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From Devin Teske <>
Subject Re: a better way to recognize module changes
Date Fri, 11 Sep 2009 22:02:28 GMT
Maybe somebody can refute what I'm seeing, but as of mod_perl-2.0.4,
Apache2::Reload is gone (so you can remove that from your list of

On Fri, 2009-09-11 at 14:26 -0700, Jonathan Swartz wrote:
> I'm thinking about an improved solution to recognizing module changes  
> in a running server, without restarting the server.
> These are the solutions I know about:
> 1) Apache2::Reload / Module::Reload
> These check whether modules have changed on each request, and if so,  
> clear their symbols and reload them inside the process.
> Problem: some modules fail to reload properly. Sometimes the failure  
> is intermittent, depending on the order of module loading and other  
> esoteric details. Moose and ORM modules seem particularly prone to  
> reload failures. For me, this level of unpredictability makes  
> *::Reload too frustrating to use.
> 2) Catalyst auto-restart
> Catalyst has an engine (Catalyst::Engine::HTTP::Prefork::Restarter)  
> which forks off a "watcher" process that waits for your modules to  
> change. When they change, it restarts the server. The usual effect is  
> that, between the time you hit "save" in your editor and reload your  
> page, the server has restarted or at least begun restarting.
> Problems: Doesn't work well if you make a few changes in a row; the  
> restart only captures your first change. Bad user experience if  
> there's an error in your module; you have to realize the server has  
> died, find the error message in some shell or log, and manually start  
> up the server again.
> 3) Perrin's MaxRequestsPerChild=1
> Perrin recently alerted me to the MaxRequestsPerChild=1 technique.  
> That is, set MaxRequestsPerChild to 1, then load any potentially- 
> changing modules in the *child*, not the parent (obviously only for  
> development environments). Each request will hit a fresh child server,  
> which will load all of your potentially-changing modules anew.
> This is the nicest solution I've seen so far. The only problem I can  
> see is its performance - each potentially-changing module has to be  
> loaded on each request. **
> 4) My idea: Combine 2 and 3
> As in 3, load any potentially-changing modules in the child. Leave  
> MaxRequestsPerChild alone. As in 2, fork off a "watcher" process that  
> waits for your modules to change. When they change, kill all the  
> server's children explicitly.
> The end result is that you get reasonable performance when your  
> modules don't change (e.g. when you are only futzing with templates),  
> but when modules do change, you should see the effects immediately.
> This should be able to work with mod_perl, fastcgi, Net::Server, etc.,  
> as long as the parent server responds appropriately to the killing of  
> all its children (by launching new ones). Apache, at least, seems to  
> be ok with this.
> What do people think? Is this worth codifying in a module, or does  
> something like this already exist?
> Thanks for any feedback
> Jon
> ** - You can try to load things only on demand, but often mod_perl  
> code is written without 'use' statements as it assumes everything is  
> loaded in the parent. You can also try to minimize the number of  
> potentially-changing modules, but then you run the risk of leaving  
> something off and having to adjust it and restart.
Devin Teske

Field Engineer
Metavante Corporation
626-573-6040 Office
510-735-5650 Mobile

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