CouchDB
  1. CouchDB
  2. COUCHDB-755

use exported functions when registering for config changes / db updates to allow hot upgrading

    Details

    • Type: Improvement Improvement
    • Status: Closed
    • Priority: Major Major
    • Resolution: Fixed
    • Affects Version/s: 0.11
    • Fix Version/s: 2.0.0
    • Component/s: Database Core
    • Labels:
      None
    • Skill Level:
      Committers Level (Medium to Hard)

      Description

      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.

        Activity

        Hide
        Adam Kocoloski added a comment -

        These new applications have been pulled in as part of 2.0

        Show
        Adam Kocoloski added a comment - These new applications have been pulled in as part of 2.0
        Hide
        Adam Kocoloski added a comment -

        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

        Show
        Adam Kocoloski added a comment - 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

          People

          • Assignee:
            Adam Kocoloski
            Reporter:
            Adam Kocoloski
          • Votes:
            0 Vote for this issue
            Watchers:
            1 Start watching this issue

            Dates

            • Created:
              Updated:
              Resolved:

              Development