<!DOCTYPE html><html lang="en"><head><meta charSet="utf-8"/><meta http-equiv="X-UA-Compatible" content="IE=edge"/><title>Topic Compaction · Apache Pulsar</title><meta name="viewport" content="width=device-width, initial-scale=1.0"/><meta name="generator" content="Docusaurus"/><meta name="description" content="Pulsar was built with highly scalable [persistent storage](/docs/en/2.4.0/concepts-architecture-overview#persistent-storage) of message data as a primary objective. Pulsar topics enable you to persistently store as many unacknowledged messages as you need while preserving message ordering. By default, Pulsar stores *all* unacknowledged/unprocessed messages produced on a topic. Accumulating many unacknowledged messages on a topic is necessary for many Pulsar use cases but it can also be very time intensive for Pulsar consumers to &quot;rewind&quot; through the entire log of messages."/><meta name="docsearch:version" content="2.4.0"/><meta name="docsearch:language" content="en"/><meta property="og:title" content="Topic Compaction · Apache Pulsar"/><meta property="og:type" content="website"/><meta property="og:url" content="https://pulsar.apache.org/"/><meta property="og:description" content="Pulsar was built with highly scalable [persistent storage](/docs/en/2.4.0/concepts-architecture-overview#persistent-storage) of message data as a primary objective. Pulsar topics enable you to persistently store as many unacknowledged messages as you need while preserving message ordering. By default, Pulsar stores *all* unacknowledged/unprocessed messages produced on a topic. Accumulating many unacknowledged messages on a topic is necessary for many Pulsar use cases but it can also be very time intensive for Pulsar consumers to &quot;rewind&quot; through the entire log of messages."/><meta name="twitter:card" content="summary"/><meta name="twitter:image" content="https://pulsar.apache.org/img/pulsar.svg"/><link rel="shortcut icon" href="/img/pulsar.ico"/><link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/atom-one-dark.min.css"/><link rel="alternate" type="application/atom+xml" href="https://pulsar.apache.org/blog/atom.xml" title="Apache Pulsar Blog ATOM Feed"/><link rel="alternate" type="application/rss+xml" href="https://pulsar.apache.org/blog/feed.xml" title="Apache Pulsar Blog RSS Feed"/><link rel="stylesheet" href="/css/code-blocks-buttons.css"/><script type="text/javascript" src="https://buttons.github.io/buttons.js"></script><script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/clipboard.js/2.0.0/clipboard.min.js"></script><script type="text/javascript" src="/js/custom.js"></script><script src="/js/scrollSpy.js"></script><link rel="stylesheet" href="/css/main.css"/><script src="/js/codetabs.js"></script></head><body class="sideNavVisible separateOnPageNav"><div class="fixedHeaderContainer"><div class="headerWrapper wrapper"><header><a href="/en"><img class="logo" src="/img/pulsar.svg" alt="Apache Pulsar"/></a><a href="/en/versions"><h3>2.4.0</h3></a><div class="navigationWrapper navigationSlider"><nav class="slidingNav"><ul class="nav-site nav-site-internal"><li class="siteNavGroupActive"><a href="/docs/en/2.4.0/getting-started-standalone" target="_self">Docs</a></li><li class=""><a href="/en/download" target="_self">Download</a></li><li class="siteNavGroupActive"><a href="/docs/en/2.4.0/client-libraries" target="_self">Clients</a></li><li class=""><a href="#restapis" target="_self">REST APIs</a></li><li class=""><a href="#cli" target="_self">Cli</a></li><li class=""><a href="/blog/" target="_self">Blog</a></li><li class=""><a href="#community" target="_self">Community</a></li><li class=""><a href="#apache" target="_self">Apache</a></li><li class=""><a href="https://pulsar-next.staged.apache.org/" target="_self">New Website (Beta)</a></li><span><li><a id="languages-menu" href="#"><img class="languages-icon" src="/img/language.svg" alt="Languages icon"/>English</a><div id="languages-dropdown" class="hide"><ul id="languages-dropdown-items"><li><a href="/docs/ja/2.4.0/concepts-topic-compaction">日本語</a></li><li><a href="/docs/fr/2.4.0/concepts-topic-compaction">Français</a></li><li><a href="/docs/ko/2.4.0/concepts-topic-compaction">한국어</a></li><li><a href="/docs/zh-CN/2.4.0/concepts-topic-compaction">中文</a></li><li><a href="/docs/zh-TW/2.4.0/concepts-topic-compaction">繁體中文</a></li><li><a href="https://crowdin.com/project/apache-pulsar" target="_blank" rel="noreferrer noopener">Help Translate</a></li></ul></div></li><script>
        const languagesMenuItem = document.getElementById("languages-menu");
        const languagesDropDown = document.getElementById("languages-dropdown");
        languagesMenuItem.addEventListener("click", function(event) {
          event.preventDefault();

