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https://issues.apache.org/jira/browse/CASSANDRA-8789?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14316905#comment-14316905
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Ariel Weisberg commented on CASSANDRA-8789:
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Doing it based on size rather then flow means you are changing order guarantees
based on something that is relatively arbitrary. For instance if some code were
steaming data as several large messages you might have the last message be
smaller because it is the tail of the data, and that could end up delivered
before the prior messages.
I think that relaxing ordering guarantees is one of those things you want to
opt into.
> Revisit how OutboundTcpConnection pools two connections for different message
> types
> -----------------------------------------------------------------------------------
>
> Key: CASSANDRA-8789
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8789
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Ariel Weisberg
> Assignee: Ariel Weisberg
> Fix For: 3.0
>
>
> I was looking at this trying to understand what messages flow over which
> connection.
> For reads the request goes out over the command connection and the response
> comes back over the ack connection.
> For writes the request goes out over the command connection and the response
> comes back over the command connection.
> Reads get a dedicated socket for responses. Mutation commands and responses
> both travel over the same socket along with read requests.
> Sockets are used uni-directional so there are actually four sockets in play
> and four threads at each node (2 inbounded, 2 outbound).
> CASSANDRA-488 doesn't leave a record what the impact of this change was. If
> someone remembers what situations were made better it would be good to know.
> I am not clear on when/how this is helpful. The consumer side shouldn't be
> blocking so the only head of line blocking issue is the time it takes to
> transfer data over the wire.
> If message size is the cause of blocking issues then the current design mixes
> small messages and large messages on the same connection retaining the head
> of line blocking.
> Read requests share the same connection as write requests (which are large),
> and write acknowledgments (which are small) share the same connections as
> write requests. The only winner is read acknowledgements.
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