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/Elixir 1.5

Agent

Agents are a simple abstraction around state.

Often in Elixir there is a need to share or store state that must be accessed from different processes or by the same process at different points in time.

The Agent module provides a basic server implementation that allows state to be retrieved and updated via a simple API.

Examples

For example, in the Mix tool that ships with Elixir, we need to keep a set of all tasks executed by a given project. Since this set is shared, we can implement it with an agent:

defmodule Mix.TasksServer do
  use Agent

  def start_link do
    Agent.start_link(fn -> MapSet.new end, name: __MODULE__)
  end

  @doc "Checks if the task has already executed"
  def executed?(task, project) do
    item = {task, project}
    Agent.get(__MODULE__, fn set ->
      item in set
    end)
  end

  @doc "Marks a task as executed"
  def put_task(task, project) do
    item = {task, project}
    Agent.update(__MODULE__, &MapSet.put(&1, item))
  end

  @doc "Resets the executed tasks and returns the previous list of tasks"
  def take_all() do
    Agent.get_and_update(__MODULE__, fn set ->
      {Enum.into(set, []), MapSet.new}
    end)
  end
end

Agents provide a segregation between the client and server APIs (similar to GenServers). In particular, the anonymous functions given to the Agent are executed inside the agent (the server). This distinction is important because you may want to avoid expensive operations inside the agent, as they will effectively block the agent until the request is fulfilled.

Consider these two examples:

# Compute in the agent/server
def get_something(agent) do
  Agent.get(agent, fn state -> do_something_expensive(state) end)
end

# Compute in the agent/client
def get_something(agent) do
  Agent.get(agent, &(&1)) |> do_something_expensive()
end

The first function blocks the agent. The second function copies all the state to the client and then executes the operation in the client. One aspect to consider is whether the data is large enough to require processing in the server, at least initially, or small enough to be sent to the client cheaply. Another factor is whether the data needs to be processed atomically: getting the state and calling do_something_expensive(state) outside of the agent means that the agent’s state can be updated in the meantime. This is specially important in case of updates as computing the new state in the client rather than in the server can lead to race conditions if multiple clients are trying to update the same state to different values.

Finally note use Agent defines a child_spec/1 function, allowing the defined module to be put under a supervision tree. The generated child_spec/1 can be customized with the following options:

  • :id - the child specification id, defauts to the current module
  • :start - how to start the child process (defaults to calling __MODULE__.start_link/1)
  • :restart - when the child should be restarted, defaults to :permanent
  • :shutdown - how to shut down the child

For example:

use Agent, restart: :transient, shutdown: 10_000

See the Supervisor docs for more information.

Name registration

An agent is bound to the same name registration rules as GenServers. Read more about it in the GenServer documentation.

A word on distributed agents

It is important to consider the limitations of distributed agents. Agents provide two APIs, one that works with anonymous functions and another that expects an explicit module, function, and arguments.

In a distributed setup with multiple nodes, the API that accepts anonymous functions only works if the caller (client) and the agent have the same version of the caller module.

Keep in mind this issue also shows up when performing “rolling upgrades” with agents. By rolling upgrades we mean the following situation: you wish to deploy a new version of your software by shutting down some of your nodes and replacing them with nodes running a new version of the software. In this setup, part of your environment will have one version of a given module and the other part another version (the newer one) of the same module.

The best solution is to simply use the explicit module, function, and arguments APIs when working with distributed agents.

Hot code swapping

An agent can have its code hot swapped live by simply passing a module, function, and arguments tuple to the update instruction. For example, imagine you have an agent named :sample and you want to convert its inner state from a keyword list to a map. It can be done with the following instruction:

{:update, :sample, {:advanced, {Enum, :into, [%{}]}}}

The agent’s state will be added to the given list of arguments ([%{}]) as the first argument.

