ReactiveX/RxJava

RxJava – Reactive Extensions for the JVM – a library for composing asynchronous and event-based programs using observable sequences for the Java VM.

Denis Stepanov

@dstepanov

Is there a way to keep the failing subscriber?

Artem Zinnatullin :slowpoke:

@artem-zinnatullin

2 Ways:

1) Don't throw exception as error signal, because it terminates Observable
2) Materialize operator

Denis Stepanov

@dstepanov

I'm using PublishSubject and I would like to catch an exception at onNext if any of the subscribers throws an exception

Artem Zinnatullin :slowpoke:

@artem-zinnatullin

@dstepanov take a look at this thread https://github.com/ReactiveX/RxJava/issues/2858#issuecomment-126121441

Matt Langston

@mattblang

has anyone used TestSubscriber for unit testing? I’m trying to assertError, but I am getting an AssertionError that Exceptions differ. However, it says java.net.SocketTimeoutException and the actual should be java.net.SocketTimeoutException

Matt Langston

@mattblang

Wups, it is because assertError needs the .class, because Throwable doesn’t implement .equals, thus doing a reference comparison

Dirk T.

@DirkToewe

Is there a Software/Tool/Plugin that allows to connect Observables similar to Matlab Simulink or Maybe in the Form of the Marble Diagrams?

I forgot the most important key adverb: VISUALLY :smile:

David Stemmer

@weefbellington

@DirkToewe maybe you could try http://rxmarbles.com/?

I'm not sure if that's exactly what you want

Bartłomiej Hołota

@bholota

I'm trying to understand defer(), should I use it in case when I'm creating Observable from list of Strings, map those into File and just after that subscribe to it? Should this observable be deffered or not?

Dorus

@Dorus

Defer is for lazy evaluation. Use it if you need lazy evaluation.

Bartłomiej Hołota

@bholota

Ok, so in cases when I want wait with emission until someone subscribe to it, right?

Dorus

@Dorus

Yes, as normal for cold observables. There are multiple ways to accomplish that, and defer is one. I believe Observable.create() and many others can do the same.

Bartłomiej Hołota

@bholota

ok, one more thing: when I will Observable.from(...).map(...).subscribe(...) and in such chaining this subscribe will catch all emitted items? It don't has to be defered? (it's hot observable right?)

Artem Zinnatullin :slowpoke:

@artem-zinnatullin

It's not hot observable

subscriber will catch all items

It needs to be deferred only if in from(something) something should be computed only after subscription to the observable

ah, sorry for indexes, markdown…

About defer: imagine you have Observable.just(networkRequest())

Artem Zinnatullin :slowpoke:

@artem-zinnatullin

the problem is that in Java all arguments computed before execution of the function, so you can not schedule networkRequest() on the other thread, it will be computed in thread that called Observable.just(networkRequest()).

To avoid that you need to have a function that wraps your call and defer it:

Observable
  .defer(() -> Observable.just(networkRequest())
  .map()
  .subscribe()

this will be cold observable and networkRequest() will be executed only after subscription.

Bartłomiej Hołota

@bholota

Ok, thanks, I need to read more about it.

Abhinav Solan

@eyeced

I am doing an insert into DB wihich are single value inserts mostly coming from numerous clients, these are around 50k values coming in from different sources every 3 seconds, I am not able to do a batch insert into the db. Is their a way with obsevables that before calling on insert I can combine all of these into a single observable stream .. and all other observables are pushing into this defined observable ?

David Stemmer

@weefbellington

@eyeced how often do you want do do the batch insert?

is there something that triggers an end-of-stream -- like a sentinal value that says "this is the last thing to be inserted into the database"?

or some sort of event that would allow you to figure out when the batch insert should occur?

Abhinav Solan

@eyeced

These are continuos values coming no end of stream

David Stemmer

@weefbellington

@eyeced alright, so what I might try is sample -- should allow you to sample a continuous stream of values at a specified interval, returning the values since the last sample as a Collection

Abhinav Solan

@eyeced

Should I create one observable like batcher in the class and then call on merge for all incoming observables and use the window / batch option with time for the batcher observable and keep on doing batch insert using that .. would this be a good solution ?

David Stemmer

@weefbellington

alternatively you could try buffer which allows you split the observable stream into fixed-sized chunks, for example if you wanted to do a batch update each time 1000 events were queued up

here is buffer: http://reactivex.io/documentation/operators/buffer.html

looks like buffer supports sampling as well so that might be what you want

@eyeced ^

Abhinav Solan

@eyeced

So is should I just create common observable and merge in all the incoming observables in that .. and use buffer with that ...

David Stemmer

@weefbellington

yep

Abhinav Solan

@eyeced

thanks a lot @weefbellington

David Stemmer

@weefbellington

sure

Abhinav Solan

@eyeced

I did a setup for cassandra cluster which was giving around 60k inserts performance .. when I used single insertes it came down 20k inserts .. I will try out with this suggestion .. this might put it back to original performance

60k / sec

David Stemmer

@weefbellington

@eyeced you could use a PublishSubject as your source observable if you are OK with your observable being "hot"

each time you need to push an event to the stream, you would call PublishSubject#onNext

Abhinav Solan

@eyeced

Ok i will try out with that too ..

David Stemmer

@weefbellington

personally I don't recommend Subjects ("hot" observables) in the general case (they can be problematic because you can start pushing data to them before anything is subscribed to them) but for some scenarios they can be useful

Dorus

@Dorus

I could swear Sample only returns the latest value (if any) every time interval, while Buffer and Window allow you to group all events in a certain timespan together (Probably buffer in this case, as that returns the collected values in a collection, while window keeps them in a nested observable so you would still need to aggregate them). But i'm coming from Rx.NET, so it might be different here with RxJava.

David Stemmer

@weefbellington

@Dorus yeah looks like you're right, buffer is what he wants here

I usually use sample to manage backpressure when I don't care about receiving every update

Dorus

@Dorus

yeah exactly

Dorus

@Dorus

Also, combining streams is simply Merge, as long as the clients dont emit errors. I see there is even a mergeDelayError in case you want to wait for all other clients to complete first. If bulk insert from a single client is easier than combining events from different clients, do the buffer operation before you merge.

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