Kevin Gallardo created CASSANDRA-15642:
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Summary: Inconsistent failure messages on distributed queries
Key: CASSANDRA-15642
URL: https://issues.apache.org/jira/browse/CASSANDRA-15642
Project: Cassandra
Issue Type: Improvement
Components: Consistency/Coordination
Reporter: Kevin Gallardo
As a follow up to some exploration I have done for CASSANDRA-15543, I realized
the following behavior in both {{ReadCallback}} and {{AbstractWriteHandler}}:
- await for responses
- when all required number of responses have come back: unblock the wait
- when a single failure happens: unblock the wait
- when unblocked, look to see if the counter of failures is > 1 and if so
return an error message based on the {{failures}} map that's been filled
Error messages that can result from this behavior can be a ReadTimeout, a
ReadFailure, a WriteTimeout or a WriteFailure.
In case of a Write/ReadFailure, the user will get back an error looking like
the following:
"Failure: Received X responses, and Y failures"
(if this behavior I describe is incorrect, please correct me)
This causes a usability problem. Since the handler will fail and throw an
exception as soon as 1 failure happens, the error message that is returned to
the user is not accurate.
(note: I am not entirely sure of the behavior around timeouts for now)
At, say, CL = QUORUM = 3, the failed request may complete first, then a
successful one completes, and another fails. If the exception is thrown fast
enough, the error message could say
"Failure: Received 0 response, and 1 failure at CL = 3"
Which 1. doesn't make a lot of sense because the CL doesn't match the previous
information, but 2. the information is incorrect. We received a successful
response, only it came after the initial failure.
>From that logic, I think it is safe to assume that the information returned in
>the error message cannot be trusted in case of a failure. We can only know
>that at least 1 node has failed, or not if the response is successful.
I am suggesting that for a big improvement in usability, the ReadCallback and
AbstractWriteResponseHandler wait for all responses to come back before
unblocking the wait, or let it timeout. This is way, the users will be able to
have some trust around the numbers returned to them. Also we would be able to
return more information this way.
Right now, an error that happens first prevents from a timeout to happen
because it fails immediately, and so potentially it hides problems with other
replicas. If we were to wait for all responses, we might get a timeout, in that
case we'd also be able to tell wether failures have happened *before* that
timeout, and have a more complete view where you can't detect both situations.
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