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Introduction #

The Observable is a data type for modeling and processing asynchronous and reactive streaming of events with non-blocking back-pressure.

The Observable is strongly inspired by ReactiveX, but with an idiomatic Scala API and influenced by the Scala ecosystem of projects such as Cats and Scalaz. It’s also compatible with the Reactive Streams specification, hence it has good interoperability.

// We need a Scheduler in scope in order to make 
// the Observable produce elements when subscribed
import monix.execution.Scheduler.Implicits.global
import monix.reactive._

import concurrent.duration._

// We first build an observable that emits a tick per second, 
// the series of elements being an auto-incremented long
val source = Observable.interval(1.second)
  // Filtering out odd numbers, making it emit every 2 seconds
  .filter(_ % 2 == 0)
  // We then make it emit the same element twice
  .flatMap(x => Observable(x, x))
  // This stream would be infinite, so we limit it to 10 items
// Observables are lazy, nothing happens until you subscribe...
val cancelable = source
  // On consuming it, we want to dump the contents to stdout
  // for debugging purposes
  // Finally, start consuming it

At its simplest, Observable is a replacement for your regular Iterable or Scala Stream, but with the ability to process asynchronous events without blocking. And in fact you can convert any Iterable into an Observable.

But Observable scales to complex problems, touching on functional reactive programming (FRP), or it can model complex interactions between producers and consumers, being a potent alternative for the actor model.

Design Summary #

A visual representation of where it sits in the design space:

  Single Multiple
Synchronous A Iterable[A]
Asynchronous Future[A] / Task[A] Observable[A]

The Monix Observable:

  • models lazy & asynchronous streaming of events
  • it is highly composable and lawful
  • it’s basically the Observer pattern on steroids
  • you can also think of it as being like a Scala Future or like a Task, except with the ability to stream multiple items instead of just one, or you can think of it as an asynchronous and non-blocking Iterable with benefits
  • models producer-consumer relationships, where you can have a single producer pushing data into one or multiple consumers
  • works best for unidirectional communications
  • allows fine-grained control over the execution model
  • doesn’t trigger the execution, or any effects until subscribe
  • allows for cancelling of active streams
  • never blocks any threads in its implementation
  • does not expose any API calls that can block threads
  • compatible with Scala.js like the rest of Monix

See comparisons with similar tools, like Akka or FS2.