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What do dispatchers do?

Dispatchers are responsible for scheduling all code that run inside the ActorSystem. Dispatchers are one of the most important parts of Proto.Actor, as they control the throughput and time share for each of the actors, giving each one a fair share of resources.

By default, all actors share a single Global Dispatcher. Unless you change the configuration, this dispatcher uses the .NET Thread Pool behind the scenes, which is optimized for most common scenarios. That means the default configuration should be good enough for most cases.

Why should I use different dispatchers?

When messages arrive in the actor’s mailbox, the dispatcher schedules the delivery of messages in batches, and tries to deliver the entire batch before releasing the thread to another actor.

There are some other common reasons to select a different dispatcher. These reasons include (but are not limited to):

  • isolating one or more actors to specific threads in order to:
    • ensure high-load actors don’t starve the system by consuming too much cpu-time;
    • ensure important actors always have a dedicated thread to do their job;
    • create bulkheads, ensuring problems created in one part of the system do not leak to others;
  • allow actors to execute in a specific SyncrhonizationContext; (.NET specific)
NOTE Consider using custom dispatchers for special cases only. The most common configuration change would be the Throughput setting on the ThreadPoolDispatcher, where you might have to tune the number of messages a specific actor type process in a single run of the dispatcher. If your system falls over and doesn’t run properly, it might be tempting to blame the dispatchers, this is very likely NOT the issue. but rather bad design of workloads and actor interactions.

Configuring dispatchers

Dispatchers are configured via Props like so:

//throughput means number of messages in a single go, before yielding the thread back
var dispatcher = new ThreadPoolDispatcher(throughput = 100);
//define a props using MyActor, and the above dispatcher
var props = Props.FromProducer(() => new MyActor()).WithDispatcher(dispatcher);

Built-in dispatchers

Some dispatchers are available out-of-the-box for convenience.

  • ThreadPoolDispatcher - a dispatcher that schedules the mailbox as a Task on the threadpool.
  • CurrentSynchronizationContext - a dispatcher that runs on the current synchronization context. e.g in WPF.
  • SynchronousDispatcher - a dispatcher that runs synchronously (blocking).