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| <article class="doc"> |
| <h1 class="page">Change Data Capture</h1> |
| <div class="sect1"> |
| <h2 id="overview"><a class="anchor" href="#overview"></a><a class="link" href="#overview">Overview</a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <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>cdc=true</code> (either |
| when <a href="../cql/ddl.html#create-table" class="page"><code>creating the table</code></a> or |
| <a href="../cql/ddl.html#alter-table" class="page"><code>altering it</code></a>). Upon CommitLogSegment creation, |
| a hard-link to the segment is created in the directory specified in |
| <code>cassandra.yaml</code>. On segment fsync to disk, if CDC data is present |
| anywhere in the segment a <segment_name>_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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| <div class="sect1"> |
| <h2 id="configuration"><a class="anchor" href="#configuration"></a><a class="link" href="#configuration">Configuration</a></h2> |
| <div class="sectionbody"> |
| <div class="sect2"> |
| <h3 id="enabling-or-disabling-cdc-on-a-table"><a class="anchor" href="#enabling-or-disabling-cdc-on-a-table"></a><a class="link" href="#enabling-or-disabling-cdc-on-a-table">Enabling or disabling CDC on a table</a></h3> |
| <div class="paragraph"> |
| <p>CDC is enable or disable through the <span class="title-ref">cdc</span> table property, |
| for instance:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-cql hljs" data-lang="cql">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;</code></pre> |
| </div> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="cassandra-yaml-parameters"><a class="anchor" href="#cassandra-yaml-parameters"></a><a class="link" href="#cassandra-yaml-parameters">cassandra.yaml parameters</a></h3> |
| <div class="paragraph"> |
| <p>The following cassandra.yaml options are available for CDC:</p> |
| </div> |
| <div class="dlist"> |
| <dl> |
| <dt class="hdlist1"><code>cdc_enabled</code> (default: false)</dt> |
| <dd> |
| <p>Enable or disable CDC operations node-wide.</p> |
| </dd> |
| <dt class="hdlist1"><code>cdc_raw_directory</code> (default: <code>$CASSANDRA_HOME/data/cdc_raw</code>)</dt> |
| <dd> |
| <p>Destination for CommitLogSegments to be moved after all corresponding |
| memtables are flushed.</p> |
| </dd> |
| <dt class="hdlist1"><code>cdc_free_space_in_mb</code>: (default: min of 4096 and 1/8th volume space)</dt> |
| <dd> |
| <p>Calculated as sum of all active CommitLogSegments that permit CDC<br> |
| all flushed CDC segments in <code>cdc_raw_directory</code>.</p> |
| </dd> |
| <dt class="hdlist1"><code>cdc_free_space_check_interval_ms</code> (default: 250)</dt> |
| <dd> |
| <p>When at capacity, we limit the frequency with which we re-calculate |
| the space taken up by <code>cdc_raw_directory</code> to prevent burning CPU |
| cycles unnecessarily. Default is to check 4 times per second.</p> |
| </dd> |
| </dl> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="reading-commitlogsegments"><a class="anchor" href="#reading-commitlogsegments"></a><a class="link" href="#reading-commitlogsegments">Reading CommitLogSegments</a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Use a |
| <a href="https://github.com/apache/cassandra/blob/e31e216234c6b57a531cae607e0355666007deb2/src/java/org/apache/cassandra/db/commitlog/CommitLogReader.java">CommitLogReader.java</a>. |
| Usage is |
| <a 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 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 href="https://github.com/apache/cassandra/blob/e31e216234c6b57a531cae607e0355666007deb2/src/java/org/apache/cassandra/db/commitlog/CommitLogReadHandler.java">CommitLogReadHandler</a>.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="warnings"><a class="anchor" href="#warnings"></a><a class="link" href="#warnings">Warnings</a></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p><strong>Do not enable CDC without some kind of consumption process in-place.</strong></p> |
| </div> |
| <div class="paragraph"> |
| <p>If CDC is enabled on a node and then on a table, the |
| <code>cdc_free_space_in_mb</code> will fill up and then writes to CDC-enabled |
| tables will be rejected unless some consumption process is in place.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="further-reading"><a class="anchor" href="#further-reading"></a><a class="link" href="#further-reading">Further Reading</a></h2> |
| <div class="sectionbody"> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><a href="https://issues.apache.org/jira/browse/CASSANDRA-8844">JIRA ticket</a></p> |
| </li> |
| <li> |
| <p><a href="https://issues.apache.org/jira/browse/CASSANDRA-12148">JIRA ticket</a></p> |
| </li> |
| </ul> |
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