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id="toc-Magic_numbers-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-WANObjectCache" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#WANObjectCache"> <div class="vector-toc-text"> <span class="vector-toc-numb">2</span> <span>WANObjectCache</span> </div> </a> <button aria-controls="toc-WANObjectCache-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle WANObjectCache subsection</span> </button> <ul id="toc-WANObjectCache-sublist" class="vector-toc-list"> <li id="toc-High_level" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#High_level"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1</span> <span>High level</span> </div> </a> <ul id="toc-High_level-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Memcached_commands" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Memcached_commands"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2</span> <span>Memcached commands</span> </div> </a> <ul id="toc-Memcached_commands-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Getting_revision/page_from_WANCache_key" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Getting_revision/page_from_WANCache_key"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.3</span> <span>Getting revision/page from WANCache key</span> </div> </a> <ul id="toc-Getting_revision/page_from_WANCache_key-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Infrastructure" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Infrastructure"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Infrastructure</span> </div> </a> <button aria-controls="toc-Infrastructure-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Infrastructure subsection</span> </button> <ul id="toc-Infrastructure-sublist" class="vector-toc-list"> <li id="toc-Mcrouter" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Mcrouter"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1</span> <span>Mcrouter</span> </div> </a> <ul id="toc-Mcrouter-sublist" class="vector-toc-list"> <li id="toc-Mcrouter_Routes" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Mcrouter_Routes"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1.1</span> <span>Mcrouter Routes</span> </div> </a> <ul id="toc-Mcrouter_Routes-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Example" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Example"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1.2</span> <span>Example</span> </div> </a> <ul id="toc-Example-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> </ul> </li> <li id="toc-Servers" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Servers"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Servers</span> </div> </a> <ul id="toc-Servers-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Runbooks" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Runbooks"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Runbooks</span> </div> </a> <ul id="toc-Runbooks-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Dashboards" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Dashboards"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Dashboards</span> </div> </a> <ul id="toc-Dashboards-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Links" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Links"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>Links</span> </div> </a> <ul id="toc-Links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Memcached for MediaWiki</span></h1> </header> <div class="vector-page-toolbar"> <div class="vector-page-toolbar-container"> <div id="left-navigation"> <nav aria-label="Namespaces"> <div id="p-associated-pages" class="vector-menu vector-menu-tabs mw-portlet mw-portlet-associated-pages" 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vector-has-collapsible-items" title="More options" > <div class="vector-menu-heading"> Actions </div> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li id="ca-more-view" class="selected vector-more-collapsible-item mw-list-item"><a href="/wiki/Memcached_for_MediaWiki"><span>Read</span></a></li><li id="ca-more-viewsource" class="vector-more-collapsible-item mw-list-item"><a href="/w/index.php?title=Memcached_for_MediaWiki&action=edit"><span>View source</span></a></li><li id="ca-more-history" class="vector-more-collapsible-item mw-list-item"><a href="/w/index.php?title=Memcached_for_MediaWiki&action=history"><span>View history</span></a></li> </ul> </div> </div> <div