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https://issues.apache.org/jira/browse/CASSANDRA-10993?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15366347#comment-15366347
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Tyler Hobbs commented on CASSANDRA-10993:
-----------------------------------------
I hit a bit of a roadblock yesteraday with combining the event loops. The
combination of {{NioEventLoop}} being declared {{final}} along with
{{AbstractNioChannel.isCompatible()}} having an [{{instanceof
NioEventLoop}}|https://github.com/netty/netty/blob/4.1/transport/src/main/java/io/netty/channel/nio/AbstractNioChannel.java#L371-L374]
check makes this impossible without modifying netty.
[~norman] what's the reasoning behind making {{NioEventLoop}} final? We'd like
to explore merging our event loop task handling into a custom Netty event loop,
but it looks like this isn't going to be an option with the current codebase.
Any recommendations?
In the meantime, I'm going to focus on the read path.
> Make read and write requests paths fully non-blocking, eliminate related
> stages
> -------------------------------------------------------------------------------
>
> Key: CASSANDRA-10993
> URL: https://issues.apache.org/jira/browse/CASSANDRA-10993
> Project: Cassandra
> Issue Type: Sub-task
> Components: Coordination, Local Write-Read Paths
> Reporter: Aleksey Yeschenko
> Assignee: Tyler Hobbs
> Fix For: 3.x
>
>
> Building on work done by [~tjake] (CASSANDRA-10528), [~slebresne]
> (CASSANDRA-5239), and others, convert read and write request paths to be
> fully non-blocking, to enable the eventual transition from SEDA to TPC
> (CASSANDRA-10989)
> Eliminate {{MUTATION}}, {{COUNTER_MUTATION}}, {{VIEW_MUTATION}}, {{READ}},
> and {{READ_REPAIR}} stages, move read and write execution directly to Netty
> context.
> For lack of decent async I/O options on Linux, we’ll still have to retain an
> extra thread pool for serving read requests for data not residing in our page
> cache (CASSANDRA-5863), however.
> Implementation-wise, we only have two options available to us: explicit FSMs
> and chained futures. Fibers would be the third, and easiest option, but
> aren’t feasible in Java without resorting to direct bytecode manipulation
> (ourselves or using [quasar|https://github.com/puniverse/quasar]).
> I have seen 4 implementations bases on chained futures/promises now - three
> in Java and one in C++ - and I’m not convinced that it’s the optimal (or
> sane) choice for representing our complex logic - think 2i quorum read
> requests with timeouts at all levels, read repair (blocking and
> non-blocking), and speculative retries in the mix, {{SERIAL}} reads and
> writes.
> I’m currently leaning towards an implementation based on explicit FSMs, and
> intend to provide a prototype - soonish - for comparison with
> {{CompletableFuture}}-like variants.
> Either way the transition is a relatively boring straightforward refactoring.
> There are, however, some extension points on both write and read paths that
> we do not control:
> - authorisation implementations will have to be non-blocking. We have control
> over built-in ones, but for any custom implementation we will have to execute
> them in a separate thread pool
> - 2i hooks on the write path will need to be non-blocking
> - any trigger implementations will not be allowed to block
> - UDFs and UDAs
> We are further limited by API compatibility restrictions in the 3.x line,
> forbidding us to alter, or add any non-{{default}} interface methods to those
> extension points, so these pose a problem.
> Depending on logistics, expecting to get this done in time for 3.4 or 3.6
> feature release.
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