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vs Kudu</a></li><li class="navListItem"><a class="navItem" href="/docs/0.17.1/comparisons/druid-vs-redshift.html">Apache Druid vs Redshift</a></li><li class="navListItem"><a class="navItem" href="/docs/0.17.1/comparisons/druid-vs-spark.html">Apache Druid vs Spark</a></li><li class="navListItem"><a class="navItem" href="/docs/0.17.1/comparisons/druid-vs-sql-on-hadoop.html">Apache Druid vs SQL-on-Hadoop</a></li><li class="navListItem"><a class="navItem" href="/docs/0.17.1/design/auth.html">Authentication and Authorization</a></li><li class="navListItem"><a class="navItem" href="/docs/0.17.1/design/broker.html">Broker</a></li><li class="navListItem"><a class="navItem" href="/docs/0.17.1/design/coordinator.html">Coordinator Process</a></li><li class="navListItem"><a class="navItem" href="/docs/0.17.1/design/historical.html">Historical Process</a></li><li class="navListItem"><a class="navItem" href="/docs/0.17.1/design/indexer.html">Indexer Process</a></li><li class="navListItem"><a 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</script></nav></div><div class="container mainContainer"><div class="wrapper"><div class="post"><header class="postHeader"><a class="edit-page-link button" href="https://github.com/apache/druid/edit/master/docs/dependencies/zookeeper.md" target="_blank" rel="noreferrer noopener">Edit</a><h1 class="postHeaderTitle">ZooKeeper</h1></header><article><div><span><!--
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<p>Apache Druid uses <a href="http://zookeeper.apache.org/">Apache ZooKeeper</a> (ZK) for management of current cluster state. The operations that happen over ZK are</p>
<ol>
<li><a href="/docs/0.17.1/design/coordinator.html">Coordinator</a> leader election</li>
<li>Segment &quot;publishing&quot; protocol from <a href="/docs/0.17.1/design/historical.html">Historical</a></li>
<li>Segment load/drop protocol between <a href="/docs/0.17.1/design/coordinator.html">Coordinator</a> and <a href="/docs/0.17.1/design/historical.html">Historical</a></li>
<li><a href="/docs/0.17.1/design/overlord.html">Overlord</a> leader election</li>
<li><a href="/docs/0.17.1/design/overlord.html">Overlord</a> and <a href="/docs/0.17.1/design/middlemanager.html">MiddleManager</a> task management</li>
</ol>
<h2><a class="anchor" aria-hidden="true" id="coordinator-leader-election"></a><a href="#coordinator-leader-election" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Coordinator Leader Election</h2>
<p>We use the Curator LeadershipLatch recipe to do leader election at path</p>
<pre><code class="hljs">${druid<span class="hljs-selector-class">.zk</span><span class="hljs-selector-class">.paths</span><span class="hljs-selector-class">.coordinatorPath</span>}/_COORDINATOR
</code></pre>
<h2><a class="anchor" aria-hidden="true" id="segment-publishing-protocol-from-historical-and-realtime"></a><a href="#segment-publishing-protocol-from-historical-and-realtime" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Segment &quot;publishing&quot; protocol from Historical and Realtime</h2>
<p>The <code>announcementsPath</code> and <code>servedSegmentsPath</code> are used for this.</p>
<p>All <a href="/docs/0.17.1/design/historical.html">Historical</a> processes publish themselves on the <code>announcementsPath</code>, specifically, they will create an ephemeral znode at</p>
<pre><code class="hljs">${druid<span class="hljs-selector-class">.zk</span><span class="hljs-selector-class">.paths</span><span class="hljs-selector-class">.announcementsPath</span>}/${druid.host}
</code></pre>
<p>Which signifies that they exist. They will also subsequently create a permanent znode at</p>
<pre><code class="hljs">${druid<span class="hljs-selector-class">.zk</span><span class="hljs-selector-class">.paths</span><span class="hljs-selector-class">.servedSegmentsPath</span>}/${druid.host}
</code></pre>
<p>And as they load up segments, they will attach ephemeral znodes that look like</p>