          if (languagesDropDown.className == "hide") {
            languagesDropDown.className = "visible";
          } else {
            languagesDropDown.className = "hide";
          }
        });
      </script></span></ul></nav></div></header></div></div><div class="navPusher"><div class="docMainWrapper wrapper"><div class="docsNavContainer" id="docsNav"><nav class="toc"><div class="toggleNav"><section class="navWrapper wrapper"><div class="navBreadcrumb wrapper"><div class="navToggle" id="navToggler"><div class="hamburger-menu"><div class="line1"></div><div class="line2"></div><div class="line3"></div></div></div><h2><i>›</i><span>Concepts and Architecture</span></h2><div class="tocToggler" id="tocToggler"><i class="icon-toc"></i></div></div><div class="navGroups"><div class="navGroup"><h3 class="navGroupCategoryTitle">Get Started</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/pulsar-2.0">Pulsar 2.0</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/getting-started-standalone">Run Pulsar locally</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/getting-started-docker">Run Pulsar in Docker</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/client-libraries">Use Pulsar with client libraries</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Concepts and Architecture</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/concepts-overview">Overview</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/concepts-messaging">Messaging</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/concepts-architecture-overview">Architecture</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/concepts-clients">Clients</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/concepts-replication">Geo Replication</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/concepts-multi-tenancy">Multi Tenancy</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/concepts-authentication">Authentication and Authorization</a></li><li class="navListItem navListItemActive"><a class="navItem" href="/docs/en/2.4.0/concepts-topic-compaction">Topic Compaction</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/concepts-tiered-storage">Tiered Storage</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/concepts-schema-registry">Schema Registry</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Pulsar Functions</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-overview">Overview</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-quickstart">Get started</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-api">API</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-deploying">Deploying functions</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-guarantees">Processing guarantees</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-state">State Storage</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-metrics">Metrics</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-worker">Admin: Pulsar Functions Worker</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-runtime">Admin: Configure Functions runtime</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/functions-debugging">Debugging</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/window-functions-context">Window Functions: Context</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Pulsar IO</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/io-overview">Overview</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/io-quickstart">Get started</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/io-managing">Managing Connectors</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/io-connectors">Builtin Connectors</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/io-develop">Developing Connectors</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/io-cdc">CDC Connector</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Pulsar SQL</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/sql-overview">Overview</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/sql-getting-started">Get Started</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/sql-deployment-configurations">Deployment and Configuration</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Deployment</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/deploy-aws">Amazon Web Services</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/deploy-kubernetes">Kubernetes</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/deploy-bare-metal">Bare metal</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/deploy-bare-metal-multi-cluster">Bare metal multi-cluster</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/deploy-monitoring">Monitoring</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Administration</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/administration-zk-bk">ZooKeeper and BookKeeper</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/administration-geo">Geo-replication</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/administration-dashboard">Dashboard</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/administration-stats">Pulsar statistics</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/administration-load-balance">Load balance</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/administration-proxy">Pulsar