id="p-tb" class="vector-menu mw-portlet mw-portlet-tb" > <div class="vector-menu-heading"> General </div> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li id="t-whatlinkshere" class="mw-list-item"><a href="/wiki/Special:WhatLinksHere/Memcached_for_MediaWiki" title="A list of all wiki pages that link here [j]" accesskey="j"><span>What links here</span></a></li><li id="t-recentchangeslinked" class="mw-list-item"><a href="/wiki/Special:RecentChangesLinked/Memcached_for_MediaWiki" rel="nofollow" title="Recent changes in pages linked from this page [k]" accesskey="k"><span>Related changes</span></a></li><li id="t-specialpages" class="mw-list-item"><a href="/wiki/Special:SpecialPages" title="A list of all special pages [q]" accesskey="q"><span>Special pages</span></a></li><li id="t-permalink" class="mw-list-item"><a href="/w/index.php?title=Memcached_for_MediaWiki&oldid=2246531" title="Permanent link to this revision of this page"><span>Permanent link</span></a></li><li id="t-info" class="mw-list-item"><a href="/w/index.php?title=Memcached_for_MediaWiki&action=info" title="More information about this page"><span>Page information</span></a></li><li id="t-cite" class="mw-list-item"><a href="/w/index.php?title=Special:CiteThisPage&page=Memcached_for_MediaWiki&id=2246531&wpFormIdentifier=titleform" title="Information on how to cite this page"><span>Cite this page</span></a></li><li id="t-urlshortener" class="mw-list-item"><a href="/w/index.php?title=Special:UrlShortener&url=https%3A%2F%2Fwikitech.wikimedia.org%2Fwiki%2FMemcached_for_MediaWiki"><span>Get shortened URL</span></a></li><li id="t-urlshortener-qrcode" class="mw-list-item"><a href="/w/index.php?title=Special:QrCode&url=https%3A%2F%2Fwikitech.wikimedia.org%2Fwiki%2FMemcached_for_MediaWiki"><span>Download QR code</span></a></li> </ul> </div> </div> <div id="p-coll-print_export" class="vector-menu mw-portlet mw-portlet-coll-print_export" > <div class="vector-menu-heading"> Print/export </div> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li id="coll-create_a_book" class="mw-list-item"><a href="/w/index.php?title=Special:Book&bookcmd=book_creator&referer=Memcached+for+MediaWiki"><span>Create a book</span></a></li><li id="coll-download-as-rl" class="mw-list-item"><a href="/w/index.php?title=Special:DownloadAsPdf&page=Memcached_for_MediaWiki&action=show-download-screen"><span>Download as PDF</span></a></li><li id="t-print" class="mw-list-item"><a href="/w/index.php?title=Memcached_for_MediaWiki&printable=yes" title="Printable version of this page [p]" accesskey="p"><span>Printable version</span></a></li> </ul> </div> </div> </div> </div> </div> </div> </nav> </div> </div> </div> <div class="vector-column-end"> <div class="vector-sticky-pinned-container"> <nav class="vector-page-tools-landmark" aria-label="Page tools"> <div id="vector-page-tools-pinned-container" class="vector-pinned-container"> </div> </nav> <nav class="vector-appearance-landmark" aria-label="Appearance"> <div id="vector-appearance-pinned-container" class="vector-pinned-container"> <div id="vector-appearance" class="vector-appearance vector-pinnable-element"> <div class="vector-pinnable-header vector-appearance-pinnable-header vector-pinnable-header-pinned" data-feature-name="appearance-pinned" data-pinnable-element-id="vector-appearance" data-pinned-container-id="vector-appearance-pinned-container" data-unpinned-container-id="vector-appearance-unpinned-container" > <div class="vector-pinnable-header-label">Appearance</div> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-pin-button" data-event-name="pinnable-header.vector-appearance.pin">move to sidebar</button> <button class="vector-pinnable-header-toggle-button vector-pinnable-header-unpin-button" data-event-name="pinnable-header.vector-appearance.unpin">hide</button> </div> </div> </div> </nav> </div> </div> <div id="bodyContent" class="vector-body" aria-labelledby="firstHeading" data-mw-ve-target-container> <div class="vector-body-before-content"> <div class="mw-indicators"> </div> <div id="siteSub" class="noprint">From Wikitech</div> </div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><style data-mw-deduplicate="TemplateStyles:r2230541">.mw-parser-output .tpl-navinfra--portal{float:none}.mw-parser-output .tpl-navinfra--portal .tpl-navsidebar-contents{margin:0 8px;display:flex;flex-flow:row wrap;gap:16px}.mw-parser-output .tpl-navinfra--portal .tpl-navsidebar-content{break-inside:avoid}.mw-parser-output .tpl-navinfra--sidebar .tpl-navsidebar-content{margin:0}.mw-parser-output .tpl-navinfra--sidebar .tpl-navsidebar-heading{font-weight:normal;margin:1px 0 0 0}.mw-parser-output .tpl-navinfra--sidebar .tpl-navsidebar-heading a{display:block;border-top:1px solid var(--border-color-muted,#eaecf0);background-color:var(--background-color-neutral-subtle,#f8f9fa);color:var(--color-base,#202122);margin:0 -8px;padding:4px 8px 0px 8px}.mw-parser-output .tpl-navinfra--sidebar .tpl-navsidebar-heading a:hover{background-color:var(--background-color-base,#fff);color:var(--color-emphasized,#000)}.mw-parser-output ul{margin-top:unset!important}</style> <style data-mw-deduplicate="TemplateStyles:r2241375">.mw-parser-output .tpl-navsidebar{max-width:22em;background:var(--background-color-base,#fff);color:var(--color-base,#202122);border:1px solid var(--border-color-base,#a2a9b1);float:right;clear:right;margin:.5em 0 1em 1em}.mw-parser-output .tpl-navsidebar-floatright{float:right;clear:right;margin:.5em 0 1em 1em}.mw-parser-output .tpl-navsidebar-floatleft{float:left;clear:left;margin:.5em 1em 1em 0}.mw-parser-output .tpl-navsidebar-floatnone{float:none;clear:both;margin:.5em 0}.mw-parser-output .tpl-navsidebar-topimage{margin:0 0 16px 0}.mw-parser-output .tpl-navsidebar-title{margin:8px 16px;border-bottom:3px solid var(--border-color-muted,#eaecf0);font-size:20px;text-align:center}.mw-parser-output .tpl-navsidebar-image{margin:0 0 8px}.mw-parser-output .tpl-navsidebar-content{margin:0 0 16px 0;padding:0 8px}.mw-parser-output .tpl-navsidebar-heading{margin:8px 0;font-weight:bold}.mw-parser-output .tpl-navsidebar-foot{padding:0 8px;margin:0;text-align:right;font-size:smaller}@media not (min-width:720px){.mw-parser-output .tpl-navsidebar{float:none;clear:both;margin:.5em 0;max-width:none}}</style><div role="navigation" class="navigation-not-searchable tpl-navsidebar tpl-navinfra--sidebar" style=""><p class="tpl-navsidebar-title"><a href="/wiki/Wikimedia_infrastructure" title="Wikimedia infrastructure">Wikimedia infrastructure</a></p><div class="tpl-navsidebar-contents"><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/Data_centers" title="Data centers">Data centers</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/Network_design" title="Network design">Networking</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/Global_traffic_routing" title="Global traffic routing">Global traffic routing</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/MediaWiki_at_WMF" title="MediaWiki at WMF">MediaWiki SRE</a></p><p class="mw-empty-elt"> </p><ul><li><a href="/wiki/Application_servers" title="Application servers">Application servers</a></li> <li><a href="/wiki/BounceHandler" title="BounceHandler">BounceHandler</a></li> <li><a href="/wiki/Application_servers/Runbook" title="Application servers/Runbook">PHP 7 and php-fpm</a></li> <li><a href="/wiki/Citoid" title="Citoid">Citoid</a></li> <li><a href="/wiki/Dumps" title="Dumps">Dumps</a></li> <li><a href="/wiki/Envoy" title="Envoy">Envoy</a></li> <li><a href="/wiki/External_storage" title="External storage">External storage</a></li> <li><a href="/wiki/MediaWiki_JobQueue" title="MediaWiki JobQueue">MediaWiki JobQueue</a></li> <li><a class="mw-selflink selflink">Memcached</a></li> <li><a href="/wiki/Memcached_for_MediaWiki/mcrouter" class="mw-redirect" title="Memcached for MediaWiki/mcrouter">Mcrouter runbook</a></li> <li><a href="/wiki/MediaWiki_HTTP_cache_headers" title="MediaWiki HTTP cache headers">MediaWiki HTTP cache headers</a></li> <li><a href="/wiki/MediaWiki_On_Kubernetes" title="MediaWiki On Kubernetes">MediaWiki On Kubernetes</a></li> <li><a href="/wiki/MediaWiki_and_EtcdConfig" title="MediaWiki and EtcdConfig">MediaWiki and EtcdConfig</a></li> <li><a href="/wiki/Mathoid" title="Mathoid">Mathoid</a></li> <li><a href="/wiki/Nutcracker" title="Nutcracker">Nutcracker</a></li> <li><a href="/wiki/Parser_cache" title="Parser cache">Parser cache</a></li> <li><a href="/wiki/Shellbox" title="Shellbox">Shellbox</a></li> <li><a href="/wiki/Redis" title="Redis">Redis</a></li></ul> </div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/MediaWiki_Engineering" title="MediaWiki Engineering">MediaWiki Engineering</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/Upload.wikimedia.org" title="Upload.wikimedia.org">Multimedia</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/Data_Engineering" class="mw-redirect" title="Data Engineering">Data