Understanding Apache RewriteMap with RewriteLock

Question

I've taken over development of a fairly heavy-duty LAMP application. The original dev used an .htaccess file with RewriteMap and a PHP script to handle certain conditions of the app.

Specifically, when certain subdomain patterns are requested by the client, the RewriteMap catches them and sends them to the appropriate application module.

I'm quite comfortable with typical mod_rewrite redirects, and I think I've got the basic RewriteMap concept figured out; but I'm struggling to find decent documentation on how RewriteLock works. According to the Apache docs:

This directive sets the filename for a synchronization lockfile which mod_rewrite needs to communicate with RewriteMap programs. Set this lockfile to a local path (not on a NFS-mounted device) when you want to use a rewriting map-program. It is not required for other types of rewriting maps.

But this is still a little vague for me. Whats the exact purpose and function of RewriteLock and how does it work?

Answer 1

RewriteLock is used with the prg: keyword. RewriteMap can be used with several keywords, to use text files (txt:), hashfiles (dbm:), randomized text (rnd:) or external mapping scripts ( this one is the prg: keyword ). In this mode the external script is launched when apache start. Then for every incoming request, when mod-rewrite is calling the prg: mapping, apache sends input to that script and reads the output stream to get the value.

RewriteLock must be used in that case to prevent parallel requests (so parallel inputs to that external process) to mix answers on this process standard output. It's a locking mechanism (a file, the given path, which is a classical token, only one user) to enforce serialization of the calls to this external mapping script. IMHO it should be transparently applied by mod-rewrite when using prg: as I never found a prg case where this locking thing is not mandatory.

Edit:

Well in fact you could use an external prg: without the rewriteLock if randomization of the output is not a problem, i.e. for a given entry you can get a response which was given for another entry, like in a script doing some advanced rnd:, your own round-robin service. But if the output must reflect the entry, then you need that semaphore, which of course can slow down the rewritemap process.

So if you're only using the hashmap or textmap you do not need to set the RewriteLock.

Edit:

You may find useful details on this thread, like the fact the lock file exists only for a few milliseconds, when apache calls the prg and waits for an answer.

Edit: On the question one strange fact is:

The original dev used an .htaccess file with RewriteMap

This is strange because RewriteMap cannot work on .htaccess files, .htaccess are configuration entries read dynamically and RewriteMap as stated here in the Context line can only be set in the main configuration or in a VirtualHost configuration. It cannot be in a Location, a Directory or a .htaccess. So chances are this will never work in a .htaccess.

Now @puk asked for an example of RewriteMap usage. Well, searching for "RewriteMap" in Stack overflow will show you several real examples:

• here in a question
• here a list of example in my answer
• another here