Actors
======

Remote functions in Ray should be thought of as functional and side-effect free.
Restricting ourselves only to remote functions gives us distributed functional
programming, which is great for many use cases, but in practice is a bit
limited.

Ray extends the dataflow model with **actors**. An actor is essentially a
stateful worker (or a service). When a new actor is instantiated, a new worker
is created, and methods of the actor are scheduled on that specific worker and
can access and mutate the state of that worker.

Suppose we've already started Ray.

.. code-block:: python

  import ray
  ray.init()

Defining and creating an actor
------------------------------

Consider the following simple example. The ``ray.remote`` decorator indicates
that instances of the ``Counter`` class will be actors.

.. code-block:: python

  @ray.remote
  class Counter(object):
      def __init__(self):
          self.value = 0

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

To actually create an actor, we can instantiate this class by calling
``Counter.remote()``.

.. code-block:: python

  a1 = Counter.remote()
  a2 = Counter.remote()

When an actor is instantiated, the following events happen.

1. A node in the cluster is chosen and a worker process is created on that node
   (by the raylet on that node) for the purpose of running methods
   called on the actor.
2. A ``Counter`` object is created on that worker and the ``Counter``
   constructor is run.

Using an actor
--------------

We can schedule tasks on the actor by calling its methods.

.. code-block:: python

  a1.increment.remote()  # ray.get returns 1
  a2.increment.remote()  # ray.get returns 1

When ``a1.increment.remote()`` is called, the following events happens.

1. A task is created.
2. The task is assigned directly to the raylet responsible for the
   actor by the driver's raylet.
3. An object ID is returned.

We can then call ``ray.get`` on the object ID to retrieve the actual value.

Similarly, the call to ``a2.increment.remote()`` generates a task that is
scheduled on the second ``Counter`` actor. Since these two tasks run on
different actors, they can be executed in parallel (note that only actor
methods will be scheduled on actor workers, regular remote functions will not
be).

On the other hand, methods called on the same ``Counter`` actor are executed
serially in the order that they are called. They can thus share state with
one another, as shown below.

.. code-block:: python

  # Create ten Counter actors.
  counters = [Counter.remote() for _ in range(10)]

  # Increment each Counter once and get the results. These tasks all happen in
  # parallel.
  results = ray.get([c.increment.remote() for c in counters])
  print(results)  # prints [1, 1, 1, 1, 1, 1, 1, 1, 1, 1]

  # Increment the first Counter five times. These tasks are executed serially
  # and share state.
  results = ray.get([counters[0].increment.remote() for _ in range(5)])
  print(results)  # prints [2, 3, 4, 5, 6]

A More Interesting Actor Example
--------------------------------

A common pattern is to use actors to encapsulate the mutable state managed by an
external library or service.

`Gym`_ provides an interface to a number of simulated environments for testing
and training reinforcement learning agents. These simulators are stateful, and
tasks that use these simulators must mutate their state. We can use actors to
encapsulate the state of these simulators.

.. _`Gym`: https://gym.openai.com/

.. code-block:: python

  import gym

  @ray.remote
  class GymEnvironment(object):
      def __init__(self, name):
          self.env = gym.make(name)
          self.env.reset()

      def step(self, action):
          return self.env.step(action)

      def reset(self):
          self.env.reset()

We can then instantiate an actor and schedule a task on that actor as follows.

.. code-block:: python

  pong = GymEnvironment.remote("Pong-v0")
  pong.step.remote(0)  # Take action 0 in the simulator.

Using GPUs on actors
--------------------

A common use case is for an actor to contain a neural network. For example,
suppose we have imported Tensorflow and have created a method for constructing
a neural net.

.. code-block:: python

  import tensorflow as tf

  def construct_network():
      x = tf.placeholder(tf.float32, [None, 784])
      y_ = tf.placeholder(tf.float32, [None, 10])

      W = tf.Variable(tf.zeros([784, 10]))
      b = tf.Variable(tf.zeros([10]))
      y = tf.nn.softmax(tf.matmul(x, W) + b)

      cross_entropy = tf.reduce_mean(-tf.reduce_sum(y_ * tf.log(y), reduction_indices=[1]))
      train_step = tf.train.GradientDescentOptimizer(0.5).minimize(cross_entropy)
      correct_prediction = tf.equal(tf.argmax(y,1), tf.argmax(y_,1))
      accuracy = tf.reduce_mean(tf.cast(correct_prediction, tf.float32))

      return x, y_, train_step, accuracy

We can then define an actor for this network as follows.

.. code-block:: python

  import os

  # Define an actor that runs on GPUs. If there are no GPUs, then simply use
  # ray.remote without any arguments and no parentheses.
  @ray.remote(num_gpus=1)
  class NeuralNetOnGPU(object):
      def __init__(self):
          # Set an environment variable to tell TensorFlow which GPUs to use. Note
          # that this must be done before the call to tf.Session.
          os.environ["CUDA_VISIBLE_DEVICES"] = ",".join([str(i) for i in ray.get_gpu_ids()])
          with tf.Graph().as_default():
              with tf.device("/gpu:0"):
                  self.x, self.y_, self.train_step, self.accuracy = construct_network()
                  # Allow this to run on CPUs if there aren't any GPUs.
                  config = tf.ConfigProto(allow_soft_placement=True)
                  self.sess = tf.Session(config=config)
                  # Initialize the network.
                  init = tf.global_variables_initializer()
                  self.sess.run(init)

To indicate that an actor requires one GPU, we pass in ``num_gpus=1`` to
``ray.remote``. Note that in order for this to work, Ray must have been started
with some GPUs, e.g., via ``ray.init(num_gpus=2)``. Otherwise, when you try to
instantiate the GPU version with ``NeuralNetOnGPU.remote()``, an exception will
be thrown saying that there aren't enough GPUs in the system.

