Comparison
Active-passivevsFailover
Active-passive
the second site sits there costing money and serving nothing, and the day you need it you find out which four things were never replicated.
Keeping a secondary location ready but idle, promoted only on failure. It is simpler than active-active because there is one writer, and its weakness is exactly that it is never exercised, so its readiness is a belief rather than an observation. Regular, scheduled failovers are the only thing that turns it from a slide into a capability.
Full entry →Failover
the primary stopped answering and something promoted a replica, and now you find out how much of the last few seconds you lost.
Promoting a replica to leader when the current leader is deemed dead. Under asynchronous replication it loses any unreplicated writes; under synchronous it may refuse to proceed. The hard part is the deeming: too eager and you failover on a network blip, too slow and the outage is the timeout.
Full entry →