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From Joshua Marantz <jmara...@google.com>
Subject Re: Sharing information across Apache child processes
Date Tue, 30 Sep 2014 13:35:52 GMT
There's a variation on this theme you might consider.  You can use a few
counters in shared memory, and save config information in a file.  If one
of the child processes learns of a config update (say by handling an HTTP
request for one from somewhere) it can update the file (atomically, say,
via write-to-temp-and-rename) and bump a shared-memory counter (atomically
via shared-memory mutex).  Other child processes can quickly poll on
counter-changes and re-read config files when they change.

The advantage of this approach over storing the whole config in
shared-memory is that we only put a fixed number of integers in shared
memory, rather than our whole configuration structure, which might be large
or vary in size depending on the config.

mod_pagespeed uses this technique
<https://code.google.com/p/modpagespeed/source/browse/trunk/src/pagespeed/kernel/cache/purge_context.h>.
Note there is no apr code directly in that file as it's abstracted behind
some C++ interfaces, but 'Variable' is really an int64's worth of mutexed
shared memory in operation.

-Josh


-

On Tue, Sep 30, 2014 at 9:20 AM, Abdi Abdirahman <abd.mohamud@gmail.com>
wrote:

> On 09/29/2014 05:14 PM, Sorin Manolache wrote:
>
>> On 2014-09-29 13:39, Rajalakshmi Iyer wrote:
>>
>>> Hello,
>>>
>>> I have a requirement whereby my application's configuration information
>>> (comprising a few complex data structures) needs to be shared across the
>>> various Apache child processes.
>>>
>>> Currently, the configuration is being individually loaded by each child
>>> process, which makes it hard for configuration changes to propagate.
>>>
>>> What is the best way / place to have a common configuration for the
>>> application?
>>>
>>> Please advise.
>>>
>>
>> I suppose you want to update the configuration without running "apache2
>> -k graceful" (or "apache2ctl graceful").
>>
>> In this case you could use a segment of memory that is shared across the
>> apache children. You'll have to create the shared segment memory before the
>> parent forks its children (for example in post_config). The shared memory
>> is then inherited by the forked children.
>>
>> You'll need a method to update the contents of the shared memory segment
>> and a multiple-readers-single-writer inter-process exclusion mechanism in
>> order to safely read and write from the shared segment.
>>
>> Sorin
>>
>>  Hello All,
>
> You can check out mod_cluster approach (https://github.com/
> modcluster/mod_cluster/tree/master/native/mod_proxy_cluster) - it uses
> shared memory as data storage and additional thread per process to check
> updates in storage. This way you'll keep per-process module config in sync
> with shared data.
> Not perfect solution though, but I doubt that there is any flawless way to
> share config structures.
>
> ________________
> Best regards,
> Abdi A.
>

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