&lt;</span> <span class="n">RemoteFilter</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="o">&gt;</span>
<span class="p">{</span>
<span class="k">static</span> <span class="kt">int32_t</span> <span class="n">GetTypeId</span><span class="p">()</span>
<span class="p">{</span>
<span class="k">return</span> <span class="n">GetBinaryStringHashCode</span><span class="p">(</span><span class="s">"RemoteFilter&lt;int32_t,std::string&gt;"</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">static</span> <span class="kt">void</span> <span class="n">GetTypeName</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span> <span class="n">dst</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">dst</span> <span class="o">=</span> <span class="s">"RemoteFilter&lt;int32_t,std::string&gt;"</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">static</span> <span class="kt">int32_t</span> <span class="n">GetFieldId</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span><span class="o">*</span> <span class="n">name</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">return</span> <span class="n">GetBinaryStringHashCode</span><span class="p">(</span><span class="n">name</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">static</span> <span class="kt">bool</span> <span class="n">IsNull</span><span class="p">(</span><span class="k">const</span> <span class="n">RemoteFilter</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;&amp;</span><span class="p">)</span>
<span class="p">{</span>
<span class="k">return</span> <span class="nb">false</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">static</span> <span class="kt">void</span> <span class="n">GetNull</span><span class="p">(</span><span class="n">RemoteFilter</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;&amp;</span> <span class="n">dst</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">dst</span> <span class="o">=</span> <span class="n">RemoteFilter</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span><span class="p">();</span>
<span class="p">}</span>
<span class="k">static</span> <span class="kt">void</span> <span class="n">Write</span><span class="p">(</span><span class="n">BinaryWriter</span><span class="o">&amp;</span> <span class="n">writer</span><span class="p">,</span> <span class="k">const</span> <span class="n">RemoteFilter</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;&amp;</span> <span class="n">obj</span><span class="p">)</span>
<span class="p">{</span>
<span class="c1">// No-op.</span>
<span class="p">}</span>
<span class="k">static</span> <span class="kt">void</span> <span class="n">Read</span><span class="p">(</span><span class="n">BinaryReader</span><span class="o">&amp;</span> <span class="n">reader</span><span class="p">,</span> <span class="n">RemoteFilter</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;&amp;</span> <span class="n">dst</span><span class="p">)</span>
<span class="p">{</span>
<span class="c1">// No-op.</span>
<span class="p">}</span>
<span class="p">};</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="kt">int</span> <span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="n">IgniteConfiguration</span> <span class="n">cfg</span><span class="p">;</span>
<span class="n">cfg</span><span class="p">.</span><span class="n">springCfgPath</span> <span class="o">=</span> <span class="s">"/path/to/configuration.xml"</span><span class="p">;</span>
<span class="c1">// Start a node.</span>
<span class="n">Ignite</span> <span class="n">ignite</span> <span class="o">=</span> <span class="n">Ignition</span><span class="o">::</span><span class="n">Start</span><span class="p">(</span><span class="n">cfg</span><span class="p">);</span>
<span class="c1">// Get binding.</span>
<span class="n">IgniteBinding</span> <span class="n">binding</span> <span class="o">=</span> <span class="n">ignite</span><span class="p">.</span><span class="n">GetBinding</span><span class="p">();</span>
<span class="c1">// Registering remote filter.</span>
<span class="n">binding</span><span class="p">.</span><span class="n">RegisterCacheEntryEventFilter</span><span class="o">&lt;</span><span class="n">RemoteFilter</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;&gt;</span><span class="p">();</span>
<span class="c1">// Get cache instance.</span>
<span class="n">Cache</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">cache</span> <span class="o">=</span> <span class="n">ignite</span><span class="p">.</span><span class="n">GetOrCreateCache</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span><span class="p">(</span><span class="s">"myCache"</span><span class="p">);</span>
<span class="c1">// Declaring custom listener.</span>
<span class="n">Listener</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">listener</span><span class="p">;</span>
<span class="c1">// Declaring filter.</span>
<span class="n">RemoteFilter</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">filter</span><span class="p">;</span>
<span class="c1">// Declaring continuous query.</span>
<span class="n">continuous</span><span class="o">::</span><span class="n">ContinuousQuery</span><span class="o">&lt;</span><span class="kt">int32_t</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">qry</span><span class="p">(</span><span class="n">MakeReference</span><span class="p">(</span><span class="n">listener</span><span class="p">),</span> <span class="n">MakeReference</span><span class="p">(</span><span class="n">filter</span><span class="p">));</span>
<span class="p">}</span></code></pre>
</div>
</div></code-tab></code-tabs>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
<div class="paragraph">
<p>In order to use remote filters, make sure the class definitions of the filters are available on the server nodes.
