WikimediaDebug

WikimediaDebug is a set of tools for debugging and profiling MediaWiki web requests in a production environment.

You can use WikimediaDebug through the accompanying browser extension, or from the command-line using the X-Wikimedia-Debug header.

So long as WikimediaDebug is on, all your requests will be handled as a Varnish cache miss.

Browser usage

Install the extension from Firefox Browser Add-ons or Chrome Web Store. The extension create a button with the Wikimedia communty icon, usually near the upper right corner of the browser window.

To make a web request to one of the debug servers, click the icon, select the right server (if you’re not deploying a change yourself, the deployer should have told you which server to select), then toggle the on/off switch. When the extension is enabled, the button should show an “On” badge.

Command-line usage

Force Varnish to skip the cache and pass request to mwdebug1001.eqiad.wmnet:

$ curl -H 'X-Wikimedia-Debug: backend=mwdebug1001.eqiad.wmnet' https://meta.wikimedia.org/wiki/Main_Page

Same as above, but profile a request using Tideways and publish the profile to XHGui:

$ curl -H 'X-Wikimedia-Debug: backend=mwdebug1001.eqiad.wmnet; profile' https://meta.wikimedia.org/wiki/Main_Page

Request a page with MediaWiki configured for read-only mode:

$ curl -H 'X-Wikimedia-Debug: backend=mwdebug1001.eqiad.wmnet; readonly' https://meta.wikimedia.org/wiki/Main_Page

Options

The following attributes can influence the backend request:

backend=…: Choose the mwdebug server that Varnish will route your request. It can be set to 1 in Beta Cluster where the only backend acts as both appserver and mwdebug server.
forceprofile (Inline profile): Capture a trace profiler and append it to the web response.
log (Verbose logs): Verbosely enable all MediaWiki debug log groups, submitted to Logstash for querying.
profile (XHGui): Record a trace for performance analysis and publish the result to XHGui.
readonly: Read-only mode requests a page from MediaWiki with read-only mode enabled, to simulate how your code behaves when the database is read-only due to replica lag or scheduled maintenance.
shorttimeout: Simulate how your code handles execution timeouts (e.g. from mediawiki/libs/RequestTimeout, T293568). When enabled, $wgRequestTimeLimit is set to 2s. This option is available via command-line use only.

Available backends

You can simulate how your code behaves in a secondary data center (Multi-DC), by selecting a host in a DC other than the current primary. You can use either of the datacenters through this list at any time, even if that datacenter is currently inactive or depooled. If you pick a backend in a datacenter that is not currently the primary DC, some actions may be read-only, disabled or slower.

It's now also possible to reach an experimental MediaWiki installation running on our Kubernetes cluster. Beware that some of the features described on this wiki page may not be available yet via this backend. Please choose a different backend unless you specifically need to verify something on Kubernetes.

The following application servers are dedicated to WikimediaDebug use:

2x mwdebug in Eqiad
2x mwdebug in Codfw
k8s-experimental (mw-on-k8s).

Request profiling

Excimer UI provides detailed and interactive flame graphs on a per-request basis. To use it, enable the "Excimer UI" option (excimer attribute). This will capture PHP trace logs from the current request (using Excimer), and send them to Excimer UI at performance.wikimedia.org, where the visualisation is powered by Speedscope. Excimer UI launched in May 2023 (T291015).

To try out Speedscope with MediaWiki data, open this example profile.

XHGui profiling

XHGui provides a call tree for a given web request with aggregations of execution time, call count, and memory usage for all called functions. The capturing and storing for XHGui is powered by php-tideways (formerly known as XHProf). Our instance is configured via wmf-config makes your profiles publicly accessible at https://performance.wikimedia.org/xhgui/.

To explore XHGui and its features, open this example profile.

Capture a profile from a browser:

Enable the "XHGui" option.
Click the "Find in XHGui" link in the WikimediaDebug popup. This takes you directly to a list of profiles matching your Request ID (usually only 1 match).
Click the sharable permalink (e.g. the timestamp link, or "GET" method link) to view the recorded profile.

Capture from the command-line using Curl:

Set the profile attribute.
Find your profile at https://performance.wikimedia.org/xhgui/.

Plaintext request profile

WikimediaDebug can deliver a plaintext-format profile directly from the web server. Enable the "Inline profile" option (forceprofile attribute) on any MediaWiki web request (including index.php, api.php, and load.php), and MediaWiki will append a plain text profile to the web response.

