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by aphyr
4663 days ago
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I concur; this is punting on the resolution problem. As far as I can determine in testing with Jepsen, there are no cases where one can safely (e.g. in a way which guarantees some causal connection of your write to a future state of the system) update a cell in Cassandra without a strong timestamp coordinator: either an external system like Zookeeper, or Cassandra 2.0 paxos transactions. Most of the production users and datastax employees I've spoken with recommend structuring your data as a write-once immutable log of operations, and to merge conflicts on read. This is exactly the same problem that Riak siblings have--except that you don't get the benefit of automatic pruning of old versions, because there's no causality tracking. GC is up to the user. |
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It depends on what you mean by "guarantees". In most real world systems, if you can actually have two concurrent writes to the same record, it is non-deterministic which write wins... which is precisely what happens in Cassandra if you have multiple concurrent writes to a cell without a strong timestamp coordinator.
There is a somewhat more complex case about scenarios where you have multiple cells tied to the same record being updated together, but even then it will look identical to if you allowed both writes to happen, but it was non-deterministic which one happened first.
While you can create an timestamp authority that arbitrates precisely what happens when, the reality is that if there wasn't an already observable mechanism for determining this, who is to say which one happened first?
Really, the only problem you run into is if there is atomic validation logic you need tied in to whether the updates happen at all, which is what Cassandra 2.0's lightweight transactions really address. Without them, you generally do need some kind of external authority or a "log concurrently and then resolve serially later" strategy. That sounds like a pain, but the latter in particular is again closer to how most of the real world operates (in particular, the world of financial transactions).