You can do this in two ways:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Add the classes to the classpath of every server node;</p>
</li>
<li>
<p><a href="/docs/latest/peer-class-loading">Enable peer class loading</a>.</p>
</li>
</ul>
</div>
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="remote-transformer">Remote Transformer</h2>
<div class="sectionbody">
<div class="paragraph">
<p>By default, continuous queries send the whole updated object to the local listener. This can lead to excessive network usage, especially if the object is very large. Moreover, applications often need only a subset of fields of the object.</p>
</div>
<div class="paragraph">
<p>To address these cases, you can use a continuous query with a transformer. A transformer is a function that is executed on remote nodes for every updated object and sends back only the results of the transformation.</p>
</div>
<code-tabs><code-tab data-tab='Java'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code data-lang="java"><span class="nc">IgniteCache</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">Person</span><span class="o">&gt;</span> <span class="n">cache</span> <span class="o">=</span> <span class="n">ignite</span><span class="o">.</span><span class="na">getOrCreateCache</span><span class="o">(</span><span class="s">"myCache"</span><span class="o">);</span>
<span class="c1">// Create a new continuous query with a transformer.</span>
<span class="nc">ContinuousQueryWithTransformer</span><span class="o">&lt;</span><span class="nc">Integer</span><span class="o">,</span> <span class="nc">Person</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;</span> <span class="n">qry</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">ContinuousQueryWithTransformer</span><span class="o">&lt;&gt;();</span>
<span class="c1">// Factory to create transformers.</span>
<span class="nc">Factory</span> <span class="n">factory</span> <span class="o">=</span> <span class="nc">FactoryBuilder</span><span class="o">.</span><span class="na">factoryOf</span><span class="o">(</span>
<span class="c1">// Return one field of a complex object.</span>
<span class="c1">// Only this field will be sent over to the local listener.</span>
<span class="o">(</span><span class="nc">IgniteClosure</span><span class="o">&lt;</span><span class="nc">CacheEntryEvent</span><span class="o">,</span> <span class="nc">String</span><span class="o">&gt;)</span>
<span class="n">event</span> <span class="o">-&gt;</span> <span class="o">((</span><span class="nc">Person</span><span class="o">)</span><span class="n">event</span><span class="o">.</span><span class="na">getValue</span><span class="o">()).</span><span class="na">getName</span><span class="o">()</span>
<span class="o">);</span>
<span class="n">qry</span><span class="o">.</span><span class="na">setRemoteTransformerFactory</span><span class="o">(</span><span class="n">factory</span><span class="o">);</span>
<span class="c1">// Listener that will receive transformed data.</span>
<span class="n">qry</span><span class="o">.</span><span class="na">setLocalListener</span><span class="o">(</span><span class="n">names</span> <span class="o">-&gt;</span> <span class="o">{</span>
<span class="k">for</span> <span class="o">(</span><span class="nc">String</span> <span class="n">name</span> <span class="o">:</span> <span class="n">names</span><span class="o">)</span>
<span class="nc">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">"New person name: "</span> <span class="o">+</span> <span class="n">name</span><span class="o">);</span>
<span class="o">});</span></code></pre>
</div>
</div></code-tab><code-tab data-tab='C#/.NET' data-unavailable='true'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code>This API is not presently available for C#/.NET.</code></pre>
</div>
</div></code-tab><code-tab data-tab='C++' data-unavailable='true'><div class="listingblock">
<div class="content">
<pre class="rouge highlight"><code>This API is not presently available for C++.</code></pre>
</div>
</div></code-tab></code-tabs>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
<div class="paragraph">
<p>In order to use transformers, make sure the class definitions of the transformers are available on the server nodes.
You can do this in two ways:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Add the classes to the classpath of every server node;</p>
</li>
<li>
<p><a href="/docs/latest/peer-class-loading">Enable peer class loading</a>.</p>
</li>
</ul>
</div>
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="examples">Examples</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The following application examples show typical usage of continuous queries.</p>
</div>
<div class="paragraph">
<p><a href="https://github.com/apache/ignite/tree/master/examples/src/main/java/org/apache/ignite/examples/datagrid/CacheContinuousQueryExample.java" target="_blank" rel="noopener">CacheContinuousQueryExample.java</a></p>
</div>
<div class="paragraph">
<p><a href="https://github.com/apache/ignite/tree/master/examples/src/main/java/org/apache/ignite/examples/datagrid/CacheContinuousAsyncQueryExample.java" target="_blank" rel="noopener">CacheContinuousAsyncQueryExample.java</a></p>
</div>
<div class="paragraph">
<p><a href="https://github.com/apache/ignite/tree/master/examples/src/main/java/org/apache/ignite/examples/datagrid/CacheContinuousQueryWithTransformerExample.java" target="_blank" rel="noopener">CacheContinuousQueryWithTransformerExample.java</a></p>
</div>
</div>
</div>
<div class="copyright">
© 2020 The Apache Software Foundation.<br/>
Apache, Apache Ignite, the Apache feather and the Apache Ignite logo are either registered trademarks or trademarks of The Apache Software Foundation.
</div>
</article>
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<ul class="sectlevel1">
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<li><a href="#initial-query">Initial Query</a></li>
<li><a href="#remote-filter">Remote Filter</a></li>
<li><a href="#remote-transformer">Remote Transformer</a></li>
<li><a href="#examples">Examples</a></li>
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