What are lightweight transactions (LWT) in Cassandra?

Lightweight transactions give Cassandra a way to do compare-and-set style operations — "only apply this write if a condition holds" — which normal writes cannot express, since ordinary writes are unconditional and last-write-wins.

INSERT INTO users (user_id, email)
VALUES ('u1', 'a@b.com')
IF NOT EXISTS;

UPDATE accounts SET balance = 150
WHERE account_id = 'acc1'
IF balance = 100;

• IF NOT EXISTS guarantees a row is only created if it doesn't already exist — useful for uniqueness constraints like usernames or reservations.
• IF <condition> applies an update only when the current stored value matches, enabling optimistic-concurrency patterns.

Under the hood, LWTs use the Paxos consensus protocol among replicas to agree on whether the condition holds and the write should proceed, rather than the normal quorum-write path. This makes them linearizable but noticeably slower and more resource-intensive than regular writes, so they should be reserved for the specific operations that truly need conditional semantics, not used as a default write pattern.

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