Elixir v1.3.0-rc.0 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, 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
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
Note that agents still provide a segregation between the client and server APIs, as seen in GenServers. In particular, all code inside the function passed to the agent is executed by the agent. This distinction is important because you may want to avoid expensive operations inside the agent, as it 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. The difference 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.
Name Registration
An Agent is bound to the same name registration rules as GenServers.
Read more about it in the GenServer
docs.
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 args tuple to the update instruction. For example, imagine
you have an agent named :sample
and you want to convert its inner state
from some dict structure 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 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
Gets an agent value via the given function
Gets an agent value via the given function
Gets and updates the agent state in one operation
Gets and updates the agent state in one operation
Starts an agent process without links (outside of a supervision tree)
Starts an agent 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 with the given module function and arguments
Stops the agent with the given reason
Updates the agent state
Updates the agent state
Link to this section Types
The agent reference
The agent name
on_start :: {:ok, pid} | {:error, {:already_started, pid} | term}
Return values of start*
functions
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 function must return the new state.
Note that cast
returns :ok
immediately, regardless of whether the
destination node or agent exists.
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 args are expected
instead of an anonymous function. The state is added as first
argument to the given list of args.
Gets an agent value via the given 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.
A timeout can also be specified (it has a default value of 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 args are expected
instead of an anonymous function. The state is added as first
argument to the given list of args.
Gets and updates the agent state in one operation.
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 (i.e. the get
value)
and the second one is the new state.
A timeout can also be specified (it has a default value of 5000).
get_and_update(agent, module, atom, [term], timeout) :: any
Gets and updates the agent state in one operation.
Same as get_and_update/3
but a module, function and args are expected
instead of an anonymous function. The state is added as first
argument to the given list of args.
start((() -> term), GenServer.options) :: on_start
Starts an agent process without links (outside of a supervision tree).
See start_link/2
for more information.
start(module, atom, [any], GenServer.options) :: on_start
Starts an agent with the given module function and arguments.
Similar to start/2
but a module, function and args are expected
instead of an anonymous function.
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 is invoked and its return
value is used as the agent state. Note that start_link
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 with reason
, the function returns
{:error, reason}
.
start_link(module, atom, [any], GenServer.options) :: on_start
Starts an agent linked to the current process with the given module function and arguments.
Same as start_link/2
but a module, function and args are expected
instead of an anonymous function.
stop(agent, reason :: term, timeout) :: :ok
Stops the agent with the given reason
.
It returns :ok
if the server terminates with the given
reason, if it 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.
Updates the agent state.
The function fun
is sent to the agent
which invokes the function
passing the agent state. The function must return the new state.
A timeout can also be specified (it has a default value of 5000).
This function always returns :ok
.
update(agent, module, atom, [term], timeout) :: :ok
Updates the agent state.
Same as update/3
but a module, function and args are expected
instead of an anonymous function. The state is added as first
argument to the given list of args.