Actors are objects which encapsulate state and behavior, they communicate exclusively by exchanging messages which are placed into the recipient’s mailbox. In a sense, actors are the most stringent form of object-oriented programming, but it serves better to view them as persons: while modeling a solution with actors, envision a group of people and assign sub-tasks to them, arrange their functions into an organizational structure and think about how to escalate failure (all with the benefit of not actually dealing with people, which means that we need not concern ourselves with their emotional state or moral issues). The result can then serve as a mental scaffolding for building the software implementation.
An ActorSystem is a heavyweight structure that will allocate 1…N Threads, so create one per logical application.
Like in an economic organization, actors naturally form hierarchies. One actor, which is to oversee a certain function in the program might want to split up its task into smaller, more manageable pieces. For this purpose, it starts child actors.
The quintessential feature of actor systems is that tasks are split up and delegated until they become small enough to be handled in one piece. In doing so, not only is the task itself clearly structured, but the resulting actors can be reasoned about in terms of which messages they should process, how they should react normally and how failure should be handled.
Compare this to layered software design which easily devolves into defensive programming with the aim of not leaking any failure out: if the problem is communicated to the right person, a better solution can be found than if trying to keep everything “under the carpet”.
Now, the difficulty in designing such a system is how to decide how to structure the work. There is no single best solution, but there are a few guidelines which might be helpful:
- If one actor carries very important data (i.e. its state shall not be lost if avoidable), this actor should source out any possibly dangerous sub-tasks to children and handle failures of these children as appropriate. Depending on the nature of the requests, it may be best to create a new child for each request, which simplifies state management for collecting the replies. This is known as the “Error Kernel Pattern” from Erlang.
- If one actor depends on another actor for carrying out its duty, it should watch that other actor’s liveness and act upon receiving a termination notice.
- If one actor has multiple responsibilities each responsibility can often be pushed into a separate child to make the logic and state more simple.
The actor system as a collaborating ensemble of actors is the natural unit for managing shared facilities like scheduling services, configuration, logging, etc. Several actor systems with different configurations may co-exist within the same JVM without problems, there is no global shared state within Pekko itself, however the most common scenario will only involve a single actor system per JVM.
Couple this with the transparent communication between actor systems — within one node or across a network connection — and actor systems are a perfect fit to form a distributed application.
- Actors should be like nice co-workers: do their job efficiently without bothering everyone else needlessly and avoid hogging resources. Translated to programming this means to process events and generate responses (or more requests) in an event-driven manner. Actors should not block (i.e. passively wait while occupying a Thread) on some external entity—which might be a lock, a network socket, etc.—unless it is unavoidable; in the latter case see
Blocking Needs Careful Management.
- Do not pass mutable objects between actors. In order to ensure that, prefer immutable messages. If the encapsulation of actors is broken by exposing their mutable state to the outside, you are back in normal Java concurrency land with all the drawbacks.
- Actors are made to be containers for behavior and state, embracing this means to not routinely send behavior within messages (which may be tempting using Scala closures). One of the risks is to accidentally share mutable state between actors, and this violation of the actor model unfortunately breaks all the properties which make programming in actors such a nice experience.
- The top-level actor of the actor system is the innermost part of your Error Kernel, it should only be responsible for starting the various sub systems of your application, and not contain much logic in itself, prefer truly hierarchical systems. This has benefits with respect to fault-handling (both considering the granularity of configuration and the performance) and it also reduces the strain on the guardian actor, which is a single point of contention if over-used.
An actor system manages the resources it is configured to use in order to run the actors which it contains. There may be millions of actors within one such system, after all the mantra is to view them as abundant and they weigh in at an overhead of only roughly 300 bytes per instance. Naturally, the exact order in which messages are processed in large systems is not controllable by the application author, but this is also not intended. Take a step back and relax while Pekko does the heavy lifting under the hood.
When you know everything is done for your application, you can have the user guardian actor stop, or call the
terminate() method of
ActorSystem. That will run
CoordinatedShutdown stopping all running actors.