Engineering</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/Search" title="Search">Search</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/SRE/Data_Persistence/Documentation" title="SRE/Data Persistence/Documentation">SRE Data Persistence</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/SRE/Infrastructure_Foundations/Documentation" title="SRE/Infrastructure Foundations/Documentation">SRE Infra Foundations</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/SRE/Observability/Documentation" title="SRE/Observability/Documentation">SRE Observability</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/Performance" title="Performance">Wikimedia Performance</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/Event_Platform" title="Event Platform">Event Platform</a></p><p class="mw-empty-elt"> </p></div><div class="tpl-navsidebar-content"> <p class="tpl-navsidebar-heading"><a href="/wiki/Release_Engineering" title="Release Engineering">Release Engineering</a></p><p class="mw-empty-elt"> </p></div></div><p class="tpl-navsidebar-foot">[<span class="noprint plainlinks"><a class="external text" href="https://wikitech.wikimedia.org/w/index.php?title=Template:Navigation_Wikimedia_infrastructure&action=edit"><span title="Edit this template">edit</span></a></span>]</p></div> <p>This page is about <b>Memcached for MediaWiki</b>. </p><p>This page is not about other Memcached clusters in production, such as those for <a href="/wiki/Thumbor" title="Thumbor">Thumbor</a>, <a href="/wiki/Help:Cloud_Services_introduction" title="Help:Cloud Services introduction">Wikimedia Cloud Services</a>, and <a href="/wiki/Swift" title="Swift">Swift</a>. </p> <figure typeof="mw:File/Thumb"><a href="/wiki/File:Wikipedia_Memcached_flow_2022.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/4/45/Wikipedia_Memcached_flow_2022.png/400px-Wikipedia_Memcached_flow_2022.png" decoding="async" width="400" height="285" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/4/45/Wikipedia_Memcached_flow_2022.png/600px-Wikipedia_Memcached_flow_2022.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/4/45/Wikipedia_Memcached_flow_2022.png/800px-Wikipedia_Memcached_flow_2022.png 2x" data-file-width="2224" data-file-height="1584"/></a><figcaption>MediaWiki's use of Memcached at WMF.</figcaption></figure> <meta property="mw:PageProp/toc"/> <div class="mw-heading mw-heading2 ext-discussiontools-init-section"><h2 id="Magic_numbers" data-mw-thread-id="h-Magic_numbers"><span data-mw-comment-start="" id="h-Magic_numbers"></span>Magic numbers<span data-mw-comment-end="h-Magic_numbers"></span></h2></div> <ul><li>WANCache (last updated: May 2020.) <ul><li>Tombstone (aka "hold-off TTL"): 11 seconds.</li> <li>Interim value: 1 second.</li></ul></li> <li>Mcrouter <ul><li>Gutter TTL: upto 10 minutes (<a rel="nofollow" class="external text" href="https://github.com/wikimedia/puppet/blob/9f9e389dac7830c7aabbf95803f51fd288cc6357/hieradata/role/common/mediawiki/appserver.yaml#L10">gutter_ttl</a>).</li> <li>On-host tier TTL: upto 10 seconds.</li></ul></li></ul> <div class="mw-heading mw-heading2 ext-discussiontools-init-section"><h2 id="WANObjectCache" data-mw-thread-id="h-WANObjectCache"><span data-mw-comment-start="" id="h-WANObjectCache"></span>WANObjectCache<span data-mw-comment-end="h-WANObjectCache"></span></h2></div> <p><b>WANObjectCache</b> (or <b>WANCache</b>) is the primary interface in MediaWiki for interacting with Memcached and mcrouter. WANCache provides a developer-friendly API that naturally follow our best practices and transparently deals with the complex requirements of operating a platform of our scale. This includes preventing cache stampedes, avoiding cache misses for hot data through probabilistic and asynchronous regeneration prior to logical expiry, avoiding network congestion, supporting multiple versions of the software to run alongside each other (and apply purges to both, whilst storing values separately), avoiding cache polution during long-running processes or when databases are experiencing replication lag. The WANCache interface came out of the <a href="/wiki/Performance/Multi-DC_MediaWiki" title="Performance/Multi-DC MediaWiki">Multi-DC MediaWiki initiative</a> which required us to take these constraints more seriously, though they generally are not unique to Multi-DC and also significantly improved resilience and correctness during the 2015-2021 single-DC period. </p><p>WANCache builds on top of <b>BagOStuff</b>, which is the lower level key-value interface to