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https://issues.apache.org/jira/browse/CASSANDRA-15642?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Kevin Gallardo updated CASSANDRA-15642:
---------------------------------------
Description:
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.
was:
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)
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.
> 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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