[
https://issues.apache.org/jira/browse/CASSANDRA-15642?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17074827#comment-17074827
]
Kevin Gallardo commented on CASSANDRA-15642:
--------------------------------------------
[~benedict] thanks for having a look at the ticket!
bq. There is no real significant increase in information by waiting, and it
delays how quickly a client may action this information
I would argue that right now that a user wouldn't even be aware that the
information is not reliable, as it's only by digging in server code that I
realized that this is happening. The error message tell you "received X errors
and X responses" and there is no indication externally to the user/client that
the information returned in not reliable.
Additionally, given these findings I don't know how the client would be able to
properly action on the information, quickly or not, if the info is not
reliable? I am not sure that giving partial info quickly is better than
cohesive info
bq. It is possible to simply report consistent information
I am not sure how that would be possible without waiting all the responses come
back or timeout, but happy to be explained if I am missing something
> 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.
--
This message was sent by Atlassian Jira
(v8.3.4#803005)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
