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<li class="toctree-l2 current"><a class="current reference internal" href="#">Change Data Capture</a><ul>
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  <div class="section" id="change-data-capture">
<h1>Change Data Capture<a class="headerlink" href="#change-data-capture" title="Permalink to this headline">¶</a></h1>
<div class="section" id="overview">
<h2>Overview<a class="headerlink" href="#overview" title="Permalink to this headline">¶</a></h2>
<p>Change data capture (CDC) provides a mechanism to flag specific tables for archival as well as rejecting writes to those
tables once a configurable size-on-disk for the CDC log is reached. An operator can enable CDC on a table by setting the
table property <code class="docutils literal notranslate"><span class="pre">cdc=true</span></code> (either when <a class="reference internal" href="../cql/ddl.html#create-table-statement"><span class="std std-ref">creating the table</span></a> or
<a class="reference internal" href="../cql/ddl.html#alter-table-statement"><span class="std std-ref">altering it</span></a>). Upon CommitLogSegment creation, a hard-link to the segment is created in the
directory specified in <code class="docutils literal notranslate"><span class="pre">cassandra.yaml</span></code>. On segment fsync to disk, if CDC data is present anywhere in the segment a
&lt;segment_name&gt;_cdc.idx file is also created with the integer offset of how much data in the original segment is persisted
to disk. Upon final segment flush, a second line with the human-readable word “COMPLETED” will be added to the _cdc.idx
file indicating that Cassandra has completed all processing on the file.</p>
<p>We we use an index file rather than just encouraging clients to parse the log realtime off a memory mapped handle as data
can be reflected in a kernel buffer that is not yet persisted to disk. Parsing only up to the listed offset in the _cdc.idx
file will ensure that you only parse CDC data for data that is durable.</p>
<p>A threshold of total disk space allowed is specified in the yaml at which time newly allocated CommitLogSegments will
not allow CDC data until a consumer parses and removes files from the specified cdc_raw directory.</p>
</div>
<div class="section" id="configuration">
<h2>Configuration<a class="headerlink" href="#configuration" title="Permalink to this headline">¶</a></h2>
<div class="section" id="enabling-or-disabling-cdc-on-a-table">
<h3>Enabling or disabling CDC on a table<a class="headerlink" href="#enabling-or-disabling-cdc-on-a-table" title="Permalink to this headline">¶</a></h3>
<p>CDC is enable or disable through the <cite>cdc</cite> table property, for instance:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>CREATE TABLE foo (a int, b text, PRIMARY KEY(a)) WITH cdc=true;

ALTER TABLE foo WITH cdc=true;

ALTER TABLE foo WITH cdc=false;
</pre></div>
</div>
</div>
<div class="section" id="cassandra-yaml-parameters">
<h3>cassandra.yaml parameters<a class="headerlink" href="#cassandra-yaml-parameters" title="Permalink to this headline">¶</a></h3>
<p>The following <cite>cassandra.yaml</cite> are available for CDC:</p>
<dl class="docutils">
<dt><code class="docutils literal notranslate"><span class="pre">cdc_enabled</span></code> (default: false)</dt>
<dd>Enable or disable CDC operations node-wide.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">cdc_raw_directory</span></code> (default: <code class="docutils literal notranslate"><span class="pre">$CASSANDRA_HOME/data/cdc_raw</span></code>)</dt>
<dd>Destination for CommitLogSegments to be moved after all corresponding memtables are flushed.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">cdc_free_space_in_mb</span></code>: (default: min of 4096 and 1/8th volume space)</dt>
<dd>Calculated as sum of all active CommitLogSegments that permit CDC + all flushed CDC segments in
<code class="docutils literal notranslate"><span class="pre">cdc_raw_directory</span></code>.</dd>
<dt><code class="docutils literal notranslate"><span class="pre">cdc_free_space_check_interval_ms</span></code> (default: 250)</dt>
<dd>When at capacity, we limit the frequency with which we re-calculate the space taken up by <code class="docutils literal notranslate"><span class="pre">cdc_raw_directory</span></code> to
prevent burning CPU cycles unnecessarily. Default is to check 4 times per second.</dd>
</dl>
</div>
</div>
<div class="section" id="reading-commitlogsegments">
<span id="id1"></span><h2>Reading CommitLogSegments<a class="headerlink" href="#reading-commitlogsegments" title="Permalink to this headline">¶</a></h2>
<p>Use a <a class="reference external" href="https://github.com/apache/cassandra/blob/e31e216234c6b57a531cae607e0355666007deb2/src/java/org/apache/cassandra/db/commitlog/CommitLogReader.java">CommitLogReader.java</a>.
Usage is <a class="reference external" href="https://github.com/apache/cassandra/blob/e31e216234c6b57a531cae607e0355666007deb2/src/java/org/apache/cassandra/db/commitlog/CommitLogReplayer.java#L132-L140">fairly straightforward</a>
with a <a class="reference external" href="https://github.com/apache/cassandra/blob/e31e216234c6b57a531cae607e0355666007deb2/src/java/org/apache/cassandra/db/commitlog/CommitLogReader.java#L71-L103">variety of signatures</a>
available for use. In order to handle mutations read from disk, implement <a class="reference external" href="https://github.com/apache/cassandra/blob/e31e216234c6b57a531cae607e0355666007deb2/src/java/org/apache/cassandra/db/commitlog/CommitLogReadHandler.java">CommitLogReadHandler</a>.</p>
</div>
<div class="section" id="warnings">
<h2>Warnings<a class="headerlink" href="#warnings" title="Permalink to this headline">¶</a></h2>
<p><strong>Do not enable CDC without some kind of consumption process in-place.</strong></p>
<p>If CDC is enabled on a node and then on a table, the <code class="docutils literal notranslate"><span class="pre">cdc_free_space_in_mb</span></code> will fill up and then writes to
CDC-enabled tables will be rejected unless some consumption process is in place.</p>
</div>
<div class="section" id="further-reading">
<h2>Further Reading<a class="headerlink" href="#further-reading" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><a class="reference external" href="https://issues.apache.org/jira/browse/CASSANDRA-8844">JIRA ticket</a></li>
<li><a class="reference external" href="https://issues.apache.org/jira/browse/CASSANDRA-12148">JIRA ticket</a></li>
</ul>
</div>
</div>


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