Asynchronous, Reactive Programming for Scala and Scala.js.

Build status Gitter


Monix is a high-performance Scala / Scala.js library for composing asynchronous, event-based programs.

It started as a proper implementation of ReactiveX, with stronger functional programming influences and designed from the ground up for back-pressure and made to cleanly interact with Scala's standard library, compatible out-of-the-box with the Reactive Streams protocol. It then expanded to include abstractions for suspending side effects and for resource handling, being one of the parents and implementors of cats-effect.

A Typelevel project, Monix proudly exemplifies pure, typeful, functional programming in Scala, while making no compromise on performance.


  • exposes the kick-ass Observable, Iterant, Task and Coeval data types, along with all the support they need
  • modular, only use what you need
  • designed for true asynchronicity, running on both the JVM and Scala.js
  • really good test coverage, code quality and API documentation as a primary project policy


See monix-sample for a project exemplifying Monix used both on the server and on the client.


The packages are published on Maven Central.

For the stable release (compatible with Cats and Cats-Effect 2.x):

libraryDependencies += "io.monix" %% "monix" % "3.2.2"


Monix 3.x is modular by design, so you can pick and choose:

  • monix-catnap exposes pure abstractions built on top of the Cats-Effect type classes; depends on monix-execution, Cats 1.x and Cats-Effect
  • monix-execution exposes the low-level execution environment, or more precisely Scheduler, Cancelable, Atomic, Local, CancelableFuture and Future based abstractions from monix-catnap.
  • monix-eval exposes Task, Coeval; depends on monix-execution
  • monix-reactive exposes Observable for modeling reactive, push-based streams with back-pressure; depends on monix-eval
  • monix-tail exposes Iterant streams for purely functional pull based streaming; depends on monix-eval and makes heavy use of Cats-Effect
  • monix provides all of the above

Versioning Scheme

The versioning scheme follows the Semantic Versioning (semver) specification, meaning stable versions have the form $major.$minor.$patch, such that:

  1. $major version updates make binary incompatible API changes
  2. $minor version updates adds functionality in a backwards-compatible manner, and
  3. $patch version updates makes backwards-compatible bug fixes

We publish intermediary versions, automatically, whenever PRs get merged to master. The project has a dynamic version setup, releases are via GitHub Actions with versions such as 3.2.2+3-1234abcd, where 3.2.2 is the base version, +3 represents the number of commits, the distance from that base version, and 1234abcd represents the commit's "sha" (hash) prefix, identifying the commit in GitHub.

We make NO GUARANTEES for these intermediary versions, but they are pretty high quality, they get published on Maven Central too, and you can depend on them for testing purposes, or if you really, really need a new feature that's not published in a stable version yet.



API Documentation:

(contributions are welcome)



The Monix project welcomes contributions from anybody wishing to participate. All code or documentation that is provided must be licensed with the same license that Monix is licensed with (Apache 2.0, see LICENSE.txt).

People are expected to follow the Scala Code of Conduct when discussing Monix on GitHub, Gitter channel, or other venues.

Feel free to open an issue if you notice a bug, have an idea for a feature, or have a question about the code. Pull requests are also gladly accepted. For more information, check out the contributor guide.

If you'd like to donate in order to help with ongoing maintenance:


Here's a (non-exhaustive) list of companies that use Monix in production. Don't see yours? Submit a PR ❤️


All code in this repository is licensed under the Apache License, Version 2.0. See LICENCE.txt.