Basically it goes something like this (oversimplifying): hset('shard.healthcheck', checkId, token) wait 500ms on every slave hget('shard.healthcheck', checkId)

Basically it goes something like this (oversimplifying): hset('shard.healthcheck', checkId, token) wait 500ms on every slave hget('shard.healthcheck', checkId) Verify the tokens match. Slaves are removed from the pool when the tokens don't, and allowed back in when the tokens match. Writes to master is disabled if a x/2 slaves are not available (where x is the number of slaves).