soundcloud/large-hadron-migrator — Issue #12: LHM Juggle
Author: duncanbeevers
Created: 2012-03-04 15:38:22+00:00
State: open

Currently LHM has two phases to a migration. During the first phase the site stays live while writes are applied to old+new tables. During the second phase the site goes down while the journal is played back.

The amount of time the site is down during the second phase depends directly on the amount of write traffic received during the first phase.

It seems conceivable that if the first phase took long enough, LHM could automate juggling the table roles, and could perform the play-back while the site was still up. Implemented correctly, this could almost completely eliminate downtime since the process would be repeatable and the playback windows would get shorter and shorter.

Is this an issue you've experienced with your dataset? I've only experienced this once, and the downtime introduced by the playback time was only like 10 minutes after a day-long migration, but it still felt unnecessary.