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Glen Marchesani
@fizzy33
so we want existing messages
then switch to a hot observable that is the tail
so it is in effect
def existingMessages: Observable[Message] = ...
def tailObserver: Observable[Message] = ...
existingMessages ++ tailObserver
where we need to get the onComplete on existingMessagess otherwise readers hang around forever
fwiw Monix has made this much much simpler
I plan to create a simple example of what you recommended to show what I ran into
just too many things needed before we release
Ilya
@squadgazzz

Seems like @squadgazzz use case is the same

my case is similar, but I know when to stop and this means I know what’s the last element.

  val obs1 = Observable
    .fromIterable(List(1, 2, 3, 4, 5, 6, 7, 8, 9))
    .doOnNext(e => Task(println(e)))
    .takeWhileInclusive(_ < 5)
    .flatMap {
      case 5 => Observable.pure(5) ++ Observable.fromIterable(List(10, 11, 12, 13, 14))
      case e => Observable.pure(e)
    }

In case I don’t know the last element we should properly use publishSelector somehow

Glen Marchesani
@fizzy33
@squadgazzz yeah I use that same cheat (knowing when to stop)
Ilya
@squadgazzz

@squadgazzz yeah I use that same cheat (knowing when to stop)

So why not to use flatMap?

Glen Marchesani
@fizzy33
I have two observables. One is the existingMessages (which is backed by a reader of the data store) and the tailObserver for new messages
I need the existingMessages observable to have onComplete eagerly called
the flatMap means the source (existingMessages in my case) never has onComplete called
so it leaves lots of readers hanging out there
Ilya
@squadgazzz
oh I see. let me think too)
Glen Marchesani
@fizzy33
Sort of the whole reason I did concatWithLast was to eagerly call onComplete on the original observable
the cool thing for me is I now have an in memory hot path for messages so the latency goes way way down
and the amount of code to do that is almost nil
I just get it for free
so concatWithLast is a very mild hack for the gains it promises
this is a TON of things listening to a large pulsar cluster
Oleg Pyzhcov
@oleg-py
not sure I even understand what it does
calls a function on a last element and then emits the results concatenated with a current one?
Glen Marchesani
@fizzy33
yes that is what it does
Oleg Pyzhcov
@oleg-py
does it not work without unsafe stuff?
def concatWithLast[A](s: Observable[A])(f: A => Observable[A]) = {
  s.publishSelector { hot =>
    Observable.from(hot.lastL.start).flatMap { fa =>
      hot ++ Observable.from(fa.join).flatMap(f)
    }
  }
}
Glen Marchesani
@fizzy33
I can try it :-)
my concatWithLast is incredibly and admittedly naive
This message was deleted
Glen Marchesani
@fizzy33
that doesn't work I haven't debugged too much to figure out why
Ilya
@squadgazzz

Can anyone please explain what's going on here?

  val obs = Observable.fromIterable(1 to 2)
  val result = obs.publishSelector { o =>
    o ++ Observable.fromIterable(11 to 12)
  }
  result.toListL.executeAsync.runToFuture.foreach(l => println(l.mkString("[", ",", "]")))
  Thread.sleep(5.seconds.toMillis)

result: [1,2,11,12]

  val obs = Observable.fromIterable(1 to 2)
  val result = obs.publishSelector { o =>
    o ++ Observable.fromIterable(11 to 12)
  } ++ Observable.fromIterable(101 to 102)
  result.toListL.executeAsync.runToFuture.foreach(l => println(l.mkString("[", ",", "]")))
  Thread.sleep(5.seconds.toMillis)

result: [11,12,101,102]

Where did obs's elements gone?

Glen Marchesani
@fizzy33
fwiw @oleg-py in all my micro tests the concatWithLast you supplied works...
Glen Marchesani
@fizzy33
What I am seeing in the production use case is all the messages are pumped before the hot ++ Observable.from(fa.join).flatMap(f) happens
Alexandru Nedelcu
@alexandru
@squadgazzz that's a good question 🙂 from the looks of it, doesn't seem right; could be some sort of bug
I smell a subscription timing issue, since publishSelector works with a PublishSubject underneath, and it might lose initial events if the underlying implementation subscribes too early.
Ilya
@squadgazzz
@alexandru how can I handle this? I have to use puplishSelector in real code. And the same time I need to append another observable later.
Glen Marchesani
@fizzy33
what about
val result0 = obs.publishSelector { o =>
    o ++ Observable.fromIterable(11 to 12)
  }
val result1 = obs.publishSelector { o => o ++ Observable.fromIterable(101 to 102) }
Alexandru Nedelcu
@alexandru
@squadgazzz I don't know why that happens, from the look of it you're not doing anything incorrectly. This needs to be investigated. Let me think.
Glen Marchesani
@fizzy33
fwiw this does have the behaviour you expect

  val obs = Observable.fromIterable(1 to 2)
  val result =
    obs
      .publishSelector { o =>
        o ++ Observable.fromIterable(11 to 12)
      }
      .publishSelector(_ ++ Observable.fromIterable(101 to 102))
  result.toListL.executeAsync.runToFuture.foreach(l => println(l.mkString("[", ",", "]")))
  Thread.sleep(5000)
but that becomes untenable to anywhere you use the obs you have to use it inside of publishSelector
Alexandru Nedelcu
@alexandru
Yeah, well, if it's a bug, we'd have to fix it.
@squadgazzz can you please open an issue? I'll take a look.
Ilya
@squadgazzz
@alexandru monix/monix#1292 thank you
@fizzy33 yeah, this works if I know about previous publishSelector call.
Cyrill Sitnikov
@InfiniteDisorder
Hello guys, have you ever experienced migrating custom akka flow stages to monix, is it possible to do it without scheduler evidence? In the example below is it possible to emit batches from OutputStream to something observable-like
val buf = new OutputStream {
     override def write(b: Int): Unit = {
        /* previously bytes were emitted to the next stage of  the flow */
    }
}
Alexandru Nedelcu
@alexandru
@InfiniteDisorder in that write, you could write those bytes into a ConcurrentSubject.
ClementWee
@ClementWee
Does Observable cancel itself on encountering an error?
Piotr Gawryś
@Avasil
@ClementWee it completes with an error if you subscribe in Task (L methods, consumeWith) and stops upstream. So it cancels in some way and it's immediate but it's not the same as canceling a subscription
Alex Michael Berry
@almibe
Is there anything in Monix to help with nested Tasks that you want to run?
ClementWee
@ClementWee
@almibe what do you mean by "nested tasks"?