Memcached and other storage backends. </p><p>See also: </p> <ul><li><a href="https://doc.wikimedia.org/mediawiki-core/master/php/classWANObjectCache.html#details" class="extiw" title="wmdoc:mediawiki-core/master/php/classWANObjectCache.html">WANObjectCache high-level documentation</a></li> <li><a href="https://gerrit.wikimedia.org/g/mediawiki/core/%2B/master/includes/libs/objectcache/README.md" class="extiw" title="git:mediawiki/core/+/master/includes/libs/objectcache/README.md">WANObjectCache metrics and internal details</a></li> <li><a href="https://www.mediawiki.org/wiki/Manual:Object_cache" class="extiw" title="mw:Manual:Object cache">mw:Manual:Object_cache</a>: what kind of data is stored here, how it gets there etc.</li></ul> <div class="mw-heading mw-heading3"><h3 id="High_level" data-mw-thread-id="h-High_level-WANObjectCache"><span data-mw-comment-start="" id="h-High_level-WANObjectCache"></span>High level<span data-mw-comment-end="h-High_level-WANObjectCache"></span></h3></div> <ul><li><b>Like a replica database.</b> There is generally no proactive setting of values during HTTP write actions. Instead, values are computed based on information from replica DBs, and computed on-demand using the <code>getWithSet(key, ttl, callable)</code> idiom. This means the application generally only expects cache values to be as up to date as a replica DB would be. Historically, it was common for MediaWiki to populate its cache during HTTP write actions instead. This meant that in a single-DC setup it could loosely be expected that the cache was as up-to-date as the master DB. As part of the multi-dc effort, this was changed starting in 2015, and thus its expectations were loosened to that of a replica DB.</li> <li><b>No synchronisation.</b> MediaWiki's WANCache layer does not require synchronisation of cached values across data centers. Instead, it considers each datacenter's Memcached cluster as independent. Each populating its own values as-needed on dc-local app servers from dc-local replica DBs.</li> <li><b>Tombstones (broadcasted purge)</b>. During HTTP write actions, MediaWiki asks WANCache to purge cache keys of which it has modified the source data. These purges take the form of short-lived Memcached keys known as "tombstones". We do not use the <code>DELETE</code> command because we want each data center to be able to populate its memcached independently, thus requiring no cross-dc master database connection, thus reading from a local replica, thus values ingested in the cache may be as stale as a replica can be. Implementing a Memcached purge as <code>DELETE</code> would mean both in the same DC and other DCs, the same key could be re-populated immediately with the same stale value we just deleted. Instead, WANCache formulates its purge as a <code>SET</code> operation that stores a placeholder value known as a "tombstone" (lasts for approx. 10 seconds for local and remote replica DBs to catch up).</li> <li><b>Interim values</b>. Upon seeing such tombstone, WANCache acts much like a cache miss, except that the newly computed value is not written back over the tombstone (as the computed value may be stale). Instead, to avoid a recompute stampede these maybe-stale values are stored as an "interim value" in a sister key which is only kept for a few seconds.</li></ul> <div class="mw-heading mw-heading3"><h3 id="Memcached_commands" data-mw-thread-id="h-Memcached_commands-WANObjectCache"><span data-mw-comment-start="" id="h-Memcached_commands-WANObjectCache"></span>Memcached commands<span data-mw-comment-end="h-Memcached_commands-WANObjectCache"></span></h3></div> <p>Intra-dc: </p> <ul><li>Read traffic from the <code>getWithSet</code> idiom results in a <code>GETS</code> command (getMulti) that fetches the main key, plus any sister keys that might exist.</li> <li>Write trafffic from the <code>getWithSet</code> idiom results in either <code>ADD</code> if the key was known to be absent, or <code>Memcached->mergeViaCas</code> if a value existed but either required (or was elected for) regeneration.