proxy</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Security</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-overview">Overview</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-tls-transport">Transport Encryption using TLS</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-tls-authentication">Authentication using TLS</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-token-client">Client Authentication using tokens</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-token-admin">Token authentication admin</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-athenz">Authentication using Athenz</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-kerberos">Authentication using Kerberos</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-jwt">Authentication using JWT</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-authorization">Authorization and ACLs</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-encryption">End-to-End Encryption</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/security-extending">Extending</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Client Libraries</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/client-libraries-java">Java</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/client-libraries-go">Go</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/client-libraries-python">Python</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/client-libraries-cpp">C++</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/client-libraries-websocket">WebSocket</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Admin API</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-overview">Overview</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-clusters">Clusters</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-tenants">Tenants</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-brokers">Brokers</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-namespaces">Namespaces</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-permissions">Permissions</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-persistent-topics">Persistent topics</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-non-persistent-topics">Non-Persistent topics</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-partitioned-topics">Partitioned topics</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/admin-api-schemas">Schemas</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Adaptors</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/adaptors-kafka">Kafka client wrapper</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/adaptors-spark">Apache Spark</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/adaptors-storm">Apache Storm</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Cookbooks</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/cookbooks-tiered-storage">Tiered Storage</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/cookbooks-compaction">Topic compaction</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/cookbooks-deduplication">Message deduplication</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/cookbooks-non-persistent">Non-persistent messaging</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/cookbooks-partitioned">Partitioned Topics</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/cookbooks-retention-expiry">Message retention and expiry</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/cookbooks-encryption">Encryption</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/cookbooks-message-queue">Message queue</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/cookbooks-bookkeepermetadata">BookKeeper Ledger Metadata</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Development</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/develop-tools">Simulation tools</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/developing-binary-protocol">Binary protocol</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/develop-schema">Custom schema storage</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/develop-load-manager">Modular load manager</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/develop-cpp">Building Pulsar C++ client</a></li></ul></div><div class="navGroup"><h3 class="navGroupCategoryTitle">Reference</h3><ul class=""><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/reference-terminology">Terminology</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/reference-cli-tools">Pulsar CLI tools</a></li><li class="navListItem"><a class="navItem" href="/docs/en/2.4.0/reference-configuration">Pulsar configuration</a></li></ul></div></div></section></div><script>
            var coll = document.getElementsByClassName('collapsible');
            var checkActiveCategory = true;
            for (var i = 0; i < coll.length; i++) {
              var links = coll[i].nextElementSibling.getElementsByTagName('*');
              if (checkActiveCategory){
                for (var j = 0; j < links.length; j++) {
                  if (links[j].classList.contains('navListItemActive')){
                    coll[i].nextElementSibling.classList.toggle('hide');
                    coll[i].childNodes[1].classList.toggle('rotate');
                    checkActiveCategory = false;
                    break;
                  }
                }
              }

