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From Juha Laiho <>
Subject Re: [users@httpd] Problem with mod_headers ?
Date Wed, 31 Dec 2008 08:50:44 GMT
André Warnier wrote:
> As a separate question : how does Apache (or mod_jk) exactly arrange to
> do that (not allow mod_headers to change the response headers set by
> Tomcat) ?
> The answer may help me decide whether I can/want to try writing my own
> mod_perl response header modification handler (or output filter).
> And as an additional comment, I do think that indeed this is debatable.
> After all, if the Apache admin decides to overwrite the headers set by
> Tomcat, it should be his (informed) decision, no ?

Note: this is based on observations from Apache 1.3; Apache 2.x may well
behave differently -- and even then, these are vague memories of experiments
done quite a long ago.

The request processing is split into different phases, and modules attach
to hooks for the phases. For some phases, there can be only one module
acting upon a single request (f.ex. the actual generation of the response),
but for others (rewriting the request, handling headers, ...), the stage
can consist of various modules that are stacked on top of each other,
working in a chain. This is what makes things difficult: the various phases
are not that well documented, and likewise, the module documentation does
not outright tell to which processing phase(s) the module attaches itself.
A final missing piece of documentation, I recall, was that the modules were
inserted to the stacks in the order they were loaded (the LoadModule/UseModule
statements in the configuration).

So, in order to get the desired outcome of a combination of modules, you'll
need to dig through them to find out to which phases they attach, and then
load them in an order that is appropriate for your needs.

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