Announcing Version 1.0


26 June 2024, Dominik Charousset [#API] [#Roadmap]


Towards a New Decade

Today, we are thrilled to announce the release of CAF version 1.0. This is a major milestone for us and we are excited to share it with you.

CAF has been in beta for over a decade. During that time, we have revised the design multiple times, continuously added features and received valuable feedback from our users. We are very grateful for the comments, suggestions and bug reports we have received over the years. Be it on GitHub, in chat, via email or in private conversations, your feedback has been invaluable to us.

For this milestone release, we have asked ourselves how CAF would look like if we started from scratch today and how we want to evolve CAF over the next decade. Of course, we did not throw away everything we have built so far. However, we have thought of all the lessons we have learned, considered the community feedback we have received and discussed our experience of teaching CAF to new users.

Our goal for CAF version 1.0 is to provide a solid foundation for the next decade. We want to make CAF more accessible to new users while keeping the flexibility and power that our experienced users have come to appreciate. Ultimately, we want to make CAF easier to learn and teach, with a coherent API design. Whether you are using CAF for cloud applications, micro services, edge computing or the IoT, we strive to make CAF a great choice for building distributed systems.

A secondary, but nonetheless important, goal is to provide more stability in the API and the ABI. We want to make it easier to package CAF for distributions while allowing us to continue to evolve the framework. To this end, we have carefully reviewed the API and have deprecated several features that we consider overly complex, error-prone or not in line with the overall design of CAF.

When looking at the changelog, you will notice that version 1.0 has more deprecations than any previous release and that we have touched APIs that are at the core of CAF. We have recently introduced the new mail API that replaces the old messaging API. Today, we would like to use this blog post to discuss further changes we have made and the reasons behind them.

Deprecating aout

The aout utility has been part of CAF for a long time. It provides a way to print to the console from multiple actors in parallel without interleaving output. However, the design of aout tries to mimic std::cout. With std::format on the horizon, we have decided to deprecate aout in favor of a printing API that is more in line with modern C++.

For printing to the console, CAF now provides a println member function that exists on actors and the actor system. The syntax is similar to std::format and allows for type-safe printing of arbitrary values. When building CAF in C++20 mode, println will use std::format under the hood. In C++17 mode, it will use a custom implementation that supports only a subset of the formatting options. In both cases, CAF will automatically pick up inspect overloads to render custom types.

Simpler Monitoring

When implementing event-based actors, a callback-based API is much more natural than calling monitor and then providing a special handler someplace else. Thus, we have deprecated the single-argument monitor function on event-based actors in favor of a new overload that takes two arguments: the actor to monitor and a callback function. The new overload also returns a disposable object that can be used to cancel the monitoring.

Alongside this change, we have also deprecated the set_down_handler function, since it becomes obsolete when using the new monitor overload. The monitored spawn flag has been deprecated for the same reason.

Better Idle Timeout Handling

Binding the idle timeout to the actor behavior has been a source of confusion for many users. Consider this example:

caf::behavior my_actor(caf::event_based_actor* self) {
  return {
    // ... handlers ...
    caf::after(10s) >> [] { /* ... */ }

Without looking at the documentation, it is not clear whether the timeout is triggered once or repeatedly. Furthermore, it is unclear what kind of reference will CAF keep to the actor. Will it keep the actor alive or will it only keep a weak reference? The answer is that CAF will keep a strong reference to the actor and trigger the timeout repeatedly. This is not always what users expect or want. To trigger a timeout only once, users have to call self->become() in the callback to install a new behavior that does not include the timeout.

To address this issue, we have deprecated this use of after and introduced a new set_idle_handler(timeout, ref_type, repeat_type, callback) function, where timeout is a duration, ref_type is either strong_ref or weak_ref and repeat_type is either once or repeat. The callback is a function that will be called when the timeout triggers. This new approach separates the idle timeout from the behavior and adds much clarity to the code.

Note that nothing changes for blocking actors. They still pass after to receive as before (it simply configures how long the function may block). The new function is only relevant for event-based actors.

Fewer Special Handlers

In the past, CAF has provided special handlers for certain message types such as exit_msg as well as a catch-all handler for unexpected messages.

This design moves message handling logic away from the behavior, making it harder to understand the actor’s behavior at a glance. It also makes it harder to reason about an actor in general, as the behavior is now spread across multiple functions.

To address this issue, we have deprecated the special handlers. Instead, actors can now simply handle these types of messages in their regular behavior and provide a message handler for message as a catch-all.

Deprecating the Legacy Unit Testing Framework

With CAF 1.0, we now include a fully-featured version of the new unit testing framework that we have been working on. Our team has been using this framework for a while now and we are very happy with it. We have ported all of our tests to the new framework and are deprecating the old testing framework as a result. The old headers are still available, but will be removed in version 2.0.

Simplifying Spawn

CAF provides multiple ways to spawn actors. We have supported and promoted the use of state-based actors for a while, but there have been several ways to implement them as well. The feedback we have received from users is that the current API is too complex and that it is not always clear which function to use.

To address this, we have streamlined the API for spawning actors and there are only two ways we recommend to spawn actors now: as a function or using a state class. For the latter, we have already shipped the new actor_from_state utility in CAF 0.19.5.

All of our examples and guides have been updated to reflect this.

However, we have decided against deprecating the old spawn functions right away. This release already has a lot of deprecations in fundamental parts of CAF and we want to give users more time to adjust to the new API. However, we will deprecate the other spawn versions with CAF 2.0 and strongly encourage users to write new code only using the best practices we have outlined.

Traits for Typed Actors

When encountering compiler errors with typed_actor, it is often hard to understand the compiler output due to the very long type names. To address this we allow users to define traits for typed actors instead of passing the signatures for the message handlers directly to the typed_actor template.

The trait is simply a user-defined class that contains a type alias signatures, as shown in the following example:

struct cell_trait {
  using signatures = caf::type_list<
    caf::result<void>(caf::put_atom, int32_t)

using cell_actor = caf::typed_actor<cell_trait>;

This quality-of-life change reduces the symbol name length in the compiler output and makes it easier to understand the error messages. While we recommend using this new approach for defining typed actors, the old way remains supported.

Improving the ABI Stability

While mostly an invisible change, we have also worked on improving the ABI stability of CAF by better separating the implementation details from the public API. With this release, we are adopting a new versioning policy: only major releases may break the ABI (or API). We will continue to provide bug fixes and new features in minor releases, but changes that affect the ABI will only be made in major releases.

Discontinuing CAFcademy

Last but not least, we have an important announcement to make regarding CAFcademy.

With CAFcademy, we have provided a free online archive of learning materials for CAF. Initially, we have also offered professional services around CAF on that website, before we have re-organized our commercial offerings around CAF with the launch of Interance in 2022.

With the company operating all things CAF now, we have decided to discontinue the CAFcademy brand, as it no longer serves a purpose. The articles from CAFcademy have been completely overhauled to reflect the current best practices and are available on The gems articles from CAFcademy have been moved to the manual and are now part of the official documentation. The line has always been blurry between the two and we believe that this consolidation will make it easier for users to find the information they need.

The CAFcademy website will be taken offline January 2025.