</li></ul> <p>Cross-dc: </p> <ul><li>Purge traffic uses the <code>/*/mw-wan/</code> prefix to tell mcrouter to broadcast this to other pools and clusters as well. The actual command is generally <code>SET</code> as it needs to induce a "hold-off" period using the tombstone (per the above). In rare cases where a hold-off is not needed (e.g. if the purge is not related to a DB write), then the broadcasted event will use <code>DELETE</code></li></ul> <div class="mw-heading mw-heading3"><h3 id="Getting_revision/page_from_WANCache_key" data-mw-thread-id="h-Getting_revision/page_from_WANCache_key-WANObjectCache"><span id="Getting_revision.2Fpage_from_WANCache_key"></span><span data-mw-comment-start="" id="h-Getting_revision/page_from_WANCache_key-WANObjectCache"></span>Getting revision/page from WANCache key<span data-mw-comment-end="h-Getting_revision/page_from_WANCache_key-WANObjectCache"></span></h3></div> <p>If you're trying to track down the specific revision text given an SqlBlobStore key, the somewhat convoluted procedure is documented at <a href="https://www.mediawiki.org/wiki/Manual:Caching#Revision_text" class="extiw" title="mw:Manual:Caching">mw:Manual:Caching#Revision_text</a>. </p> <div class="mw-heading mw-heading2 ext-discussiontools-init-section"><h2 id="Infrastructure" data-mw-thread-id="h-Infrastructure"><span data-mw-comment-start="" id="h-Infrastructure"></span>Infrastructure<span data-mw-comment-end="h-Infrastructure"></span></h2></div> <p>There are two logical pools of memcached servers for MediaWiki: </p> <ul><li><b>Main</b>: The main pool for has 18 <a href="https://en.wikipedia.org/wiki/Shard_(database_architecture)" class="extiw" title="en:Shard (database architecture)">shards</a> and runs on the <code>mc10XX</code> hosts (in <a href="/wiki/Eqiad_cluster" class="mw-redirect" title="Eqiad cluster">Eqiad</a>) and <code>mc20XX</code> hosts (in <a href="/wiki/Codfw_cluster" class="mw-redirect" title="Codfw cluster">Codfw</a>).</li> <li><b>Gutter</b>: The gutter pool has 3 shards per DC, and hosted on <code>mc-gp100x</code> and <code>mc-gp200x</code> hosts (launch task: <a href="https://phabricator.wikimedia.org/T244852" class="extiw" title="phab:T244852">T244852</a>).</li></ul> <p>MediaWiki connects to memcached through a proxy called <b><a rel="nofollow" class="external text" href="https://github.com/facebook/mcrouter">mcrouter</a></b> which provides a number of benefits. </p> <div class="mw-heading mw-heading3"><h3 id="Mcrouter" data-mw-thread-id="h-Mcrouter-Infrastructure"><span data-mw-comment-start="" id="h-Mcrouter-Infrastructure"></span><a href="/wiki/Mw-mcrouter" title="Mw-mcrouter">Mcrouter</a><span data-mw-comment-end="h-Mcrouter-Infrastructure"></span></h3></div> <p>Each kubernetes node is running an instance of <a href="/wiki/Mw-mcrouter" title="Mw-mcrouter">mw-mcrouter</a>, the proxy that does: </p> <ul><li>consistent shards data across the memcached servers</li> <li>connection pooling</li> <li>failover to <code>gutter pool</code> in case of a server unavailability</li> <li>cross-dc replication via TLS <a href="https://phabricator.wikimedia.org/T271967" class="extiw" title="phab:T271967">T271967</a></li></ul> <p>Any mediawiki pod will have its memcached requests routed to the <a href="/wiki/Mw-mcrouter" title="Mw-mcrouter">mw-mcrouter</a> pod running on the same node. </p><p><b>Note: mw-wikifunctions is not using mw-mcrouter, but rather an in-pod mcrouter container.</b> </p> <div class="mw-heading mw-heading4"><h4 id="Mcrouter_Routes" data-mw-thread-id="h-Mcrouter_Routes-Mcrouter"><span data-mw-comment-start="" id="h-Mcrouter_Routes-Mcrouter"></span>Mcrouter Routes<span data-mw-comment-end="h-Mcrouter_Routes-Mcrouter"></span></h4></div> <p>Each MediaWiki api/appserver accesses memcached through its local Mcrouter instance <sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>. Mcrouter introduces the concepts of <a rel="nofollow" class="external text" href="https://github.com/facebook/mcrouter/wiki/Prefix-routing-setup">routes</a> and pools and each route applies consistent hashing on the key name to know where to send it, i.e. which of the 18 shards for memcached. </p><p>There are several routes available in our configuration, which are addressable via a route prefix that mcrouter <b>strips from the key</b> before forwarding the memcached command. </p> <ol><li><b>Main route</b>. This route is declared as <code>/$region/mw/</code> but is not addressed by MediaWiki as such. It routes to the dc-local "Main" pool shards. If a shard is perceived as unavailable from an appserver ("TKO") the local mcrouter forwards all commands (incl gets, sets, and locks) to a shard of the "Gutter" pool instead (launch task: <a href="https://phabricator.wikimedia.org/T244852" class="extiw" title="phab:T244852">T244852</a>). <ul><li>This route is used by the majority of traffic, through <code>WANObjectCache::getWithSet</code> calls in MediaWiki.