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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-10528) Proposal: Integrate RxJava
Date Wed, 13 Jan 2016 13:14:40 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-10528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15096160#comment-15096160
] 

Benedict commented on CASSANDRA-10528:
--------------------------------------

Conceptually this approach appears to be identical to completable futures, only with the imposition
that everything is a stream.  Since not everything is a stream, it looks unnecessarily complex
to enforce this model on everything we do, and the anonymous functions separated by a comma
don’t do wonders to mitigate this.

The most problematic part of RxJava, to me, is the way flow control is implemented.  It looks
ripe to get ugly very quickly, with many context jumps, so debugging / understanding what
is happening is likely to be non-trivial as the full functionality is transitioned.  The act
of sub/pub is already an extra step to mentally model from the status quo, but we not only
sub/pub, but sub/pub/ask, i.e. each item we want to get, we have to call a method to say we
are now ready for, and then our method we previously registered will get called with the result.
 The number of methods all interacting to orchestrate this is ballooning, and it has to be
_nested_.

This mechanism of control flow also requires that we have meaningful numerical units by which
to split work, and it’s not clear that we do.  The Row is the best we can probably use,
but incurring all of these tiny (possibly virtual) method invocations for every row limits
the scope for performance improvements, and it doesn’t prevent us being hurt by giant rows
- at some future date we need to be resilient to that problem too, and buying into a model
that may struggle to manage seems short-sighted to me.

This hasn’t even accounted for some of the most difficult to model behaviours in an async
world, like Rapid Read Protection.  Nor how it would integrate with the Netty event loop (noting
we need to remain responsive to network events).  A POC really needs to demonstrate it can
handle the hardest parts of a transition.

All told I’m personally not sold on the payoff from buying into this framework.

(side note: the modest performance gain seen above is the same as simply jacking concurrent_reads
>= native_transport_max_threads)