Example in the browser: Open any page on Wikipedia and view source (example URL). Then, enable WikimediaDebug with the "Inline profile" option, and reload the browser tab. At the end of the response should be a call-graph summary, like this:

/*
 100.00% 1437.125      1 - main()
  87.21% 1253.268      1 - ResourceLoader::respond
  79.31% 1139.756   1509 - ResourceLoaderModule::getVersionHash
  77.88% 1119.292      3 - ResourceLoader::getCombinedVersion
  ..

Example using cURL:

# Production
$ curl -H 'X-Wikimedia-Debug: 1; forceprofile' 'https://test.wikipedia.org/w/load.php?modules=startup&only=scripts&raw=1'

# Beta Cluster
$ curl -H 'X-Wikimedia-Debug: 1; forceprofile' 'https://en.wikipedia.beta.wmflabs.org/w/load.php?modules=startup&only=scripts&raw=1'

Plaintext CLI profile

You can generate a plaintext-format profile from any MediaWiki maintenance script, by setting the --profiler=text option. Note that in Wikimedia production, maintenance script must be run via the mwscript command. For performance reasons, the required debug tools are only installed on mwdebug servers, so while we normally run Maintenance scripts from an mwmaint host, when debugging make sure to connect your termimal to a debug host first. (in the Beta Cluster, you can use any MW server).

krinkle@mwdebug$ mwscript showSiteStats.php --wiki=nlwiki --profiler=text
Number of articles:  2122688
Number of users   :  1276507

<!--
100.00% 114.964    1 - main()
 …
 22.42% 25.776     1 - ShowSiteStats::execute
 16.61% 19.096     2 - Wikimedia\Rdbms\LoadBalancer::getServerConnection
  4.80% 5.522      1 - Maintenance::shutdown
  4.41% 5.065      1 - Wikimedia\Rdbms\Database::initConnection
  3.07% 3.530      1 - DeferredUpdates::doUpdates
  2.66% 3.061      1 - Wikimedia\Rdbms\Database::select
  2.38% 2.739      1 - Wikimedia\Rdbms\Database::query
  1.95% 2.240      1 - section.SELECT * FROM `site_stats` LIMIT N 
  1.48% 1.700      1 - Wikimedia\Rdbms\DatabaseMysqli::mysqlConnect
  …
-->

Debug logging

Setting the log attribute in the X-Wikimedia-Debug header (“Verbose log” checkbox in the extension) will cause MediaWiki to be maximally verbose, recording all log messages on all channels (regardless of whether or not they are otherwise enabled in wmf-config).

These messages will end up in Logstash at https://logstash.wikimedia.org/app/dashboards#/view/mwdebug and in /srv/mw-log/XWikimediaDebug.log on the mwlog host. See Logs#mw-log for more information.

To view the logs of a specific web request only, the browser extension adds a "Find in Logstash" link in the footer on any wiki as well, which opens goes to Logstash with a filter for the request ID of the current page view. You can also construct this URL manually, by navigating to https://logstash.wikimedia.org/app/dashboards#/view/x-debug and entering a search query like reqId:"…"

Staging changes

This section will walk you through a common WikimediaDebug debugging workflow.

It worked on your machine. It worked on the Beta Cluster. And the unit tests pass. But will it work in production? You can reduce the risk of unexpected breakage by deploying your change to a debugging server in production and using WikimediaDebug to verify its behavior.

Follow the instructions on How to deploy code, but stop after you have completed step 2 ("Get the code on the deployment host"). Now sync your change to one of the debug backends in a new tap by SSHing into it and running scap pull:

you@laptop:~$ ssh mwdebug1001.eqiad.wmnet
you@mwdebug1001:~$ scap pull

Now, enable the WikimediaDebug extension in your browser, and select the same backend you just pulled your code to. Your browser requests will be routed to this backend, allowing you to verify that your changes are working correctly prior to deployment.

Limitations

X-Wikimedia-Debug does not currently work on the Wikitech wiki, which is served by servers separated from the rest of the Wikimedia Foundation production cluster. Try it on Wikipedia instead!

Code steward

Maintained by Performance Team
Live chat (IRC): #wikimedia-perf ^connect
Issue tracker: Phabricator (Report an issue)

Release process

Documented in README.md.