</li> <li>MediaWiki doesn't use the <code>/$region/mw/</code> prefix. Instead <code>/$region/mw/</code> is the default route and MediaWiki sends these commands without any routing prefix.</li> <li>Switchover to and from the gutterpool is decided by Mcrouter locally (per-appserver), it is not centrally coordinated. The keys stored in a gutter server have a reduced TTL.</li></ul></li> <li><b>WAN route</b>. This route is declared as <code>/$region/mw-wan/</code>. It routes to the dc-local "Main" pool shards as well as the "Proxies" for all non-local DCs. <ul><li>This route is for internal use by MediaWiki's WANObjectCache to broadcast its purges ("tombstones"). This happens from calls to <code>WANObjectCache::purge</code> (invalidates a single key) or <code>WANObjectCache::touchCheckKey</code> (effectively invalidate many keys, through a shared "check" key; somewhat like the Varnish <a href="/wiki/XKey" title="XKey">XKey</a> mechanism).</li> <li>This route is not used for storing "regular" values is not exposed to any generic <code>WANObjectCache::getWithSet</code> or <code>BagOStuff</code> calls.</li></ul></li></ol> <div class="mw-heading mw-heading4"><h4 id="Example" data-mw-thread-id="h-Example-Mcrouter"><span data-mw-comment-start="" id="h-Example-Mcrouter"></span>Example<span data-mw-comment-end="h-Example-Mcrouter"></span></h4></div><p> The memcached key <code>WANCache:v:metawiki:translate-groups</code> (belongs to the <a href="https://www.mediawiki.org/wiki/Extension:Translate" class="extiw" title="mw:Extension:Translate">Translate extension</a>) is formatted by the WANCache library. When Translate wants to get the value of this key, WANCache will send a <code>GET</code> command from MediaWiki to <code>localhost:11213</code>, where mcrouter is listening. The command is then further routed to <code>mc1022</code> (based on key hashing). MediaWiki it totally ignorant about the <code>mc[1,2]0XX</code> host, it only knows about sending commands to a localhost port. A mcrouter admin command helps figure out where keys are hashed/routed to:</p><div class="mw-highlight mw-highlight-lang-bash mw-content-ltr" dir="ltr"><pre><span></span>elukey@mw1345:~$<span class="w"> </span><span class="nb">echo</span><span class="w"> </span><span class="s2">"get __mcrouter__.route(get,WANCache:v:metawiki:translate-groups)"</span><span class="w"> </span><span class="p">|</span><span class="w"> </span>nc<span class="w"> </span>localhost<span class="w"> </span><span class="m">11213</span><span class="w"> </span>-q<span class="w"> </span><span class="m">2</span> VALUE<span class="w"> </span>__mcrouter__.route<span class="o">(</span>get,WANCache:v:metawiki:translate-groups<span class="o">)</span><span class="w"> </span><span class="m">0</span><span class="w"> </span><span class="m">16</span> <span class="m">10</span>.64.0.83:11211 END elukey@mw1345:~$<span class="w"> </span>dig<span class="w"> </span>-x<span class="w"> </span><span class="m">10</span>.64.0.83<span class="w"> </span>+short mc1022.eqiad.wmnet. </pre></div><p>Some things to notice: </p><ul><li>The special prefix <code>__mcrouter__.route</code> is intercepted by mcrouter. These are admin commands, for which proxy returns directly without contacting the memcached hosts. This function returns the target of the consistent hashing of the key name.</li> <li>Mcrouter listens on port 11213 on all MediaWiki <a href="/wiki/Application_servers" title="Application servers">app servers</a>, meanwhile on every <code>mc10XX</code> host memcached listens on port 11211.</li></ul><p> To get a key and dump it to a file it is sufficient to:</p><div class="mw-highlight mw-highlight-lang-bash mw-content-ltr" dir="ltr"><pre><span></span>elukey@mw1345:~$<span class="w"> </span><span class="nb">echo</span><span class="w"> </span><span class="s2">"get WANCache:v:metawiki:translate-groups"</span><span class="w"> </span><span class="p">|</span><span class="w"> </span>nc<span class="w"> </span>localhost<span class="w"> </span><span class="m">11213</span><span class="w"> </span>-q<span class="w"> </span><span class="m">2</span><span class="w"> </span>><span class="w"> </span>dump.txt elukey@mw1345:~$<span class="w"> </span>du<span class="w"> </span>-hs<span class="w"> </span>dump.txt 380K<span class="w"> </span>dump.txt </pre></div><p>In this case the key's value is pretty big, and it needs PHP to be interpreted correctly (to unserialize it), but nonetheless we got some useful