Summary

Types

agent()

The agent reference

name()

The agent name

on_start()

Return values of start* functions

state()

The agent state

Functions

cast(agent, fun)

Performs a cast (fire and forget) operation on the agent state

cast(agent, module, fun, args)

Performs a cast (fire and forget) operation on the agent state

get(agent, fun, timeout \\ 5000)

Gets an agent value via the given anonymous function

get(agent, module, fun, args, timeout \\ 5000)

Gets an agent value via the given function

get_and_update(agent, fun, timeout \\ 5000)

Gets and updates the agent state in one operation via the given anonymous function

get_and_update(agent, module, fun, args, timeout \\ 5000)

Gets and updates the agent state in one operation via the given function

start(fun, options \\ [])

Starts an agent process without links (outside of a supervision tree)

start(module, fun, args, options \\ [])

Starts an agent without links with the given module, function, and arguments

start_link(fun, options \\ [])

Starts an agent linked to the current process with the given function

start_link(module, fun, args, options \\ [])

Starts an agent linked to the current process

stop(agent, reason \\ :normal, timeout \\ :infinity)

Synchronously stops the agent with the given reason

update(agent, fun, timeout \\ 5000)

Updates the agent state via the given anonymous function

update(agent, module, fun, args, timeout \\ 5000)

Updates the agent state via the given function

Types

agent()

agent() :: pid | {atom, node} | name

The agent reference

name()

name() :: atom | {:global, term} | {:via, module, term}

The agent name

on_start()

on_start ::
  {:ok, pid} |
  {:error, {:already_started, pid} | term}

Return values of start* functions

state()

state() :: term

The agent state

Functions

cast(agent, fun)

cast(agent, (state -> state)) :: :ok

Performs a cast (fire and forget) operation on the agent state.

The function fun is sent to the agent which invokes the function passing the agent state. The return value of fun becomes the new state of the agent.

Note that cast returns :ok immediately, regardless of whether agent (or the node it should live on) exists.

cast(agent, module, fun, args)

cast(agent, module, atom, [term]) :: :ok

Performs a cast (fire and forget) operation on the agent state.

Same as cast/2 but a module, function, and arguments are expected instead of an anonymous function. The state is added as first argument to the given list of arguments.

get(agent, fun, timeout \\ 5000)

get(agent, (state -> a), timeout) :: a when a: var

Gets an agent value via the given anonymous function.

The function fun is sent to the agent which invokes the function passing the agent state. The result of the function invocation is returned from this function.

timeout is an integer greater than zero which specifies how many milliseconds are allowed before the agent executes the function and returns the result value, or the atom :infinity to wait indefinitely. If no result is received within the specified time, the function call fails and the caller exits.

Examples

iex> {:ok, pid} = Agent.start_link(fn -> 42 end)
iex> Agent.get(pid, fn state -> state end)
42

get(agent, module, fun, args, timeout \\ 5000)

get(agent, module, atom, [term], timeout) :: any

Gets an agent value via the given function.

Same as get/3 but a module, function, and arguments are expected instead of an anonymous function. The state is added as first argument to the given list of arguments.

get_and_update(agent, fun, timeout \\ 5000)

get_and_update(agent, (state -> {a, state}), timeout) :: a when a: var

Gets and updates the agent state in one operation via the given anonymous function.

The function fun is sent to the agent which invokes the function passing the agent state. The function must return a tuple with two elements, the first being the value to return (that is, the “get” value) and the second one being the new state of the agent.

timeout is an integer greater than zero which specifies how many milliseconds are allowed before the agent executes the function and returns the result value, or the atom :infinity to wait indefinitely. If no result is received within the specified time, the function call fails and the caller exits.

Examples

iex> {:ok, pid} = Agent.start_link(fn -> 42 end)
iex> Agent.get_and_update(pid, fn state -> {state, state + 1} end)
42
iex> Agent.get(pid, fn state -> state end)
43

get_and_update(agent, module, fun, args, timeout \\ 5000)

get_and_update(agent, module, atom, [term], timeout) :: any

Gets and updates the agent state in one operation via the given function.

