Elixir v1.9.1 Agent View Source

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, the following agent implements a counter:

defmodule Counter do
  use Agent

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

  def value do
    Agent.get(__MODULE__, & &1)
  end

  def increment do
    Agent.update(__MODULE__, &(&1 + 1))
  end
end

Usage would be:

Counter.start_link(0)
#=> {:ok, #PID<0.123.0>}

Counter.value()
#=> 0

Counter.increment()
#=> :ok

Counter.increment()
#=> :ok

Counter.value()
#=> 2

Thanks to the agent server process, the counter can be safely incremented concurrently.

Agents provide a segregation between the client and server APIs (similar to GenServers). In particular, the functions passed as arguments to the calls to Agent functions are invoked 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.

How to supervise

An Agent is most commonly started under a supervision tree. When we invoke use Agent, it automatically defines a child_spec/1 function that allows us to start the agent directly under a supervisor. To start an agent under a supervisor with an initial counter of 0, one may do:

children = [
  {Counter, 0}
]

Supervisor.start_link(children, strategy: :one_for_all)

While one could also simply pass the Counter as a child to the supervisor, such as:

children = [
  Counter # Same as {Counter, []}
]

Supervisor.start_link(children, strategy: :one_for_all)

The definition above wouldn't work for this particular example, as it would attempt to start the counter with an initial value of an empty list. However, this may be a viable option in your own agents. A common approach is to use a keyword list, as that would allow setting the initial value and giving a name to the counter process, for example:

def start_link(opts) do
  {initial_value, opts} = Keyword.pop(opts, :initial_value, 0)
  Agent.start_link(fn -> initial_value end, opts)
end

and then you can use Counter, {Counter, name: :my_counter} or even {Counter, initial_value: 0, name: :my_counter} as a child specification.

use Agent also accepts a list of options which configures the child specification and therefore how it runs under a supervisor. The generated child_spec/1 can be customized with the following options:

:id - the child specification identifier, defaults to the current module
:restart - when the child should be restarted, defaults to :permanent
:shutdown - how to shut down the child, either immediately or by giving it time to shut down

For example:

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

See the "Child specification" section in the Supervisor module for more detailed information. The @doc annotation immediately preceding use Agent will be attached to the generated child_spec/1 function.

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.

Link to this section 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.

child_spec(arg)

Returns a specification to start an agent under a supervisor.

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.

Link to this section Types

agent()

Specs

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

The agent reference

name()

Specs

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

The agent name

on_start()

Specs

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

Return values of start* functions

state()

Specs

state() :: term()

The agent state

Link to this section Functions

cast(agent, fun)

Specs

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)

Specs

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.

child_spec(arg)

(since 1.5.0)

Returns a specification to start an agent under a supervisor.

See the "Child specification" section in the Supervisor module for more detailed information.

get(agent, fun, timeout \\ 5000)

Specs

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)

Specs

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)

Specs

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.

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)

Specs

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 \\ [])

Specs

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 \\ [])

Specs

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 \\ [])

Specs

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 in the server process, and should return the initial 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 \\ [])

Specs

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)

Specs

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)

Specs

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.

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)

Specs

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.