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From "Adam Kocoloski (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (COUCHDB-755) use exported functions when registering for config changes / db updates to allow hot upgrading
Date Wed, 29 May 2013 15:58:20 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-755?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13669361#comment-13669361
] 

Adam Kocoloski commented on COUCHDB-755:
----------------------------------------

The following two OTP applications replace couch_config and couch_db_update and only allow
exported functions as callbacks:

https://github.com/cloudant/config
https://github.com/cloudant/couch_event
                
> use exported functions when registering for config changes / db updates to allow hot
upgrading
> ----------------------------------------------------------------------------------------------
>
>                 Key: COUCHDB-755
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-755
>             Project: CouchDB
>          Issue Type: Improvement
>          Components: Database Core
>    Affects Versions: 0.11
>            Reporter: Adam Kocoloski
>
> This is a common idiom in our current codebase:
> couch_config:register(fun("couchdb", "os_process_timeout", NewTimeout) ->
>     couch_os_process:set_timeout(Pid, list_to_integer(NewTimeout))
> end),
> It proves problematic for hot code upgrades.  The issue is that the anonymous fun is
held by the couch_config process but belongs to the module in which it is defined (here, couch_external_server.erl).
 If an expert user loads a new version of couch_external_server, the anonymous fun will belong
to the 'old' version of the module.  If she upgrades the module again, couch_config will be
killed.  That's no good.
> The solution is to export the function that handles the configuration change or db update
event.  For instance
> -export(config_change/3).
> ...
> couch_config:register(fun ?MODULE:config_change/3),
> ...
> config_change("couchdb", "os_process_timeout", NewTimeout) ->
>     couch_os_process:set_timeout(Pid, list_to_integer(NewTimeout)).
> I have a patch for this that we've been using for some time in Cloudant, just need to
clean it up and apply it to trunk.

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