Same as get_and_update/3 but a module, function, and arguments are expected instead of an anonymous function. The state is added as first argument to the given list of arguments.

start(fun, options \\ [])

start((() -> term), GenServer.options) :: on_start

Starts an agent process without links (outside of a supervision tree).

See start_link/2 for more information.

Examples

iex> {:ok, pid} = Agent.start(fn -> 42 end)
iex> Agent.get(pid, fn(state) -> state end)
42

start(module, fun, args, options \\ [])

start(module, atom, [any], GenServer.options) :: on_start

Starts an agent without links with the given module, function, and arguments.

See start_link/4 for more information.

start_link(fun, options \\ [])

start_link((() -> term), GenServer.options) :: on_start

Starts an agent linked to the current process with the given function.

This is often used to start the agent as part of a supervision tree.

Once the agent is spawned, the given function fun is invoked and its return value is used as the agent state. Note that start_link/2 does not return until the given function has returned.

Options

The :name option is used for registration as described in the module documentation.

If the :timeout option is present, the agent is allowed to spend at most the given number of milliseconds on initialization or it will be terminated and the start function will return {:error, :timeout}.

If the :debug option is present, the corresponding function in the :sys module will be invoked.

If the :spawn_opt option is present, its value will be passed as options to the underlying process as in Process.spawn/4.

Return values

If the server is successfully created and initialized, the function returns {:ok, pid}, where pid is the PID of the server. If an agent with the specified name already exists, the function returns {:error, {:already_started, pid}} with the PID of that process.

If the given function callback fails, the function returns {:error, reason}.

Examples

iex> {:ok, pid} = Agent.start_link(fn -> 42 end)
iex> Agent.get(pid, fn state -> state end)
42

iex> {:error, {exception, _stacktrace}} = Agent.start(fn -> raise "oops" end)
iex> exception
%RuntimeError{message: "oops"}

start_link(module, fun, args, options \\ [])

start_link(module, atom, [any], GenServer.options) :: on_start

Starts an agent linked to the current process.

Same as start_link/2 but a module, function, and arguments are expected instead of an anonymous function; fun in module will be called with the given arguments args to initialize the state.

stop(agent, reason \\ :normal, timeout \\ :infinity)

stop(agent, reason :: term, timeout) :: :ok

Synchronously stops the agent with the given reason.

It returns :ok if the agent terminates with the given reason. If the agent terminates with another reason, the call will exit.

This function keeps OTP semantics regarding error reporting. If the reason is any other than :normal, :shutdown or {:shutdown, _}, an error report will be logged.

Examples

iex> {:ok, pid} = Agent.start_link(fn -> 42 end)
iex> Agent.stop(pid)
:ok

update(agent, fun, timeout \\ 5000)

update(agent, (state -> state), timeout) :: :ok

Updates the agent state via the given anonymous function.

The function fun is sent to the agent which invokes the function passing the agent state. The return value of fun becomes the new state of the agent.

This function always returns :ok.

timeout is an integer greater than zero which specifies how many milliseconds are allowed before the agent executes the function and returns the result value, or the atom :infinity to wait indefinitely. If no result is received within the specified time, the function call fails and the caller exits.

Examples

iex> {:ok, pid} = Agent.start_link(fn -> 42 end)
iex> Agent.update(pid, fn state -> state + 1 end)
:ok
iex> Agent.get(pid, fn state -> state end)
43

update(agent, module, fun, args, timeout \\ 5000)

update(agent, module, atom, [term], timeout) :: :ok

Updates the agent state via the given function.

Same as update/3 but a module, function, and arguments are expected instead of an anonymous function. The state is added as first argument to the given list of arguments.

© 2012–2017 Plataformatec
Licensed under the Apache License, Version 2.0.
https://hexdocs.pm/elixir/1.5.0/Agent.html