              coll[i].addEventListener('click', function() {
                var arrow = this.childNodes[1];
                arrow.classList.toggle('rotate');
                var content = this.nextElementSibling;
                content.classList.toggle('hide');
              });
            }

            document.addEventListener('DOMContentLoaded', function() {
              createToggler('#navToggler', '#docsNav', 'docsSliderActive');
              createToggler('#tocToggler', 'body', 'tocActive');

              var headings = document.querySelector('.toc-headings');
              headings && headings.addEventListener('click', function(event) {
                var el = event.target;
                while(el !== headings){
                  if (el.tagName === 'A') {
                    document.body.classList.remove('tocActive');
                    break;
                  } else{
                    el = el.parentNode;
                  }
                }
              }, false);

              function createToggler(togglerSelector, targetSelector, className) {
                var toggler = document.querySelector(togglerSelector);
                var target = document.querySelector(targetSelector);

                if (!toggler) {
                  return;
                }

                toggler.onclick = function(event) {
                  event.preventDefault();

                  target.classList.toggle(className);
                };
              }
            });
        </script></nav></div><div class="container mainContainer docsContainer"><div class="wrapper"><div class="post"><header class="postHeader"><a class="edit-page-link button" href="https://github.com/apache/pulsar/edit/master/site2/docs/concepts-topic-compaction.md" target="_blank" rel="noreferrer noopener">Edit</a><h1 id="__docusaurus" class="postHeaderTitle">Topic Compaction</h1></header><article><div><span><p>Pulsar was built with highly scalable <a href="/docs/en/2.4.0/concepts-architecture-overview#persistent-storage">persistent storage</a> of message data as a primary objective. Pulsar topics enable you to persistently store as many unacknowledged messages as you need while preserving message ordering. By default, Pulsar stores <em>all</em> unacknowledged/unprocessed messages produced on a topic. Accumulating many unacknowledged messages on a topic is necessary for many Pulsar use cases but it can also be very time intensive for Pulsar consumers to &quot;rewind&quot; through the entire log of messages.</p>
<blockquote>
<p>For a more practical guide to topic compaction, see the <a href="/docs/en/2.4.0/cookbooks-compaction">Topic compaction cookbook</a>.</p>
</blockquote>
<p>For some use cases consumers don't need a complete &quot;image&quot; of the topic log. They may only need a few values to construct a more &quot;shallow&quot; image of the log, perhaps even just the most recent value. For these kinds of use cases Pulsar offers <strong>topic compaction</strong>. When you run compaction on a topic, Pulsar goes through a topic's backlog and removes messages that are <em>obscured</em> by later messages, i.e. it goes through the topic on a per-key basis and leaves only the most recent message associated with that key.</p>
<p>Pulsar's topic compaction feature:</p>
<ul>
<li>Allows for faster &quot;rewind&quot; through topic logs</li>
<li>Applies only to <a href="/docs/en/2.4.0/concepts-architecture-overview#persistent-storage">persistent topics</a></li>
<li>Triggered automatically when the backlog reaches a certain size or can be triggered manually via the command line. See the <a href="/docs/en/2.4.0/cookbooks-compaction">Topic compaction cookbook</a></li>
<li>Is conceptually and operationally distinct from <a href="/docs/en/2.4.0/concepts-messaging#message-retention-and-expiry">retention and expiry</a>. Topic compaction <em>does</em>, however, respect retention. If retention has removed a message from the message backlog of a topic, the message will also not be readable from the compacted topic ledger.</li>
</ul>
<blockquote>
<h4><a class="anchor" aria-hidden="true" id="topic-compaction-example-the-stock-ticker"></a><a href="#topic-compaction-example-the-stock-ticker" 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>Topic compaction example: the stock ticker</h4>
<p>An example use case for a compacted Pulsar topic would be a stock ticker topic. On a stock ticker topic, each message bears a timestamped dollar value for stocks for purchase (with the message key holding the stock symbol, e.g. <code>AAPL</code> or <code>GOOG</code>). With a stock ticker you may care only about the most recent value(s) of the stock and have no interest in historical data (i.e. you don't need to construct a complete image of the topic's sequence of messages per key). Compaction would be highly beneficial in this case because it would keep consumers from needing to rewind through obscured messages.</p>
</blockquote>
