TopolearnConsistency level decides whether reads see the latest writes and whether requests succeed when nodes are down.
Each read and write specifies a consistency level (CL): how many replicas must acknowledge before the coordinator answers.
ONE,TWO,THREE: that many replicas,QUORUM: a majority of all replicas across data centers (2 of 3 with RF 3),LOCAL_QUORUM: a majority in the coordinator's data center, the usual choice for multi-DC clusters,EACH_QUORUM(writes): a quorum in every data center;ALL: every replica,LOCAL_ONE: one replica in the local data center;ANY(writes only) accepts even a stored hint.
Writes are always sent to all replicas; the CL only decides how many acknowledgements to wait for. Reads contact enough replicas to satisfy the CL and return the newest value by timestamp.
The rule of thumb: if R + W > RF, every read overlaps at least one replica that has the latest write, so you read your writes (strong consistency, ignoring clock skew). With RF 3, QUORUM + QUORUM (2 + 2 > 3) is strongly consistent and tolerates one replica down. ONE + ONE is fastest and most available but may return stale data.
Replicas that missed writes catch up through hinted handoff (the coordinator stores hints for a down node, for 3 hours by default), read repair and anti-entropy repair.
A failed write at a given CL isn't necessarily rolled back: some replicas may have applied it. Make writes idempotent so retrying is safe.
Example
-- cqlsh: the session consistency level applies to following statements
CONSISTENCY LOCAL_QUORUM;
INSERT INTO shop.users_by_email (email, user_id, name)
VALUES ('ana@example.com', 5b6962dd-3f90-4c93-8f61-eabfa4a803e2, 'Ana');
CONSISTENCY ONE; -- fast, possibly stale read
SELECT name FROM shop.users_by_email WHERE email = 'ana@example.com';
-- With RF = 3:
-- W=QUORUM(2) + R=QUORUM(2) = 4 > 3 -> reads see the latest acknowledged write
-- W=ONE(1) + R=ONE(1) = 2 -> may read stale data
-- W=ALL(3) + R=ONE(1) = 4 -> consistent, but any down node fails writesRemember
- CL = how many replicas must acknowledge, chosen per request
- R + W > RF gives read-your-writes consistency
- LOCAL_QUORUM is the usual multi-DC default
- Hints, read repair and repair bring replicas back in sync
Watch out for
- Using ALL for safety: one unavailable replica makes requests fail.