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How Does Apache Decide Order to Execute Modules?

 3:19 am on May 11, 2013 (gmt 0)

I have a question for y'all. Modules such as mod_dir, mod_alias, mod_rewrite... how does Apache determine what order to process them?

The most intuitive guess -- that it's determined by the order the modules are loaded -- doesn't seem to be the case. I've tried making mod_rewrite the first module load, and I've tried making it the last module loaded, but neither seems to make any difference.

I've also sifted through the documentation looking for the answer, but I haven't found it yet.



 5:00 am on May 11, 2013 (gmt 0)

In many cases, it's actually the order in which they are loaded. In some cases, a module depends on another being loaded first, in which case execution may be stalled.


 5:36 am on May 11, 2013 (gmt 0)

Any chance you could provide some sort of verifiable test or documentation? I ask because mod_rewrite redirections will execute before mod_alias redirections, regardless of their order in the configuration file, yet neither of these modules depend on the other.


 7:32 am on May 11, 2013 (gmt 0)

In many cases, it's actually the order in which they are loaded.

I had the impression this changed between 1.3 and 2.2-- and I think I've hunted down the source [webmasterworld.com] of this impression.

Don't look at me. I didn't pick that shade of yellow.

I'm sure he said the same thing many other times over the years, but this is the one that bobbed to the surface. It helps explain why apache docs themselves are so stubbornly elusive in regard to what would seem like a common and obvious question: It's because the correct and complete answer is "It depends" ;)


 8:42 am on May 11, 2013 (gmt 0)

Thanks. That helped me figure out what I should be looking for.

Here's where the documentation explains calling order:

For the uber curious, here's where mod_rewrite defines its calling order (line 5109):
That mod_rewrite defines itself as hook_first and mod_alias defines itself as hook_middle is why rewrite directives happen before redirect directives.

mod_info provides a much more readable way to see the priority of each module for each hook:
Translate Name:
00 mod_rewrite.c
10 mod_alias.c
30 core.c


 12:43 pm on May 11, 2013 (gmt 0)

there's also this:

The use of RewriteRule to perform this task may be appropriate if there are other RewriteRule directives in the same scope. This is because, when there are Redirect and RewriteRule directives in the same scope, the RewriteRule directives will run first, regardless of the order of appearance in the configuration file.


 5:20 pm on May 11, 2013 (gmt 0)

> there's also this:

Absolutely. In fact, that's the paragraph that got me asking this question. I wanted to know _why_ one always runs before the other, and how Apache decides that order.


 7:40 pm on May 11, 2013 (gmt 0)

Note that there are two more values, APR_HOOK_REALLY_FIRST and APR_HOOK_REALLY_LAST.

:: snrk ::

I guess this is the key part:
When a module knows that it must be run before (or after) some other modules, it can specify them by name. The second (third) argument is a NULL-terminated array of strings consisting of the names of modules that must be run before (after) the current module. For example, suppose we want "mod_xyz.c" and "mod_abc.c" to run before we do, then we'd hook as follows:
static void register_hooks()
static const char * const aszPre[] = { "mod_xyz.c", "mod_abc.c", NULL };
ap_hook_do_something(my_something_doer, aszPre, NULL, APR_HOOK_MIDDLE);

Which all comes back to this forum's boilerplate: If you are on shared hosting, you probably don't know and definitely can't change the modules' execution order.

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