Stateless Brokers

In Tansu, brokers are stateless and equal:

• Brokers do not replicate storage, so there is no concept of an in-sync replica
• Every broker is the leader of each topic partition
• Every broker is the coordinator of each consumer group
• Every broker is the coordinator of each transaction

In Tansu brokers act without consensus (e.g., Raft, ZooKeeper). When a request is made on a consumer group, the receiving broker optimistically executes the operation using its current state. At the end of the operational the new state is persisted to the storage engine. If the state was out of date, the storage engine rejects the request, responding with its current state. The broker then retries the operation with the newer state. The storage engine continues to reject requests that weren't executed on the current state.