Advanced Patterns & Architecture

One primary takes all writes as the source of truth and replicates to replicas that serve reads; a dead replica just loses read capacity, but a dead primary halts writes until a replica is promoted.

A facade is one simple front door that coordinates a complicated subsystem behind it, so clients call one easy method instead of wiring together many classes themselves.

Never optimize a metric in isolation -- approve the latency win only if it buys a real outcome, improves tail latency, and the team can reliably operate the complexity it costs.

Raft makes a cluster agree on one ordered log via an elected leader, majority-quorum commits, and safety rules that preserve committed entries across leader changes -- and it is the backbone of etcd, Consul, and Kafka KRaft.

The scheduler decides when and atomically moves a job to QUEUED; workers decide how via a queue; a visibility timeout recovers crashed workers; backoff + DLQ bounds retries; a state check honors cancellation; and sharding spreads hot buckets.