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From André Warnier (tomcat)>
Subject Re: Shared var between processes
Date Fri, 20 Oct 2017 09:17:57 GMT
On 20.10.2017 10:50, Ben RUBSON wrote:
> On 20 Oct 2017 10:38, André Warnier (tomcat) wrote:
>> On 19.10.2017 22:02, John Dunlap wrote:
>>> To piggy back onto this question, what is the best way to do this such that the
values of
>>> the variables are different for every virtual host? In our model, all virtual
hosts shared
>>> the same interpreter pool but we need to have different configuration variables
on a per
>>> virtualhost basis. Currently, we are using PerlSetVar for this purpose.
>> And that seems the right way to me.
> What about a startup file, as proposed by Adam, which would return John's configuration
> variables according to $ENV{'SERVER_NAME'} ? (using a switch/case for example)
> Each virtualhost would then have its own configuration variables, having a uniq startup
> file for the whole server.
> Would it be more performant than the PerlSetVar solution ? (doc sometimes warns about
> PerlSet* performance)
I believe that there is much more of a performance hit, when asking the server to set up 
an environment ($ENV) for sub-processes, than via the PerlSetVar mechanism.

In any case, we use the PerlSetVar mechanism extensively, for dozens of servers and/or 
and there has never been any noticeable performance issue linked to this.

Think about the mechanism : the Apache httpd configuration files get read and parsed 
*once*, at the time of the server start (or server "reload"). After that, everything is 
stored in some large pre-parsed configuration table, with sub-sections for each 
VirtualHost. This table is copied to (or shared by) each Apache sub-process (or thread), 
When one of these sub-processes makes a dir_config() call, this just looks into this 
pre-processed configuration table.
In comparison, the mechanism involving the environment has to set up this environment 
repeatedly (at least each time a new Apache sub-process is started). (It also would have 
to do this for each cgi-bin script call, if it was not handled under mod_perl).
That is bound to be less efficient than the configuration method.

Also, I do not see what advantage there would be for having a set of separate 
supplementary files that need to be read and parsed separately, as compared to having that

stuff in the Apache configuration files in the first place.
(Except maybe if you do not have full access to the Apache configuration files, in terms 
of permissions e.g.; but that can be solved in other ways).
It seems much more maintainable (and documentable) to have everything in one place.

The only real issue with PerlSetVar, is that of course it only is "retrievable" by perl things
running under mod_perl. (You could not retrieve such a value from a shell script running 
as a cgi-bin for example; so if you have a mixture of such things, you may /still/ need to

use $ENV, for the non-perl things).

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