Distributed systems fail in pieces.

One service times out while the rest of the request is still running. One replica falls behind while another keeps serving reads. One region loses a dependency while another stays healthy. A deployment breaks only the new version. A retry succeeds twice.

Good distributed systems are not systems that never fail. They are systems that limit the damage, protect the rules that must never be broken, recover in a predictable way, and show operators what is happening.

The hard part is not memorizing patterns. The hard part is asking the right questions:

This chapter covers common failure modes and the patterns engineers use to keep them from turning into full incidents.