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From "T Jake Luciani (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-10528) Proposal: Integrate RxJava
Date Wed, 14 Oct 2015 20:11:05 GMT
T Jake Luciani created CASSANDRA-10528:
------------------------------------------

             Summary: Proposal: Integrate RxJava
                 Key: CASSANDRA-10528
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-10528
             Project: Cassandra
          Issue Type: Improvement
            Reporter: T Jake Luciani
             Fix For: 3.x



The purpose of this ticket is to discuss the merits of integrating the [https://github.com/ReactiveX/RxJava|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 [http://blog.danlew.net/2014/09/15/grokking-rxjava-part-1/|here]
as well as their [https://github.com/ReactiveX/RxJava/wiki/Additional-Reading|wiki].  The
primary concept of the [http://reactivex.io/documentation/observable.html|Obervable] which
is essentially a stream of stuff you can subscribe to and act on, chain, etc. This is quite
similar to [http://www.oracle.com/technetwork/articles/java/ma14-java-se-8-streams-2177646.html|Java
8 streams api] (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 on 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.

{{Picture}}

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 [https://github.com/ReactiveX/RxJavaDebug|debugged] 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 [http://www.nurkiewicz.com/2014/11/converting-between-completablefuture.html|use
both] if Observables are overkill.


I hope this ticket sparks some good discussion!








      















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