View Source Agent (Elixir v1.14.1)
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
GenServer
s). 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
Functions
Performs a cast (fire and forget) operation on the agent state.
Performs a cast (fire and forget) operation on the agent state.
Returns a specification to start an agent under a supervisor.
Gets an agent value via the given anonymous function.
Gets an agent value via the given function.
Gets and updates the agent state in one operation via the given anonymous function.
Gets and updates the agent state in one operation via the given function.
Starts an agent process without links (outside of a supervision tree).
Starts an agent without links with the given module, function, and arguments.
Starts an agent linked to the current process with the given function.
Starts an agent linked to the current process.
Synchronously stops the agent with the given reason
.
Updates the agent state via the given anonymous function.
Updates the agent state via the given function.
Link to this section Types
The agent reference
The agent name
Return values of start*
functions
@type state() :: term()
The agent state
Link to this section Functions
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.
Examples
iex> {:ok, pid} = Agent.start_link(fn -> 42 end)
iex> Agent.cast(pid, fn state -> state + 1 end)
:ok
iex> Agent.get(pid, fn state -> state end)
43
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.
Examples
iex> {:ok, pid} = Agent.start_link(fn -> 42 end)
iex> Agent.cast(pid, Kernel, :+, [12])
:ok
iex> Agent.get(pid, fn state -> state end)
54
Returns a specification to start an agent under a supervisor.
See the "Child specification" section in the Supervisor
module for more detailed information.
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
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.
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
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.
@spec 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
@spec 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.
@spec 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"}
@spec 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.
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
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
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.
Examples
iex> {:ok, pid} = Agent.start_link(fn -> 42 end)
iex> Agent.update(pid, Kernel, :+, [12])
:ok
iex> Agent.get(pid, fn state -> state end)
54