Accord

Accord is Cassandra’s protocol for linearizable, multi-partition transactions. It exists because Cassandra’s classic Dynamo-style model is designed for highly available, last-write-wins replication, not ACID transactions spanning multiple partitions or read-modify-write workflows. Before Accord, Cassandra’s Paxos-backed lightweight transactions were limited to a much narrower set of conditional operations.

Accord does not replace the rest of Cassandra’s consistency model. Most reads and writes still use the familiar Dynamo-inspired path described in Dynamo. Accord adds a separate consensus layer for the subset of operations that need transactional guarantees, and Cassandra integrates that layer with CQL, storage, messaging, and recovery.

In Cassandra 5 and 6, Accord is the foundation for explicit transaction blocks and for the longer-term migration away from Paxos-only transactional behavior. If you only need the "why" and "where does this fit" view, stop here. If you are changing protocol logic, continue to the deeper pages: