CODEWITHSUNDEEP
concurrencycassandra
user1128016
user1128016

Reputation: 1538

Concurrent writes in Cassandra: Are conflicts possible?

Does Cassandra guarantee consistency of replicas in case of concurrent writes? For example, if N=3, W=3 and there are 3 concurrent writers, is it possible to end up with 3 different values on each replica?

Is it a Cassandra-specific problem or does the canonical Dynamo design also has this problem, despite its use of vector clocks?

Upvotes: 5

Views: 3142

Answers (2)

le-doude
le-doude

Reputation: 3367

Just to add on tom.wilkie's answer If you want to guarantee a good consistency to your data with the latest value being kept try to read AND write always at LOCAL_QUORUM or QUORUM consistency.

Upvotes: 0

tom.wilkie
tom.wilkie

Reputation: 2866

Cassandra uses client-provided timestamps in this case, to ensure each replica keeps the 'latest' value. In your example, where you write to each replica, even when replicas receive the writes in different order, they will use the timestamp provided with the writes to decide which one to keep. Writing the same key with an older timestamp to a replica will just be ignored.

This mechanism isn't just needed to cope with concurrent writes - Cassandra can receive writes out of order over long periods of time (ie replying hints to a recently down node). To cope with this, when Cassandra compacts of SSTables and encounters two keys that are the same, it will use the timestamps to decide which one is kept.

Similarly, Cassandra has a feature called read repair. On read, Cassandra will compare the timestamp given by each replica and return the value associated with the latest timestamp to the client. It will then write this value back to any replicas which were out of date (this can have a performance impact, so the chance of it doing the subsequent write is tuneable).

Upvotes: 3

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