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  1. Cassandra
  2. CASSANDRA-6178

Consider allowing timestamp at the protocol level ... and deprecating server side timestamps

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    • Type: Improvement
    • Status: Resolved
    • Priority: Normal
    • Resolution: Not A Problem
    • Fix Version/s: None
    • Component/s: None
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      Description

      Generating timestamps server side by default for CQL has been done for convenience, so that end-user don't have to provide one with every query. However, doing it server side has the downside that updates made sequentially by one single client (thread) are no guaranteed to have sequentially increasing timestamps. Unless a client thread is always pinned to one specific server connection that is, but no good client driver out (that is, including thrit driver) there does that because that's contradictory to abstracting fault tolerance to the driver user (and goes again most sane load balancing strategy).

      Very concretely, this means that if you write a very trivial test program that sequentially insert a value and then erase it (or overwrite it), then, if you let CQL pick timestamp server side, the deletion might not erase the just inserted value (because the delete might reach a different coordinator than the insert and thus get a lower timestamp). From the user point of view, this is a very confusing behavior, and understandably so: if timestamps are optional, you'd hope that they are least respect the sequentiality of operation from a single client thread.

      Of course we do support client-side assigned timestamps so it's not like the test above is not fixable. And you could argue that's it's not a bug per-se. Still, it's a very confusing "default" behavior for something very simple, which suggest it's not the best default.

      You could also argue that inserting a value and deleting/overwriting right away (in the same thread) is not something real program often do. And indeed, it's likely that in practice server-side timestamps work fine for most real application. Still, it's too easy to get counter-intuitive behavior with server-side timestamps and I think we should consider moving away from them.

      So what I'd suggest is that we push back the job of providing timestamp client side. But to make it easy for the driver to generate it (rather than the end user), we should allow providing said timestamp at the protocol level.

      As a side note, letting the client provide the timestamp would also have the advantage of making it easy for the driver to retry failed operations with their initial timestamp, so that retries are truly idempotent.

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              • Assignee:
                Unassigned
                Reporter:
                slebresne Sylvain Lebresne
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                  Updated:
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