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https://issues.apache.org/jira/browse/CASSANDRA-15642?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17077707#comment-17077707
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Benedict Elliott Smith commented on CASSANDRA-15642:
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bq. fails-fast in case of at least 1 failure and if the failure happens first,
the rest is ignored.
bq. and AbstractWriteHandler
I think I understand the source of our confusion. I've been talking about
writes for which this is already true, and interpreted you as wanting to wait
for "complete" (i.e. ALL) responses. However I think your claims only hold
true for reads, and only some reads; not all reads and writes:
https://github.com/apache/cassandra/blob/0e0d288ab7e87e7d4a7542c955dd06701798bd06/src/java/org/apache/cassandra/service/AbstractWriteResponseHandler.java#L263
(this actually has another bug, by the looks of it, in that it uses
{{liveAndDown}} instead of {{live}}; but that's irrelevant here)
We always wait until there are sufficient failures to guarantee we can never
succeed, which under normal circumstances (i.e. for anything but CL.ALL) on
write should already mean waiting for {{>= blockFor}} responses?
So, if you want to file a ticket for it I think I can make us both happy: we
should always fail a query as soon as we know it cannot succeed, but for read
queries a failure should result in a "speculative" read (as might be triggered
by elapsed time). Instead today it will result in immediate failure if
speculation has not yet happened.
> 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
> Priority: Normal
>
> 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 may not be accurate.
> (note: I am not entirely sure of the behavior in case of timeouts for now)
> For example, say a request at CL = QUORUM = 3, a 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 number of
> results in the message, so you end up thinking "what happened with the rest
> of the required CL?"
> 2. the information is incorrect. We did receive 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. Only information
> users should extract out of it is that at least 1 node has failed.
> For a big improvement in usability, the {{ReadCallback}} and
> {{AbstractWriteResponseHandler}} could instead 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 information returned to them.
> Additionally, an error that happens first prevents 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 diagnostic where you can't detect both
> errors at the same time.
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