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From: Jan Lehnardt <jan@apache.org>
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Subject: Re: [PLUGINS] Plugin Hooks
Date: Thu, 8 Aug 2013 12:26:35 +0200
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how would this look in code?

On Aug 8, 2013, at 12:21 , Jason Smith <jhs@apache.org> wrote:

> Perhaps a custom behaviour to help catch API problems at compile time?
>=20
>    -behaviour(couchdb_plugin).
>=20
>=20
>=20
> On Thu, Aug 8, 2013 at 3:47 PM, Jan Lehnardt <jan@apache.org> wrote:
>=20
>> Heya,
>>=20
>> I=92m toying with the idea of moving some of my experimental into
>> bona-fide plugins. One of them is my log_to_db branch that on top of
>> writing log messages to a text file also writes a document to a log
>> database.
>>=20
>> Conceptually, this is the perfect use of a plugin: the feature is not
>> useful in the general case, because *any* activity creates write load
>> on a single database, but for certain low-volume installations, this
>> might be a useful feature (I wouldn=92t have written it, if I hadn=92t
>> needed it at some point) so allowing users to enable it as a plugin
>> would be nice.
>>=20
>> But regardless of whether my plugin is useful, it illustrates an
>> interesting point:
>>=20
>> A log_to_db plugin would need to register for logging events or, if =
it
>> doesn=92t want to duplicate all the logging-level logic in couch_log, =
it
>> would need some way of injecting a function call into
>> `couch_log:log().`. We could of course try and find a way where a
>> plugin would be able to provide an API compatible version of a =
CouchDB
>> module and swap it out for it=92s custom one, but that=92s hardly a =
great
>> idea.
>>=20
>> Other software has the notion of =93hooks=94 (some may call it =
something
>> else) where at well defined points in the main code base, external
>> functions get called with certain parameters. To make things dynamic,
>> there might be a way for plugins to register to be called by those
>> hooks and the main code then asks the registry whether there are any
>> plugin functions to call.
>>=20
>> In the log_to_db example, we=92d have something like this:
>>=20
>> couch_log_to_db.erl:
>>=20
>>    init() ->
>>        couch_hooks:register(couch_log_hook, log_hook_fun/1),
>>        ok.
>>=20
>>    log_hook_fun(Log) ->
>>        % do the log_to_db magic
>>        ok.
>>=20
>>=20
>> couch_hooks.erl:
>>=20
>>    register(Hook, Fun) ->
>>        % store the Fun with the Hook somewhere
>>        ok.
>>=20
>>    call(Hook, Args) ->
>>         % retrieve Fun for Hook from somewhere
>>        Fun(Args).
>>=20
>> couch_log.erl:
>>=20
>> % in log()
>>=20
>>    ...
>>    couch_hooks:call(couch_log_hook, Args),
>>    ...
>>=20
>> The main code would define what the hook name and arguments are and =
the
>> plugin would have to conform. The plugin registry would just manage =
the
>> registration and calling of functions for a hook, but nothing more.
>>=20
>> * * *
>>=20
>> This is just my first stab at this not thinking about it too much and =
I
>> likely miss some subtleties in Erlang that make this not work (hot =
code
>> upgrades e.g.).
>>=20
>>=20
>> How do you think we should implement a hooks feature in CouchDB?
>>=20
>>=20
>> Thanks!
>> Jan
>> --
>>=20
>>=20
>>=20
>>=20
>>=20


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