# Actors

Actors enable stateful computations within a Dask workflow. They are useful for some rare algorithms that require additional performance and are willing to sacrifice resilience.

An actor is a pointer to a user-defined-object living on a remote worker. Anyone with that actor can call methods on that remote object.

## Example

Here we create a simple `Counter` class, instantiate that class on one worker, and then call methods on that class remotely.

    class Counter:
        """ A simple class to manage an incrementing counter """
        n = 0

        def __init__(self):
            self.n = 0

        def increment(self):
            self.n += 1
            return self.n

        def add(self, x):
            self.n += x
            return self.n

    from dask.distributed import Client          # Start a Dask Client
    client = Client()

    future = client.submit(Counter, actor=True)  # Create a Counter on a worker
    counter = future.result()                    # Get back a pointer to that object

    counter
    # <Actor: Counter, key=Counter-1234abcd>

    future = counter.increment()                 # Call remote method
    future.result()                              # Get back result
    # 1

    future = counter.add(10)                     # Call remote method
    future.result()                              # Get back result
    # 11

## Motivation

Actors are motivated by some of the challenges of using pure task graphs.

Normal Dask computations are composed of a graph of functions. This approach has a few limitations that are good for resilience, but can negatively affect performance:

1. State: The functions should not mutate their inputs in-place or rely on global state. They should instead operate in a pure-functional manner, consuming inputs and producing separate outputs.

2. Central Overhead: The execution location and order is determined by the centralized scheduler. Because the scheduler is involved in every decision it can sometimes create a central bottleneck.

Some workloads may need to update state directly, or may involve more tiny tasks than the scheduler can handle (the scheduler can coordinate about 4000 tasks per second).

Actors side-step both of these limitations:

1. State: Actors can hold on to and mutate state. They are allowed to update their state in-place.

2. Overhead: Operations on actors do not inform the central scheduler, and so do not contribute to the 4000 task/second overhead. They also avoid an extra network hop and so have lower latencies.

## Create an Actor

You create an actor by submitting a Class to run on a worker using normal Dask computation functions like `submit`, `map`, `compute`, or `persist`, and using the `actors=` keyword (or `actor=` on `submit`).

    future = client.submit(Counter, actors=True)

You can use all other keywords to these functions like `workers=`, `resources=`, and so on to control where this actor ends up.

This creates a normal Dask future on which you can call `.result()` to get the Actor once it has successfully run on a worker.

    >>> counter = future.result()
    >>> counter
    <Actor: Counter, key=...>

A `Counter` object has been instantiated on one of the workers, and this `Actor` object serves as our proxy to that remote object. It has the same methods and attributes.

    >>> dir(counter)
    ['add', 'increment', 'n']

## Call Remote Methods

However accessing an attribute or calling a method will trigger a communication to the remote worker, run the method on the remote worker in a separate thread pool, and then communicate the result back to the calling side. For attribute access these operations block and return when finished, for method calls they return an `BaseActorFuture` immediately.

    >>> future = counter.increment()  # Immediately returns a BaseActorFuture
    >>> future.result()               # Block until finished and result arrives
    1

`BaseActorFuture` are similar to normal Dask `Future` objects, but not as fully featured. They currently only support the `result` method and nothing else. They don’t currently work with any other Dask functions that expect futures, like `as_completed`, `wait`, or `client.gather`. They can’t be placed into additional submit or map calls to form dependencies. They communicate their results immediately (rather than waiting for result to be called) and cache the result on the future itself.

## Access Attributes

If you define an attribute at the class level then that attribute will be accessible to the actor.

    class Counter:
        n = 0   # Recall that we defined our class with `n` as a class variable

        ...

    >>> counter.n                     # Blocks until finished
    1

Attribute access blocks automatically. It’s as though you called `.result()`.

## Execution on the Worker

When you call a method on an actor, your arguments get serialized and sent to the worker that owns the actor’s object. If you do this from a worker this communication is direct. If you do this from a Client then this will be direct if the Client has direct access to the workers (create a client with `Client(..., direct_to_workers=True)` if direct connections are possible) or by proxying through the scheduler if direct connections from the client to the workers are not possible.

The appropriate method of the Actor’s object is then called in a separate thread, the result captured, and then sent back to the calling side. Currently workers have only a single thread for actors, but this may change in the future.

The result is sent back immediately to the calling side, and is not stored on the worker with the actor. It is cached on the `BaseActorFuture` object.

## Calling from coroutines and async/await

If you use actors within a coroutine or async/await function then actor methods and attribute access will return Tornado futures

    async def f():
        counter = await client.submit(Counter, actor=True)

        await counter.increment()
        n = await counter.n

## Coroutines and async/await on the Actor

If you define an `async def` function on the actor class then that method will run on the Worker’s event loop thread rather than a separate thread.

    def Waiter:
        def __init__(self):
            self.event = asyncio.Event()

        async def set(self):
            self.event.set()

        async def wait(self):
            await self.event.wait()

    waiter = client.submit(Waiter, actor=True).result()
    waiter.wait().result()  # waits until set, without consuming a worker thread

## Performance

Worker operations currently have about 1ms of latency, on top of any network latency that may exist. However other activity in a worker may easily increase these latencies if enough other activities are present.

## Limitations

Actors offer advanced capabilities, but with some cost:

1. No Resilience: No effort is made to make actor workloads resilient to worker failure. If the worker dies while holding an actor that actor is lost forever.

2. No Diagnostics: Because the scheduler is not informed about actor computations no diagnostics are available about these computations.

3. No Load balancing: Actors are allocated onto workers evenly, without serious consideration given to avoiding communication.

4. No dynamic clusters: Actors cannot be migrated to other workers. A worker holding an actor can’t be retired, neither through `retire_workers()` nor through `Adaptive`.