Explain the write path in Cassandra?

A write in Cassandra is optimized to be fast and durable without doing any disk seeks or read-before-write checks.

flowchart LR
A[Client sends write] --> B[Coordinator node]
B --> C[Commit Log - append only, durability]
B --> D[Memtable - in-memory, per table]
D -->|threshold reached| E[Flush to SSTable on disk]
C -.-> F[Replayed on crash recovery]

1. The client sends the write to any node, which becomes the coordinator for that request.
2. The coordinator forwards the write to all replicas that own the partition, based on the token ring.
3. Each replica appends the mutation to its local commit log first, purely for crash recovery.
4. The same mutation is applied to an in-memory memtable, sorted by clustering key.
5. The coordinator waits only for the number of acknowledgements required by the requested consistency level before replying to the client.
6. Later, once the memtable fills up, it is flushed to an immutable SSTable on disk, and the corresponding commit log segment can be discarded.

Because writes are just an append plus a sorted in-memory insert, Cassandra can sustain very high write throughput compared to databases that read existing rows first or update in place.

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