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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-9318) Bound the number of in-flight requests at the coordinator
Date Wed, 14 Sep 2016 09:25:20 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-9318?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15489927#comment-15489927
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Sylvain Lebresne commented on CASSANDRA-9318:
---------------------------------------------

bq. If back pressure kicks in, then it may be that mutations are applied by all replicas but
the application receives a timeout nonetheless.

That's *not* specific to back-pressure and can perfectly happen today and that's fine. Again,
if your application *require* that the server answers within say 500ms, the server should
do so and not randomly fail that deadline because back-pressure kicks-in. If the deadline
you set as a user is too low and you get timeouts too often, then it's your problem and you
should either reconsider your deadline because it's unrealistic, or increase your cluster
capacity. But I genuinely don't understand how not doing what the user explicitly asks us
to do would ever make sense.

bq. 1) We keep the timeout strict and change the back-pressure implementation to wait at most
the given timeout, and fail otherwise.

It's possible I'm totally missing a point you and Stefania are trying to make, but it seems
to me to be the only reasonable way. The timeout is a deadline the user asks us to respect,
it's the whole point of it, so it should always be respected as strictly as possible. If the
user sets it too low, his mistake (and don't get me wrong, we should certainly educate user
to avoid that mistake through documentation as much as possible).

More generally though, that discussion seems to suggest that back-pressure would make it harder
for C* to respect a reasonable timeout. I'll admit that sounds counter-intuitive to me as
a functioning back-pressure should make it _easier_ by smoothing things over when there is
too much pressure.

> Bound the number of in-flight requests at the coordinator
> ---------------------------------------------------------
>
>                 Key: CASSANDRA-9318
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9318
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Local Write-Read Paths, Streaming and Messaging
>            Reporter: Ariel Weisberg
>            Assignee: Sergio Bossa
>         Attachments: 9318-3.0-nits-trailing-spaces.patch, backpressure.png, limit.btm,
no_backpressure.png
>
>
> It's possible to somewhat bound the amount of load accepted into the cluster by bounding
the number of in-flight requests and request bytes.
> An implementation might do something like track the number of outstanding bytes and requests
and if it reaches a high watermark disable read on client connections until it goes back below
some low watermark.
> Need to make sure that disabling read on the client connection won't introduce other
issues.



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