Debugging in production
Debugging a web request
Use X-Wikimedia-Debug to make a request bypass Varnish cache and route to a specific debug server.
You can make a self-request directly to a web server by ssh-ing to a production serve and using Curl, like so:
mwdebug1002$ curl -i --connect-to ::$HOSTNAME 'https://test.wikipedia.org/w/load.php' HTTP/1.1 200 OK Server: mwdebug1002.eqiad.wmnet … /* This file is theWeb entry point for MediaWiki's ResourceLoader: … */
Or over HTTP:
mwdebug1002$ curl -i --connect-to ::$HOSTNAME 'http://test.wikipedia.org/wiki/Main_Page' HTTP/1.1 302 Found Server: mwdebug1002.eqiad.wmnet Location: https://test.wikipedia.org/wiki/Main_Page
mwdebug1001$ curl -i --connect-to ::$HOSTNAME 'http://www.wikimedia.org/' HTTP/1.1 200 OK Server: mwdebug1001.eqiad.wmnet … <!DOCTYPE html> <html lang="mul" dir="ltr"> <head> <meta charset="utf-8"> <title>Wikimedia</title> <meta name="description" content="Wikimedia is a global movement whose mission is to bring free educational content to the world."> …
And over HTTP as if from an external HTTPS request (This is currently the only way to debug in Beta Cluster, since internal HTTPS is not available there):
deployment-mediawiki11$ curl -i --connect-to ::$HOSTNAME -H 'X-Forwarded-Proto: https' 'http://en.wikipedia.beta.wmflabs.org/wiki/Main_Page' HTTP/1.1 200 OK Server: deployment-mediawiki11.deployment-prep.eqiad1.wikimedia.cloud … <!DOCTYPE html> …
Note about Host header: Prior to 2015, the more traditional approach of using
curl 'http://localhost/wiki/Main_Page' -H 'Host: test.wikipedia.org' was supported, but per T190111 this is no longer possible because connections via "localhost" are handled by a higher priority VirtualHost in Apache that serves responses for the health status checks (not related to MediaWiki).
Note about FQDN address: Prior to 2019, it was common to workaround the above "localhost" issue by using the internal FQDN (mw0000.eqiad.wmnet) or its internal IP address instead. This is easiest via
$(hostname -f), e.g. like
curl -i -H 'Host: test.wikipedia.org' "http://$HOSTNAME/w/load.php". While this still works today for HTTP requests, it does not work reliably for HTTPS requests since the web server in question has no certificate for the internal hostname, though this could be bypassed with
curl --insecure (or
curl -k for short).
Note about --resolve option: Prior to 2020, other documentation pages recommended
--resolve as the main strategy, e.g.
curl -i --resolve "test.wikipedia.org:443:$(hostname -i)" 'https://test.wikipedia.org/w/load.php'. This still works perfectly today and is functionally equivalent to the current recommendation with
--resolve option is no longer recommended because it is too easy to misuse and not realize that it was silently ignored. For example, if you specify "resolve" with a different hostname than your URL (with redirects, there can be many host names involved), curl will silently connect to the main production edge for your first and only request, which is easy to miss if you don't enable verbose
-v mode and check what server it actually connected to. This can be mitigated by using a wildcard hostname like
--resolve "*:443:$(hostname -i)" but that still requires getting the port right, which means over HTTP, it would silently get ignored again, plus it requires the IP address and thus the extra hostname command. The
--connect-to option has the benefit of allowing both host and port to be omitted, and supports a hostname as destination (instead of IP address), thus allowing the simpler and more memorable
Pushing code to a debug server
Developers can put code updates on one of the mwdebug hosts, before deploying to the entire production cluster, see Pre-deployment testing in production .
Note that any changes you make this way will be overwritten by cluster-wide deployments. So, long-term changes should go into a block wrapped in an
if ( $wgDBname === 'testwiki' ) (to prevent them from accidentally running on all wikis!). Short-term changes (anything not committed to the git repo) should either be committed and rolled out, or reverted as soon as possible.