<h2><a class="anchor" aria-hidden="true" id="how-topic-compaction-works"></a><a href="#how-topic-compaction-works" 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>How topic compaction works</h2>
<p>When topic compaction is triggered <a href="/docs/en/2.4.0/cookbooks-compaction">via the CLI</a>, Pulsar will iterate over the entire topic from beginning to end. For each key that it encounters the compaction routine will keep a record of the latest occurrence of that key.</p>
<p>After that, the broker will create a new <a href="/docs/en/2.4.0/concepts-architecture-overview#ledgers">BookKeeper ledger</a> and make a second iteration through each message on the topic. For each message, if the key matches the latest occurrence of that key, then the key's data payload, message ID, and metadata will be written to the newly created ledger. If the key doesn't match the latest then the message will be skipped and left alone. If any given message has an empty payload, it will be skipped and considered deleted (akin to the concept of <a href="https://en.wikipedia.org/wiki/Tombstone_(data_store)">tombstones</a> in key-value databases). At the end of this second iteration through the topic, the newly created BookKeeper ledger is closed and two things are written to the topic's metadata: the ID of the BookKeeper ledger and the message ID of the last compacted message (this is known as the <strong>compaction horizon</strong> of the topic). Once this metadata is written compaction is complete.</p>
<p>After the initial compaction operation, the Pulsar <a href="/docs/en/2.4.0/reference-terminology#broker">broker</a> that owns the topic is notified whenever any future changes are made to the compaction horizon and compacted backlog. When such changes occur:</p>
<ul>
<li>Clients (consumers and readers) that have read compacted enabled will attempt to read messages from a topic and either:
<ul>
<li>Read from the topic like normal (if the message ID is greater than or equal to the compaction horizon) or</li>
<li>Read beginning at the compaction horizon (if the message ID is lower than the compaction horizon)</li>
</ul></li>
</ul>
</span></div></article></div><div class="docs-prevnext"><a class="docs-prev button" href="/docs/en/2.4.0/concepts-authentication"><span class="arrow-prev">← </span><span>Authentication and Authorization</span></a><a class="docs-next button" href="/docs/en/2.4.0/concepts-tiered-storage"><span>Tiered Storage</span><span class="arrow-next"> →</span></a></div></div></div><nav class="onPageNav"><ul class="toc-headings"><li><a href="#how-topic-compaction-works">How topic compaction works</a></li></ul></nav></div><footer class="nav-footer" id="footer"><section class="copyright">Copyright © 2022 The Apache Software Foundation. All Rights Reserved. Apache, Apache Pulsar and the Apache feather logo are trademarks of The Apache Software Foundation.</section><span><script>
      const community = document.querySelector("a[href='#community']").parentNode;
      const communityMenu =
        '<li>' +
        '<a id="community-menu" href="#">Community <span style="font-size: 0.75em">&nbsp;▼</span></a>' +
        '<div id="community-dropdown" class="hide">' +
          '<ul id="community-dropdown-items">' +
            '<li><a href="/en/contact">Contact</a></li>' +
            '<li><a href="/en/contributing">Contributing</a></li>' +
            '<li><a href="/en/coding-guide">Coding guide</a></li>' +
            '<li><a href="/en/events">Events</a></li>' +
            '<li><a href="https://twitter.com/Apache_Pulsar" target="_blank">Twitter &#x2750</a></li>' +
            '<li><a href="https://github.com/apache/pulsar/wiki" target="_blank">Wiki &#x2750</a></li>' +
            '<li><a href="https://github.com/apache/pulsar/issues" target="_blank">Issue tracking &#x2750</a></li>' +
            '<li><a href="https://pulsar-summit.org/" target="_blank">Pulsar Summit &#x2750</a></li>' +
            '<li>&nbsp;</li>' +
            '<li><a href="/en/resources">Resources</a></li>' +
            '<li><a href="/en/team">Team</a></li>' +
            '<li><a href="/en/powered-by">Powered By</a></li>' +
          '</ul>' +
        '</div>' +
        '</li>';

      community.innerHTML = communityMenu;

      const communityMenuItem = document.getElementById("community-menu");
      const communityDropDown = document.getElementById("community-dropdown");
      communityMenuItem.addEventListener("click", function(event) {
        event.preventDefault();

        if (communityDropDown.className == 'hide') {
          communityDropDown.className = 'visible';
        } else {
          communityDropDown.className = 'hide';
        }
      });
    </script></span></footer></div><script>window.twttr=(function(d,s, id){var js,fjs=d.getElementsByTagName(s)[0],t=window.twttr||{};if(d.getElementById(id))return t;js=d.createElement(s);js.id=id;js.src='https://platform.twitter.com/widgets.js';fjs.parentNode.insertBefore(js, fjs);t._e = [];t.ready = function(f) {t._e.push(f);};return t;}(document, 'script', 'twitter-wjs'));</script></body></html>