I was trying to understand how apache could communicate with php-fpm through unix sockets, when I came across this example:
My question is, why is it necessary to pipe the socket into the fcgi://localhost/? How does it work exactly?

Where does apache take the meaning of 'fcgi' from in this context? From proxy_fcgi_module? Or from fastcgi_module? Or both?

FPM is not "part" of Apache but an external service. The daemon usually listens on a UNIX socket, meaning not exposed to the 'net but confined to localhost and w/o TCP protocol overhead. To be able to hand off jobs to it you must also speak the right protocol and that's FCGI.


No, fastcgi_module and proxy_fcgi_module are different DSO's: https://wiki.apache.org/httpd/PHP-FPM and https://httpd.apache.org/docs/2.4/mo...roxy_fcgi.html are your friends.

I loved the phrase "hand off jobs"... really on the nose ) (I know - or I think I know - it wasn't intentional, but still quite funny!)

Well, I kind of already read what you're linking to and other similar resources (not that a reread would hurt). My question is a little bit less naive than it seems to be, or at least that's what I like to believe. In a way it has more to do with the way unix sockets work. If php-fpm uses unix-sockets, isn't it redundant to pipe it into localhost? I mean, could you normally set up a communication session between two apps through unix sockets some other way than on the same localhost? I'm guessing 'localhost' is more related to 'fcgi', meaning that in order to bind the php-fpm unix socket to the fcgi, you simply need to add the fcgi syntax that presupposes specifying localhost - which otherwise would use a tcp socket. Does that make sense?

So apache knows to interpret the special meaning of 'fcgi' using the proxy_fcgi_module, right? But I had used mod_php and fastcgi_module, then it would have used this latter module to interpret it? Or it wouldn't make sense using it in this context?

It is as that's exactly how it works: dump a job, hope it timely returns a usable payload.


Yes but we look at it from different angles. Indeed where the socket lives doesn't matter, it could have been "loadbalancer.fpm-pool-3.yourdo.main", but it isn't redundant. (Remove it and see if it still works? ;-p) And "fcgi://" is only related to location in the sense of the "incantation" needed to summon this lesser daemon in its known location: you shan't talk dav:// to it, nor bitcoin:// nor ed2k://, only fcgi://.


If you'd use the latter then you'd prolly use a different syntax but it would know how to interpret things, yes.