[ejabberd] Looking for advices on migrating to the odbc version of some modules...

Felix GV felix at mate1inc.com
Tue Nov 6 23:37:56 MSK 2012


Hello,

We deployed an ejabberd cluster a couple weeks ago, and the size of our
roster and last_activity tables seem to have become problematic.

We sometimes want to do rolling restarts on the cluster, and the nodes now
take 5 minutes+ each to start. We think this is probably because of the
size of Mnesia.

I have tested a set up where an ejabberd cluster uses mod_last_odbc and
mod_roster_odbc with the native MySQL driver, and that seems to work fine
for new data, but I need to find a way to migrate the data that currently
exists in Mnesia to MySQL.

The documentation says we can't run both versions of a module.

We were thinking of doing something along those lines:

   - Disconnect all users (while keeping the nodes up)
   - Using a custom erlang module to read the data from the last_activity
   and roster Mnesia tables and write it to the corresponding MySQL tables.
   - Changing the nodes' config so that it uses the odbc versions of the
   modules.
   - Restarting the nodes (or doing the module unloading and loading
   live...)
   - Once we're certain everything's fine: delete the old Mnesia data.

We don't mind having a bit of downtime if that makes things simpler and
more reliable, although we certainly are interested in doing this without
downtime if there is a proven solution to do so.

I haven't found an already-implemented way of doing such data migration,
but if it exists, I'd like to know about it! Otherwise, we'll roll out our
own implementation of the data migration module, in which case, any advice
or best practices recommendation would be greatly appreciated!

Thanks :)

--
Felix
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