CODEWITHSUNDEEP
cassandraeventual-consistency
maxim_ge
maxim_ge

Reputation: 1263

When read-your-own-writes can fail?

I use RL/WL=QUORUM and send two updates, is it possible that next SELECT reads my first update, in some circumstances? 

CREATE TABLE aggr(
    id  int,
    mysum int,
    PRIMARY KEY(id)
)

INSERT INTO aggr(id, mysum) VALUES(1, 2)
INSERT INTO aggr(id, mysum) VALUES(1, 3)
SELECT mysum FROM aggr WHERE id=1 -- expect mysum=3 here, but is it a must?

As I can judge from here it is possible even to lost part of the second update if two updates come within same timestamp.

If I work around timestamp problem, can I be sure that I always read what I wrote last time?

Upvotes: 0

Views: 59

Answers (2)

Chris Lohfink
Chris Lohfink

Reputation: 16420

No, assuming your using client side monotonic timestamps (current default, wasn't in past). But it is possible with other settings. I am assuming here that its a single client issuing those two writes. If the 2 inserts are coming from two different servers it all depends on their timestamps.

This is the default for java driver 3.x but if using a version of cassandra pre CQL3 (2.0) you need to provide them with USING TIMESTAMP in your query since the protocol didn't support it. Otherwise the two writes can go to different coordinators, and if the coordinators have clock drift between them the 1st insert may be considered "newer" than the 2nd. With client side timestamps though (should be the default on your driver if using new versions) thats not the case.

Upvotes: 2

Simon Fontana Oscarsson
Simon Fontana Oscarsson

Reputation: 2134

If you do your updates synchronously with CL=QUORUM the second update will always overwrite the first one. A lower consistency level on any of the requests would not guarantee this.

Upvotes: 0

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