When editing files on a debug server directly, remember to clear the PHP7 opcache afterwards. Without this, changes to files on disk might not take affect.
mwdebug1001$ php7adm /opcache-free
When using Scap to pull down a change from the deployment host, this happens automatically.
Use X-Wikimedia-Debug in a browser to route one of your regular web requests to the debug server you have staged code on.
From a maintenance host, use the
sql command, or use
mwscript mysql.php directly.
sql enwiki -h for help to see how you can connect to non-core databases, like ExternalStorage, or specific database hosts (e.g. db0123).
In particular, take note that in MediaWiki some of our DB clusters have a different name. For example "x1" and "x2" are known as "extension1" and "extension2", for the purposes of the
sql --cluster parameter and internal values of
$wgLBFactoryConf that this corresponds with.
$ sql test2wiki # Connected to s3.test2wiki database on a live replica in production. $ sql centralauth # Connected to s7.centralauth $ mwscript mysql.php --wiki aawiki --wikidb centralauth # (idem) $ sql wikishared # Connected to x1.centralauth $ mwscript mysql.php --wiki aawiki --cluster extension1 --wikidb wikishared # (idem) $ sql parsercache --raw-host 10.64.0.57 # Connect to an arbitrary that is not pooled or otherwise not in $wgLBFactoryConf # such as parser cache hosts. $ mwscript mysql.php --wiki aawiki --raw-host 10.64.0.5 --wikidb parsercache # (idem) $ mwscript mysql.php --wiki aawiki --cluster extension2 --list-hosts db0001 db0002 db0003
Debugging a maintenance script
This information is outdated. (last update: 2018)
ssh to a mwdebug host, then:
source /usr/local/lib/mw-deployment-vars.sh sudo -u "$MEDIAWIKI_WEB_USER" php -m debug "$MEDIAWIKI_DEPLOYMENT_DIR/multiversion/MWScript.php" someScript.php --wiki=testwiki --scriptSpecificParameters "goHere"
To debug how log messages are sent to Logstash from MediaWiki php-fpm servers, read Application_servers/Runbook#Logging.
Debugging in shell
To open a command-line shell to PHP, log in an mwdebug server or the Maintenance server and run:
$ mwscript eval.php dbname_here
Where dname is e.g. aawiki. You can call arbitrary MW code from here, and use
return .. as shortcut for
var_dump( .. ); to read out any information.
Ad-hoc log messages
The recommended approach to ad-hoc logging in production is
wfDebugLog( 'AdHocDebug', 'Hi...' );. This will reliably send the message to Logstash from both web-facing contexts, jobrunners, and CLI maintenance scripts, and does so without running the risk of unintentionally disclosing sensitive data attached to objects in memory.
If the data will be logged from a mwdebug host via CLI or via WikimediaDebug, then the message will show up at Logstash dashboard: mwdebug
If the data is expected to come from a different host (e.g. only reproducible there, or waiting for the condition to be hit organically), then the message will show up at Logstash dashboard: mediawiki where you can query for channel:AfHocDebug, or page through the channel list and zoom in on the appropiate channel.
Ad-hoc command line logging
To reproduce an issue programmatically, it is recommended to follow #Debugging in shell instead without modifying source code on disk or running modified programs.
If an issue is difficult to reproduce and you need to modify a maintenance script to log some information quickly you can use the
wfDebugLog() approach above. Alternatively, to keep the information local and not write to Logstash, you can also choose one of the following:
error_log('Hi ...', 4);
error_log Type 4 corresponds to STDERR in CLI. For web requests via Apache, STDERR is not defined and these go to syslog instead. For such web requests, these will end up in Logstash as
type:apache2 message:"Got error 'PHP message: Hi...". For mwdebug hosts, these end up on the mwdebug Logstash, but take note that these will not match type:mediawiki queries and do not show up on the general "mediawiki" or "mediawiki-errors" dashboards in Logstash. For other hosts, you may find these on the apache2log Logstash dashboard
Syslog will end up on disk, readable via
sudo tail /var/log/syslog and is also readable without sudo on the syslog Logstash dashboard, possibly querying with e.g.
message:Hi to find specific entries.