Durability Summary

This document has described how durability guarantees affect a shard’s write availability in the event of hardware or network failures. In summary:

• A durability guarantee that requires no acknowledgements from the shard’s replicas gives you the best chance that the shard can continue servicing write requests in the event of an outage. However, this durability guarantee can also result in the shard operating with two masters, which leads to data loss once hardware problems are resolved. This is not a recommended configuration.

• A durability guarantee requiring a simple majority of primary zone replicas to acknowledge the write operation guards against two masters accidently operating at one time. However, it also means that the shard will be incapable of servicing write requests if more than a majority of the replicas are offline due to a hardware failure.

• A durability guarantee requiring all primary zone replicas to acknowledge the write operation guards against any possibility of data loss. However, it also means that the shard will be unable to service write requests if even one of the replicas is unavailable for any reason.