When the actor is created, it will have access to a list of the IDs of the GPUs
that it is allowed to use via ``ray.get_gpu_ids()``. This is a list of integers,
like ``[]``, or ``[1]``, or ``[2, 5, 6]``. Since we passed in
``ray.remote(num_gpus=1)``, this list will have length one.

We can put this all together as follows.

.. code-block:: python

  import os
  import ray
  import tensorflow as tf
  from tensorflow.examples.tutorials.mnist import input_data

  ray.init(num_gpus=8)

  def construct_network():
      x = tf.placeholder(tf.float32, [None, 784])
      y_ = tf.placeholder(tf.float32, [None, 10])

      W = tf.Variable(tf.zeros([784, 10]))
      b = tf.Variable(tf.zeros([10]))
      y = tf.nn.softmax(tf.matmul(x, W) + b)

      cross_entropy = tf.reduce_mean(-tf.reduce_sum(y_ * tf.log(y), reduction_indices=[1]))
      train_step = tf.train.GradientDescentOptimizer(0.5).minimize(cross_entropy)
      correct_prediction = tf.equal(tf.argmax(y,1), tf.argmax(y_,1))
      accuracy = tf.reduce_mean(tf.cast(correct_prediction, tf.float32))

      return x, y_, train_step, accuracy

  @ray.remote(num_gpus=1)
  class NeuralNetOnGPU(object):
      def __init__(self, mnist_data):
          self.mnist = mnist_data
          # Set an environment variable to tell TensorFlow which GPUs to use. Note
          # that this must be done before the call to tf.Session.
          os.environ["CUDA_VISIBLE_DEVICES"] = ",".join([str(i) for i in ray.get_gpu_ids()])
          with tf.Graph().as_default():
              with tf.device("/gpu:0"):
                  self.x, self.y_, self.train_step, self.accuracy = construct_network()
                  # Allow this to run on CPUs if there aren't any GPUs.
                  config = tf.ConfigProto(allow_soft_placement=True)
                  self.sess = tf.Session(config=config)
                  # Initialize the network.
                  init = tf.global_variables_initializer()
                  self.sess.run(init)

      def train(self, num_steps):
          for _ in range(num_steps):
              batch_xs, batch_ys = self.mnist.train.next_batch(100)
              self.sess.run(self.train_step, feed_dict={self.x: batch_xs, self.y_: batch_ys})

      def get_accuracy(self):
          return self.sess.run(self.accuracy, feed_dict={self.x: self.mnist.test.images,
                                                         self.y_: self.mnist.test.labels})


  # Load the MNIST dataset and tell Ray how to serialize the custom classes.
  mnist = input_data.read_data_sets("MNIST_data", one_hot=True)

  # Create the actor.
  nn = NeuralNetOnGPU.remote(mnist)

  # Run a few steps of training and print the accuracy.
  nn.train.remote(100)
  accuracy = ray.get(nn.get_accuracy.remote())
  print("Accuracy is {}.".format(accuracy))

Passing Around Actor Handles (Experimental)
-------------------------------------------

Actor handles can be passed into other tasks. To see an example of this, take a
look at the `asynchronous parameter server example`_. To illustrate this with
a simple example, consider a simple actor definition. This functionality is
currently **experimental** and subject to the limitations described below.

.. code-block:: python

  @ray.remote
  class Counter(object):
      def __init__(self):
          self.counter = 0

      def inc(self):
          self.counter += 1

      def get_counter(self):
          return self.counter

We can define remote functions (or actor methods) that use actor handles.

.. code-block:: python

  @ray.remote
  def f(counter):
      while True:
          counter.inc.remote()

If we instantiate an actor, we can pass the handle around to various tasks.

.. code-block:: python

  counter = Counter.remote()

  # Start some tasks that use the actor.
  [f.remote(counter) for _ in range(4)]

  # Print the counter value.
  for _ in range(10):
      print(ray.get(counter.get_counter.remote()))

Current Actor Limitations
-------------------------

We are working to address the following issues.

1. **Actor lifetime management:** Currently, when the original actor handle for
   an actor goes out of scope, a task is scheduled on that actor that kills the
   actor process (this new task will run once all previous tasks have finished
   running). This could be an issue if the original actor handle goes out of
   scope, but the actor is still being used by tasks that have been passed the
   actor handle.
2. **Returning actor handles:** Actor handles currently cannot be returned from
   a remote function or actor method. Similarly, ``ray.put`` cannot be called on
   an actor handle.
3. **Reconstruction of evicted actor objects:** If ``ray.get`` is called on an
   evicted object that was created by an actor method, Ray currently will not
   reconstruct the object. For more information, see the documentation on
   `fault tolerance`_.
4. **Deterministic reconstruction of lost actors:** If an actor is lost due to
   node failure, the actor is reconstructed on a new node, following the order
   of initial execution. However, new tasks that are scheduled onto the actor
   in the meantime may execute in between re-executed tasks. This could be an
   issue if your application has strict requirements for state consistency.

.. _`asynchronous parameter server example`: http://ray.readthedocs.io/en/latest/example-parameter-server.html
.. _`fault tolerance`: http://ray.readthedocs.io/en/latest/fault-tolerance.html