> Proposal: Integrate RxJava
> --------------------------
>
>                 Key: CASSANDRA-10528
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-10528
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: T Jake Luciani
>            Assignee: T Jake Luciani
>             Fix For: 3.x
>
>         Attachments: rxjava-stress.png
>
>
> The purpose of this ticket is to discuss the merits of integrating the [RxJava|https://github.com/ReactiveX/RxJava]
framework into C*.  Enabling us to incrementally make the internals of C* async and move away
from SEDA to a more modern thread per core architecture. 
> Related tickets:
>    * CASSANDRA-8520
>    * CASSANDRA-8457
>    * CASSANDRA-5239
>    * CASSANDRA-7040
>    * CASSANDRA-5863
>    * CASSANDRA-6696
>    * CASSANDRA-7392
> My *primary* goals in raising this issue are to provide a way of:
>     *  *Incrementally* making the backend async
>     *  Avoiding code complexity/readability issues
>     *  Avoiding NIH where possible
>     *  Building on an extendable library
> My *non*-goals in raising this issue are:
>     
>    * Rewrite the entire database in one big bang
>    * Write our own async api/framework
>     
> -------------------------------------------------------------------------------------
> I've attempted to integrate RxJava a while back and found it not ready mainly due to
our lack of lambda support.  Now with Java 8 I've found it very enjoyable and have not hit
any performance issues. A gentle introduction to RxJava is [here|http://blog.danlew.net/2014/09/15/grokking-rxjava-part-1/]
as well as their [wiki|https://github.com/ReactiveX/RxJava/wiki/Additional-Reading].  The
primary concept of RX is the [Obervable|http://reactivex.io/documentation/observable.html]
which is essentially a stream of stuff you can subscribe to and act on, chain, etc. This is
quite similar to [Java 8 streams api|http://www.oracle.com/technetwork/articles/java/ma14-java-se-8-streams-2177646.html]
(or I should say streams api is similar to it).  The difference is java 8 streams can't be
used for asynchronous events while RxJava can.
> Another improvement since I last tried integrating RxJava is the completion of CASSANDRA-8099
which provides is a very iterable/incremental approach to our storage engine.  *Iterators
and Observables are well paired conceptually so morphing our current Storage engine to be
async is much simpler now.*
> In an effort to show how one can incrementally change our backend I've done a quick POC
with RxJava and replaced our non-paging read requests to become non-blocking.
> https://github.com/apache/cassandra/compare/trunk...tjake:rxjava-3.0
> As you can probably see the code is straight-forward and sometimes quite nice!
> *Old*
> {code}
> private static PartitionIterator fetchRows(List<SinglePartitionReadCommand<?>>
commands, ConsistencyLevel consistencyLevel)
>     throws UnavailableException, ReadFailureException, ReadTimeoutException
>     {
>         int cmdCount = commands.size();
>         SinglePartitionReadLifecycle[] reads = new SinglePartitionReadLifecycle[cmdCount];
>         for (int i = 0; i < cmdCount; i++)
>             reads[i] = new SinglePartitionReadLifecycle(commands.get(i), consistencyLevel);
>         for (int i = 0; i < cmdCount; i++)
>             reads[i].doInitialQueries();
>         for (int i = 0; i < cmdCount; i++)
>             reads[i].maybeTryAdditionalReplicas();
>         for (int i = 0; i < cmdCount; i++)
>             reads[i].awaitRes
> ultsAndRetryOnDigestMismatch();
>         for (int i = 0; i < cmdCount; i++)
>             if (!reads[i].isDone())
>                 reads[i].maybeAwaitFullDataRead();
>         List<PartitionIterator> results = new ArrayList<>(cmdCount);
>         for (int i = 0; i < cmdCount; i++)
>         {
>             assert reads[i].isDone();
>             results.add(reads[i].getResult());
>         }
>         return PartitionIterators.concat(results);
>     }
> {code}
>  *New*
> {code}
> private static Observable<PartitionIterator> fetchRows(List<SinglePartitionReadCommand<?>>
commands, ConsistencyLevel consistencyLevel)
>     throws UnavailableException, ReadFailureException, ReadTimeoutException
>     {
>         return Observable.from(commands)
>                          .map(command -> new SinglePartitionReadLifecycle(command,
consistencyLevel))
>                          .flatMap(read -> read.getPartitionIterator())
>                          .toList()
>                          .map(results -> PartitionIterators.concat(results));
>     }
> {code}
> Since the read call is now non blocking (no more future.get()) we can remove one thread
pool hop from the native netty request pool which yields a non-trivial improvement to read
performance.
> !rxjava-stress.png|width=800px!
> http://cstar.datastax.com/tests/id/ae648c12-729a-11e5-8625-0256e416528f
> At the same time the current Iterator based api still works by calling {{.toBlocking()}}
on the observable. So for example the existing thrift read call requires little modification
> On the async side we get the added benefits of RxJava:
>   * Customizable backpressure strategies (for dealing with streams that can't be processed
quickly enough)
>   * Cancelling of work due to timeouts is a 1 line change
>   * When a Subscriber disconnects from the stream they Observable stops as well
>   * Batching/windowing of work can be added in one line
>   * Observers and Subscribers can do work across any thread at any stage of the pipeline
>   * Observables can be [debugged|https://github.com/ReactiveX/RxJavaDebug] and [tested|http://reactivex.io/RxJava/javadoc/rx/observers/TestSubscriber.html]
> Another plus is the community surrounding RxJava specifically our good friends at netflix
have authored and used it extensively. Docs and examples are good.
> In order to get the most out of this we will need to take this api further into the code.
MessagingService, Disk Access/Page, Cache, Thread per core... but again I want to hammer home
this will be able to be achieved incrementally. 
> On the bad side this is:
>   *  Locking into a "framework"  
>   *  Will inevitably hit bugs / performance issues we need fixed upstream
>   * Some of the more advanced API uses look pretty mentally taxing/hard to grasp
> Which brings us to the Alternatives, primarily being to just use CompletableFutures.
> We certainly could but if you look at the code changes I had to make to make the SP calls
asynchronous I think you will realize you would need to pass
> all kinds of state around to get the read command callback to start the netty write.
 Vs observables which make that pipeline declarative. Also more advanced things like backpressure
and message passing between N:M producers and consumers becomes complex.  This isn't to say
we can't [use both|http://www.nurkiewicz.com/2014/11/converting-between-completablefuture.html]
if Observables are overkill.
> I hope this ticket sparks some good discussion!
>       



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