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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 |
| <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> |
| <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> |
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