id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
33700	APPEND_SLASH adds significant latency to all requests not ending in / (even if successful)	Anders Kaseorg	Anders Kaseorg	"Originally, `APPEND_SLASH` worked by looking for 404 responses and replacing them with redirects, so as not to unnecessarily impact the performance of successful responses. However, commit 9390da7fb6e251eaa9a785692f987296cb14523f in 1.9.5/1.10 changed this to check `should_redirect_with_slash()` on every request, resulting in a moderately expensive extra `urlconf` lookup for every request not ending with `/`, whether or not it succeeds as written.

This performance impact was not considered in the commit message or the corresponding ticket #26293, so I assume it was an oversight. That ticket asserted “This doesn't really make sense, since the two settings are not interdependent”, which is incorrect—performance was the reason for the interdependence.

The overhead was found to be significant enough in Zulip to merit [https://github.com/zulip/zulip/commit/229090a3a58de15eb2a76a8e513fa5d9a190170c subclassing] `CommonMiddleware` to skip it in certain conditions.

Here’s a [https://gist.github.com/andersk/0140318d32f1657ff40cad9e7e353bd2 minimal test project] with an exaggerated number of routes so the overhead can be easily observed.

{{{
$ ./manage.py runserver
}}}

{{{
$ wrk http://127.0.0.1:8000/url9999
Running 10s test @ http://127.0.0.1:8000/url9999
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   232.40ms   73.85ms 570.86ms   69.16%
    Req/Sec    21.70      9.47    40.00     63.35%
  426 requests in 10.01s, 64.90KB read
Requests/sec:     42.56
Transfer/sec:      6.48KB
$ sed -i 's/# APPEND_SLASH = False/APPEND_SLASH = False/' slash_test_settings.py
$ wrk http://127.0.0.1:8000/url9999
Running 10s test @ http://127.0.0.1:8000/url9999
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   139.80ms   52.07ms 352.19ms   69.09%
    Req/Sec    36.46     12.23    60.00     58.12%
  714 requests in 10.01s, 108.79KB read
Requests/sec:     71.32
Transfer/sec:     10.87KB
}}}"	Cleanup/optimization	closed	HTTP handling	4.0	Normal	fixed	CommonMiddleware	Sam Emett Speer Keryn Knight Florian Apolloner Simon Charette	Ready for checkin	1	0	0	0	0	0