Agent

In this chapter, we will create a module named KV.Bucket. This module will be responsible for storing our key-value entries in a way it can be read and modified by other processes.

If you have skipped the Getting Started guide or if you have read it long ago, be sure to re-read the chapter about Processes. We will use it as starting point.

The trouble with state

Elixir is an immutable language where nothing is shared by default. If we want to provide state, where we create buckets putting and reading values from multiple places, we have two main options in Elixir:

We have already talked about processes, while ETS is something new that we will explore later in this guide. When it comes to processes though, we rarely hand-roll our own, instead we use the abstractions available in Elixir and OTP:

  • Agent - Simple wrappers around state.
  • GenServer - “Generic servers” (processes) that encapsulate state, provide sync and async calls, support code reloading, and more.
  • GenEvent - “Generic event” managers that allow publishing events to multiple handlers.
  • Task - Asynchronous units of computation that allow spawning a process and potentially retrieving its result at a later time.

We will explore most of these abstractions in this guide. Keep in mind that they are all implemented on top of processes using the basic features provided by the VM, like send, receive, spawn and link.

Agents

Agents are simple wrappers around state. If all you want from a process is to keep state, agents are a great fit. Let’s start an iex session inside the project with:

$ iex -S mix

And play a bit with agents:

iex> {:ok, agent} = Agent.start_link fn -> [] end
{:ok, #PID<0.57.0>}
iex> Agent.update(agent, fn list -> ["eggs" | list] end)
:ok
iex> Agent.get(agent, fn list -> list end)
["eggs"]
iex> Agent.stop(agent)
:ok

We started an agent with an initial state of an empty list. We updated the agent’s state, adding our new item to the head of the list. The second argument of Agent.update/3 is a function that takes the agent’s current state as input and returns its desired new state. Finally, we retrieved the whole list. The second argument of Agent.get/3 is a function that takes the state as input and returns the value that Agent.get/3 itself will return. Once we are done with the agent, we can call Agent.stop/1 to terminate the agent process.

Let’s implement our KV.Bucket using agents. But before starting the implementation, let’s first write some tests. Create a file at test/kv/bucket_test.exs (remember the .exs extension) with the following:

defmodule KV.BucketTest do
  use ExUnit.Case, async: true

  test "stores values by key" do
    {:ok, bucket} = KV.Bucket.start_link
    assert KV.Bucket.get(bucket, "milk") == nil

    KV.Bucket.put(bucket, "milk", 3)
    assert KV.Bucket.get(bucket, "milk") == 3
  end
end

Our first test starts a new KV.Bucket and perform some get/2 and put/3 operations on it, asserting the result. We don’t need to explicitly stop the agent because it is linked to the test process and the agent is shut down automatically once the test finishes. This will always work unless the process is named.

Also note that we passed the async: true option to ExUnit.Case. This option makes this test case run in parallel with other test cases that set up the :async option. This is extremely useful to speed up our test suite by using multiple cores in our machine. Note though the :async option must only be set if the test case does not rely or change any global value. For example, if the test requires writing to the filesystem, registering processes, accessing a database, you must not make it async to avoid race conditions in between tests.

Regardless of being async or not, our new test should obviously fail, as none of the functionality is implemented.

In order to fix the failing test, let’s create a file at lib/kv/bucket.ex with the contents below. Feel free to give a try at implementing the KV.Bucket module yourself using agents before peeking at the implementation below.

defmodule KV.Bucket do
  @doc """
  Starts a new bucket.
  """
  def start_link do
    Agent.start_link(fn -> %{} end)
  end

  @doc """
  Gets a value from the `bucket` by `key`.
  """
  def get(bucket, key) do
    Agent.get(bucket, &Map.get(&1, key))
  end

  @doc """
  Puts the `value` for the given `key` in the `bucket`.
  """
  def put(bucket, key, value) do
    Agent.update(bucket, &Map.put(&1, key, value))
  end
end

We are using a map to store our keys and values. The capture operator, &, is introduced in the Getting Started guide.

Now that the KV.Bucket module has been defined, our test should pass! You can try it yourself by running: mix test.

ExUnit callbacks

Before moving on and adding more features to KV.Bucket, let’s talk about ExUnit callbacks. As you may expect, all KV.Bucket tests will require a bucket to be started during setup and stopped after the test. Luckily, ExUnit supports callbacks that allow us to skip such repetitive tasks.

Let’s rewrite the test case to use callbacks:

defmodule KV.BucketTest do
  use ExUnit.Case, async: true

  setup do
    {:ok, bucket} = KV.Bucket.start_link
    {:ok, bucket: bucket}
  end

  test "stores values by key", %{bucket: bucket} do
    assert KV.Bucket.get(bucket, "milk") == nil

    KV.Bucket.put(bucket, "milk", 3)
    assert KV.Bucket.get(bucket, "milk") == 3
  end
end

We have first defined a setup callback with the help of the setup/1 macro. The setup/1 callback runs before every test, in the same process as the test itself.

Note that we need a mechanism to pass the bucket pid from the callback to the test. We do so by using the test context. When we return {:ok, bucket: bucket} from the callback, ExUnit will merge the second element of the tuple (a dictionary) into the test context. The test context is a map which we can then match in the test definition, providing access to these values inside the block:

test "stores values by key", %{bucket: bucket} do
  # `bucket` is now the bucket from the setup block
end

You can read more about ExUnit cases in the ExUnit.Case module documentation and more about callbacks in ExUnit.Callbacks docs.

Other agent actions

Besides getting a value and updating the agent state, agents allow us to get a value and update the agent state in one function call via Agent.get_and_update/2. Let’s implement a KV.Bucket.delete/2 function that deletes a key from the bucket, returning its current value:

@doc """
Deletes `key` from `bucket`.

Returns the current value of `key`, if `key` exists.
"""
def delete(bucket, key) do
  Agent.get_and_update(bucket, &Map.pop(&1, key))
end

Now it is your turn to write a test for the functionality above! Also, be sure to explore the documentation for the Agent module to learn more about them.

Client/Server in agents

Before we move on to the next chapter, let’s discuss the client/server dichotomy in agents. Let’s expand the delete/2 function we have just implemented:

def delete(bucket, key) do
  Agent.get_and_update(bucket, fn dict->
    Map.pop(dict, key)
  end)
end

Everything that is inside the function we passed to the agent happens in the agent process. In this case, since the agent process is the one receiving and responding to our messages, we say the agent process is the server. Everything outside the function is happening in the client.

This distinction is important. If there are expensive actions to be done, you must consider if it will be better to perform these actions on the client or on the server. For example:

def delete(bucket, key) do
  :timer.sleep(1000) # puts client to sleep
  Agent.get_and_update(bucket, fn dict ->
    :timer.sleep(1000) # puts server to sleep
    Map.pop(dict, key)
  end)
end

When a long action is performed on the server, all other requests to that particular server will wait until the action is done, which may cause some clients to timeout.

In the next chapter we will explore GenServers, where the segregation between clients and servers is made even more apparent.

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