I asked this question on stackoverflow:
http://stackoverflow.com/questions/12528330/one-wsgi-app-eats-all-apache-clients
{{{
We have a SOAP mod_wsgi (apache) app which gets heavy load sometimes. The same Apache servers some other wsgi-apps.
Unfortunately you can set MaxClients only at server level, not per wsgi-app.
We get:
server reached MaxClients setting, consider raising the MaxClients setting
Is there a way to stop this wsgi app from eating all apache workers?
I want to return 503 "Service Unavailable" only to the SOAP client who connects
to the SOAP wsgi app.
Apache config snippet:
WSGIDaemonProcess soap_app threads=1 processes=3
WSGIScriptAlias /soap_app /home/soap_app/django_wsgi.py
<Location "/soap_app/">
WSGIProcessGroup soap_app
WSGIApplicationGroup %{GLOBAL}
</Location>
There are only 3 wsgi daemon processes for the soap app. But it occupies much
more apache workers.
Update: We use apache prefork mpm. There are N apache worker. And for mod_wsgi we use prefork, too. There are M mod_wsgi
worker processes. The apache worker count can be controlled by MaxClients. The mod_wsgi worker count is controlled by
the above config.
I think you can't handle this inside the python wsgi app (django). I guess it needs to be done by the mod_wsgi or apache
config.
}}}
Here the answer:
{{{
All the request are served by an Apache child (controlled by MaxClients) but every time a request hits the soap_app url
the Apache child will wait for one of the the 3 WSGIDaemonProcess. If you receive request to soap_app faster than you
can serve them with 3 processes eventually you will run out of Apache child.
The only way I see to control the number of Apache child dedicated to soap_app is tho use mod_proxy and proxy the
soap_app request to another "service". The proxy pass directive allows you to define the number of concurrent request to
be serve, that will be equal to the number of Apache child you want to use for soap_app.
The "service" to serve the soap_app request could be another VirtualHost of the same Apache (never tested it) or a
gunicorn instance with the soap_app application
}}}
I use mod_wsgi since several years now and was happy with it and want to stay
with it.
I see this solution at the moment: route only the URLs which can get too much
traffic (soap client) through mod_proxy,
and route the traffic to the same mod_wsgi application again. But this would be
recursive ...
Or I handle this at application level: I don't want more then 3 workers running at one time. I give the mod_wsgi daemon
4, and in a middleware I check how many workers are busy. If three are busy the request would return at once with 503
"Service Unavailable". If the 503 response is quicker then new requests come in, the MaxClients limit should not be
reached. But how can an application know how many workers are busy at the moment? Touching files in a directory or
memcached incr+decr is not reliable.
Do you understand my problem? Would could be done?
Thomas Güttler
--
Thomas Guettler, http://www.thomas-guettler.de/
E-Mail: guettli (*) thomas-guettler + de
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