Pin the Writer

A collaborative document product runs active-active in three regions. Every user is assigned a home region, and all writes for that user's documents are routed there while reads are served locally from any region. What does this design primarily buy?

Automatic region failover, because any region can take over writes for a key without a promotion step or any change to the routing layer

Write locality: one writer per key means cross-region write conflicts cannot occur, at the cost of a long round trip for users outside their home region

Strong consistency for reads everywhere, because a single writer per key makes every regional replica reflect the latest committed write as soon as it is applied

Lower write latency for every user, because the nearest region accepts the write locally and forwards it on to the home region afterwards