information (like the size of the key). This could be useful when it is necessary to quickly get how big a key is, rather than knowing its content. </p><div class="mw-heading mw-heading2 ext-discussiontools-init-section"><h2 id="Servers" data-mw-thread-id="h-Servers"><span data-mw-comment-start="" id="h-Servers"></span>Servers<span data-mw-comment-end="h-Servers"></span></h2></div> <p>Servers list can be found in <a href="https://gerrit.wikimedia.org/r/plugins/gitiles/operations/puppet/%2B/refs/heads/production/hieradata/common/profile/mediawiki/mcrouter_wancache.yaml" class="extiw" title="gerrit:plugins/gitiles/operations/puppet/+/refs/heads/production/hieradata/common/profile/mediawiki/mcrouter wancache.yaml">puppet</a>. This list is passed to <a href="/wiki/Mw-mcrouter" title="Mw-mcrouter">mw-mcrouter</a> after being parsed by some <a href="https://gerrit.wikimedia.org/r/plugins/gitiles/operations/puppet/%2B/refs/heads/production/modules/mediawiki/manifests/mcrouter/yaml_defs.pp" class="extiw" title="gerrit:plugins/gitiles/operations/puppet/+/refs/heads/production/modules/mediawiki/manifests/mcrouter/yaml defs.pp">wonderful code</a> </p> <div class="mw-heading mw-heading2 ext-discussiontools-init-section"><h2 id="Runbooks" data-mw-thread-id="h-Runbooks"><span data-mw-comment-start="" id="h-Runbooks"></span>Runbooks<span data-mw-comment-end="h-Runbooks"></span></h2></div> <ul><li><a href="/wiki/Memcached_for_MediaWiki/Memcached_server_failure" class="mw-redirect" title="Memcached for MediaWiki/Memcached server failure">Memcached server failure</a></li> <li><a href="/wiki/Performance/Runbooks/Analyze_memcached" title="Performance/Runbooks/Analyze memcached">Performance/Runbooks/Analyze memcached</a> (How to use memkeys or cachedump)</li></ul> <div class="mw-heading mw-heading2 ext-discussiontools-init-section"><h2 id="Dashboards" data-mw-thread-id="h-Dashboards"><span data-mw-comment-start="" id="h-Dashboards"></span>Dashboards<span data-mw-comment-end="h-Dashboards"></span></h2></div> <ul><li><a class="external text" href="https://grafana-rw.wikimedia.org/d/ltSHWhHIk/mcrouter-on-k8s">Mcrouter-on-k8s</a></li> <li><a class="external text" href="https://logstash.wikimedia.org/app/dashboards#/view/memcached?_g=(filters%3A!()%2CrefreshInterval%3A(pause%3A!t%2Cvalue%3A0)%2Ctime%3A(from%3Anow-3h%2Cto%3Anow))">mw-memcached errors</a></li> <li><a class="external text" href="https://grafana-rw.wikimedia.org/d/000000316/memcache">Memcached Dashboard</a></li> <li><a class="external text" href="https://grafana-rw.wikimedia.org/d/000000316/memcache?orgId=1&var-datasource=eqiad%20prometheus%2Fops&var-cluster=memcached_gutter&var-instance=All">Memcached Gutterpool status</a></li></ul> <div class="mw-heading mw-heading2 ext-discussiontools-init-section"><h2 id="Links" data-mw-thread-id="h-Links"><span data-mw-comment-start="" id="h-Links"></span>Links<span data-mw-comment-end="h-Links"></span></h2></div> <ul><li><a href="https://gerrit.wikimedia.org/r/plugins/gitiles/operations/docker-images/production-images/%2B/refs/heads/master/images/mcrouter/" class="extiw" title="gerrit:plugins/gitiles/operations/docker-images/production-images/+/refs/heads/master/images/mcrouter/">mcrouter image</a></li> <li><a href="https://gerrit.wikimedia.org/r/plugins/gitiles/operations/puppet/%2B/refs/heads/production/hieradata/role/common/deployment_server/kubernetes.yaml" class="extiw" title="gerrit:plugins/gitiles/operations/puppet/+/refs/heads/production/hieradata/role/common/deployment server/kubernetes.yaml">mcrouter image versions</a></li> <li><a href="https://gerrit.wikimedia.org/r/plugins/gitiles/operations/puppet/%2B/refs/heads/production/hieradata/common/profile/mediawiki/mcrouter_wancache.yaml" class="extiw" title="gerrit:plugins/gitiles/operations/puppet/+/refs/heads/production/hieradata/common/profile/mediawiki/mcrouter wancache.yaml">list of mediawiki memcached production servers</a></li></ul> <ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><a href="#cite_ref-1">↑</a></span> <span class="reference-text"><a rel="nofollow" class="external text" href="https://engineering.fb.com/web/introducing-mcrouter-a-memcached-protocol-router-for-scaling-memcached-deployments/">Introducing mcrouter: A memcached protocol router for scaling memcached deployments</a></span> </li> </ol>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?type=1x1" alt="" width="1" height="1" style="border: none; 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