The guarantee
Total order broadcast is a messaging primitive where every node delivers the same set of messages in the same order. It is also called atomic broadcast. Two properties define it.
- Reliable delivery if one node delivers a message, all working nodes eventually deliver it.
- Totally ordered delivery all nodes deliver messages in an identical order.
Why it is powerful
If every replica processes the same commands in the same order starting from the same state, they stay identical. This is the heart of state machine replication, the technique behind replicated databases and logs.
How it relates to consensus
Total order broadcast is equivalent to consensus. Deciding the next message in the global order is exactly agreeing on one value, repeated forever. So implementing it requires a consensus protocol like Raft or Paxos under the hood.
Building it
A common approach routes all messages through a leader sequencer that assigns increasing numbers, with the consensus layer ensuring the sequence survives failures and never forks.
Key idea
Total order broadcast delivers identical messages in identical order to all nodes, making replicas deterministic and proving equivalent to consensus.