Consensus or Single-Leader

1. Two Problems That Get Confused

Almost every distributed system has two separate questions to answer, and most over-engineering in this area comes from answering them with one mechanism.

• Where is the data kept, and how does it stay durable? This is replication.
• Who is allowed to accept writes right now? This is agreement.

Single-leader replication answers the first. One node is the primary, it takes every write, and replicas copy from it. That gives you durability, read scaling and a straightforward consistency story, and it is what almost every relational database does.