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From "Adam Kocoloski (JIRA)" <j...@apache.org>
Subject [jira] Created: (COUCHDB-755) use exported functions when registering for config changes / db updates to allow hot upgrading
Date Tue, 04 May 2010 02:34:55 GMT
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
             Fix For: 1.1


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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