<pre><code class="hljs">${druid<span class="hljs-selector-class">.zk</span><span class="hljs-selector-class">.paths</span><span class="hljs-selector-class">.servedSegmentsPath</span>}/${druid.host}/_segment_identifier_
</code></pre>
<p>Processes like the <a href="/docs/0.17.1/design/coordinator.html">Coordinator</a> and <a href="/docs/0.17.1/design/broker.html">Broker</a> can then watch these paths to see which processes are currently serving which segments.</p>
<h2><a class="anchor" aria-hidden="true" id="segment-load-drop-protocol-between-coordinator-and-historical"></a><a href="#segment-load-drop-protocol-between-coordinator-and-historical" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Segment load/drop protocol between Coordinator and Historical</h2>
<p>The <code>loadQueuePath</code> is used for this.</p>
<p>When the <a href="/docs/0.17.1/design/coordinator.html">Coordinator</a> decides that a <a href="/docs/0.17.1/design/historical.html">Historical</a> process should load or drop a segment, it writes an ephemeral znode to</p>
<pre><code class="hljs">${druid<span class="hljs-selector-class">.zk</span><span class="hljs-selector-class">.paths</span><span class="hljs-selector-class">.loadQueuePath</span>}/_host_of_historical_process/_segment_identifier
</code></pre>
<p>This znode will contain a payload that indicates to the Historical process what it should do with the given segment. When the Historical process is done with the work, it will delete the znode in order to signify to the Coordinator that it is complete.</p>
</span></div></article></div><div class="docs-prevnext"><a class="docs-prev button" href="/docs/0.17.1/dependencies/metadata-storage.html"><span class="arrow-prev"></span><span>Metadata storage</span></a><a class="docs-next button" href="/docs/0.17.1/ingestion/index.html"><span>Ingestion</span><span class="arrow-next"></span></a></div></div></div><nav class="onPageNav"><ul class="toc-headings"><li><a href="#coordinator-leader-election">Coordinator Leader Election</a></li><li><a href="#segment-publishing-protocol-from-historical-and-realtime">Segment &quot;publishing&quot; protocol from Historical and Realtime</a></li><li><a href="#segment-load-drop-protocol-between-coordinator-and-historical">Segment load/drop protocol between Coordinator and Historical</a></li></ul></nav></div><footer class="nav-footer druid-footer" id="footer"><div class="container"><div class="text-center"><p><a href="/technology">Technology</a> · <a href="/use-cases">Use Cases</a> · <a href="/druid-powered">Powered by Druid</a> · <a href="/docs/0.17.1/latest">Docs</a> · <a href="/community/">Community</a> · <a href="/downloads.html">Download</a> · <a href="/faq">FAQ</a></p></div><div class="text-center"><a title="Join the user group" href="https://groups.google.com/forum/#!forum/druid-user" target="_blank"><span class="fa fa-comments"></span></a> · <a title="Follow Druid" href="https://twitter.com/druidio" target="_blank"><span class="fab fa-twitter"></span></a> · <a title="Download via Apache" href="https://www.apache.org/dyn/closer.cgi?path=/incubator/druid/{{ site.druid_versions[0].versions[0].version }}/apache-druid-{{ site.druid_versions[0].versions[0].version }}-bin.tar.gz" target="_blank"><span class="fas fa-feather"></span></a> · <a title="GitHub" href="https://github.com/apache/druid" target="_blank"><span class="fab fa-github"></span></a></div><div class="text-center license">Copyright © 2019 <a href="https://www.apache.org/" target="_blank">Apache Software Foundation</a>.<br/>Except where otherwise noted, licensed under <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA 4.0</a>.<br/>Apache Druid, Druid, and the Druid logo are either registered trademarks or trademarks of The Apache Software Foundation in the United States and other countries.</div></div></footer></div><script type="text/javascript" src="https://cdn.jsdelivr.net/docsearch.js/1/docsearch.min.js"></script><script>
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