HTTP timeouts

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This documents HTTP timeouts involved in a we requests from users to a service behind WMF traffic layers.

TLS

The entry point for a user is ats-tls, which node depends on the service and user IP address:

TLS termination layer	SSL handshake timeout	connect timeout (origin server)	TTFB (origin server)	successive reads (origin server)	Keepalive timeout (client)
ats-tls	60 seconds	3 seconds	180 seconds	180 seconds	120 seconds
nginx (deprecated)	60 seconds (nginx default value)	10 seconds (nginx default value)	180 seconds	180 seconds (same config parameter as TTFB)	60 seconds

Currently a big difference between nginx and ats-tls can be found on how they handle POST requests. nginx buffers the whole request completely before relying it to the origin (varnish-frontend) while ats-tls doesn't buffer it and relays the connection to varnish-frontend as soon as possible. On nginx, the timeout to fulfil the POST body is 60 seconds between read operations, this is the default value and it isn't explicitly configured.

Caching

Our caching system is split in two layers (frontend, and backend). There is one implementation of the frontend layer (varnish) and one implementation of the backend layer (ats-be).

caching layer	connect timeout	TTFB	successive reads
varnish-frontend	3 seconds (text) / 5 seconds (upload)	65 seconds (text) / 35 seconds (upload)	33 seconds (text) / 60 seconds (upload)
ats-backend	10 seconds	180 seconds	180 seconds

App server

See also: MediaWiki at WMF#Timeouts

After leaving the backend caching layer, the request reaches the appserver. Here are described the timeouts that apply to appservers and api:

As of March 2020

layer	request timeout
Nginx (TLS)	180 seconds (appserver, api, parsoid) / 1200 seconds (jobrunner) / 86400 seconds (videoscaler). Configured byproxy_read_timeout. Time to first byte. Wall clock time.
Envoy (TLS/ats-be requests)	1 second (connect timeout) / 65 seconds (route timeout)
Apache	202 seconds (appserver, api, parsoid) / 1202 seconds (jobrunner) / 86402 seconds (videoscaler). Configured by Timeout. Entire request-response, including connection time. Wall clock time.
php-fpm	201 seconds (appservers) / 201 seconds (api) / 201 seconds (parsoid) / 86400 seconds (jobrunner, videoscaler). Configured by profile::mediawiki::php::request_timeout. Wall clock time.
PHP	210 seconds (appserver, api, parsoid) / 1200 seconds (jobrunner, videoscaler). Configured by max_execution_time. CPU time (not including syscalls and C functions from extensions).
MediaWiki	60 seconds (GET) / 200 seconds (POST) / 200 seconds (jobrunner) / 86400 seconds (videoscaler). This is configured using php-excimer

Notes

The app server timeouts might be larger than the ones on the caching layer, this is mainly to properly service internal clients.

php-fpm

The request_timeout setting the maximum time php-fpm will spend processing a request before terminating the worker process. This exists as a last-resort to kill PHP processes even if a long-running C function is not yielding to Excimer and/or if PHP raised max_execution_time at run-time.

PHP

The max_execution_time setting in php.ini measures CPU time (not wall clock time), and does not include syscalls.

Note that unlike all other settings, for videoscalers this setting is far lower than the higher-level timeouts (20min vs 24h). This is a compromise to prevent regular jobs from being able to spend 24h on the CPU, which would be very unexpected (as they share the same php-fpm configuration). Videoscaling jobs are expected to spend most of their time transcoding videos, which happens through syscalls so this is fine.

MediaWiki

This is controlled by the ExcimerTimer interval value, in wmf-config/set-time-limit. Upon reaching the timeout, php-excimer will throw a WMFTimeoutException exception once the current syscall returns.