| --- |
| layout: docpage |
| |
| title: "Documentation" |
| |
| is_homepage: false |
| is_sphinx_doc: true |
| |
| doc-parent: "Operating Cassandra" |
| |
| doc-title: "Change Data Capture" |
| doc-header-links: ' |
| <link rel="top" title="Apache Cassandra Documentation v4.0-alpha4" href="../index.html"/> |
| <link rel="up" title="Operating Cassandra" href="index.html"/> |
| <link rel="next" title="Backups" href="backups.html"/> |
| <link rel="prev" title="Compression" href="compression.html"/> |
| ' |
| doc-search-path: "../search.html" |
| |
| extra-footer: ' |
| <script type="text/javascript"> |
| var DOCUMENTATION_OPTIONS = { |
| URL_ROOT: "", |
| VERSION: "", |
| COLLAPSE_INDEX: false, |
| FILE_SUFFIX: ".html", |
| HAS_SOURCE: false, |
| SOURCELINK_SUFFIX: ".txt" |
| }; |
| </script> |
| ' |
| |
| --- |
| <div class="container-fluid"> |
| <div class="row"> |
| <div class="col-md-3"> |
| <div class="doc-navigation"> |
| <div class="doc-menu" role="navigation"> |
| <div class="navbar-header"> |
| <button type="button" class="pull-left navbar-toggle" data-toggle="collapse" data-target=".sidebar-navbar-collapse"> |
| <span class="sr-only">Toggle navigation</span> |
| <span class="icon-bar"></span> |
| <span class="icon-bar"></span> |
| <span class="icon-bar"></span> |
| </button> |
| </div> |
| <div class="navbar-collapse collapse sidebar-navbar-collapse"> |
| <form id="doc-search-form" class="navbar-form" action="../search.html" method="get" role="search"> |
| <div class="form-group"> |
| <input type="text" size="30" class="form-control input-sm" name="q" placeholder="Search docs"> |
| <input type="hidden" name="check_keywords" value="yes" /> |
| <input type="hidden" name="area" value="default" /> |
| </div> |
| </form> |
| |
| |
| |
| <ul class="current"> |
| <li class="toctree-l1"><a class="reference internal" href="../getting_started/index.html">Getting Started</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../new/index.html">New Features in Apache Cassandra 4.0</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../architecture/index.html">Architecture</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../cql/index.html">The Cassandra Query Language (CQL)</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../data_modeling/index.html">Data Modeling</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../configuration/index.html">Configuring Cassandra</a></li> |
| <li class="toctree-l1 current"><a class="reference internal" href="index.html">Operating Cassandra</a><ul class="current"> |
| <li class="toctree-l2"><a class="reference internal" href="snitch.html">Snitch</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="topo_changes.html">Adding, replacing, moving and removing nodes</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="repair.html">Repair</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="read_repair.html">Read repair</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="hints.html">Hints</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="compaction/index.html">Compaction</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="bloom_filters.html">Bloom Filters</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="compression.html">Compression</a></li> |
| <li class="toctree-l2 current"><a class="current reference internal" href="#">Change Data Capture</a><ul> |
| <li class="toctree-l3"><a class="reference internal" href="#overview">Overview</a></li> |
| <li class="toctree-l3"><a class="reference internal" href="#configuration">Configuration</a></li> |
| <li class="toctree-l3"><a class="reference internal" href="#reading-commitlogsegments">Reading CommitLogSegments</a></li> |
| <li class="toctree-l3"><a class="reference internal" href="#warnings">Warnings</a></li> |
| <li class="toctree-l3"><a class="reference internal" href="#further-reading">Further Reading</a></li> |
| </ul> |
| </li> |
| <li class="toctree-l2"><a class="reference internal" href="backups.html">Backups</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="bulk_loading.html">Bulk Loading</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="metrics.html">Monitoring</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="security.html">Security</a></li> |
| <li class="toctree-l2"><a class="reference internal" href="hardware.html">Hardware Choices</a></li> |
| </ul> |
| </li> |
| <li class="toctree-l1"><a class="reference internal" href="../tools/index.html">Cassandra Tools</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../troubleshooting/index.html">Troubleshooting</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../development/index.html">Contributing to Cassandra</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../faq/index.html">Frequently Asked Questions</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../plugins/index.html">Third-Party Plugins</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../bugs.html">Reporting Bugs</a></li> |
| <li class="toctree-l1"><a class="reference internal" href="../contactus.html">Contact us</a></li> |
| </ul> |
| |
| |
| |
| </div><!--/.nav-collapse --> |
| </div> |
| </div> |
| </div> |
| <div class="col-md-8"> |
| <div class="content doc-content"> |
| <div class="content-container"> |
| |
| <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> |
| </div> |
| |
| |
| |
| |
| <div class="doc-prev-next-links" role="navigation" aria-label="footer navigation"> |
| |
| <a href="backups.html" class="btn btn-default pull-right " role="button" title="Backups" accesskey="n">Next <span class="glyphicon glyphicon-circle-arrow-right" aria-hidden="true"></span></a> |
| |
| |
| <a href="compression.html" class="btn btn-default" role="button" title="Compression" accesskey="p"><span class="glyphicon glyphicon-circle-arrow-left" aria-hidden="true"></span> Previous</a> |
| |
| </div> |
| |
| </div> |
| </div> |
| </div> |
| </div> |
| </div> |