View Source GenServer behaviour (Elixir v1.17.1)

A behaviour module for implementing the server of a client-server relation.

A GenServer is a process like any other Elixir process and it can be used to keep state, execute code asynchronously and so on. The advantage of using a generic server process (GenServer) implemented using this module is that it will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into a supervision tree.

graph BT
    C(Client #3) ~~~ B(Client #2) ~~~ A(Client #1)
    A & B & C -->|request| GenServer
    GenServer -.->|reply| A & B & C

Example

The GenServer behaviour abstracts the common client-server interaction. Developers are only required to implement the callbacks and functionality they are interested in.

Let's start with a code example and then explore the available callbacks. Imagine we want to implement a service with a GenServer that works like a stack, allowing us to push and pop elements. We'll customize a generic GenServer with our own module by implementing three callbacks.

init/1 transforms our initial argument to the initial state for the GenServer. handle_call/3 fires when the server receives a synchronous pop message, popping an element from the stack and returning it to the user. handle_cast/2 will fire when the server receives an asynchronous push message, pushing an element onto the stack:

defmodule Stack do
  use GenServer

  # Callbacks

  @impl true
  def init(elements) do
    initial_state = String.split(elements, ",", trim: true)
    {:ok, initial_state}
  end

  @impl true
  def handle_call(:pop, _from, state) do
    [to_caller | new_state] = state
    {:reply, to_caller, new_state}
  end

  @impl true
  def handle_cast({:push, element}, state) do
    new_state = [element | state]
    {:noreply, new_state}
  end
end

We leave the process machinery of startup, message passing, and the message loop to the GenServer behaviour and focus only on the stack implementation. We can now use the GenServer API to interact with the service by creating a process and sending it messages:

# Start the server
{:ok, pid} = GenServer.start_link(Stack, "hello,world")

# This is the client
GenServer.call(pid, :pop)
#=> "hello"

GenServer.cast(pid, {:push, "elixir"})
#=> :ok

GenServer.call(pid, :pop)
#=> "elixir"

We start our Stack by calling start_link/2, passing the module with the server implementation and its initial argument with a comma-separated list of elements. The GenServer behaviour calls the init/1 callback to establish the initial GenServer state. From this point on, the GenServer has control so we interact with it by sending two types of messages on the client. call messages expect a reply from the server (and are therefore synchronous) while cast messages do not.

Each call to GenServer.call/3 results in a message that must be handled by the handle_call/3 callback in the GenServer. A cast/2 message must be handled by handle_cast/2. GenServer supports 8 callbacks, but only init/1 is required.

use GenServer

When you use GenServer, the GenServer module will set @behaviour GenServer and define a child_spec/1 function, so your module can be used as a child in a supervision tree.

Client / Server APIs

Although in the example above we have used GenServer.start_link/3 and friends to directly start and communicate with the server, most of the time we don't call the GenServer functions directly. Instead, we wrap the calls in new functions representing the public API of the server. These thin wrappers are called the client API.

Here is a better implementation of our Stack module:

defmodule Stack do
  use GenServer

  # Client

  def start_link(default) when is_binary(default) do
    GenServer.start_link(__MODULE__, default)
  end

  def push(pid, element) do
    GenServer.cast(pid, {:push, element})
  end

  def pop(pid) do
    GenServer.call(pid, :pop)
  end

  # Server (callbacks)

  @impl true
  def init(elements) do
    initial_state = String.split(elements, ",", trim: true)
    {:ok, initial_state}
  end

  @impl true
  def handle_call(:pop, _from, state) do
    [to_caller | new_state] = state
    {:reply, to_caller, new_state}
  end

  @impl true
  def handle_cast({:push, element}, state) do
    new_state = [element | state]
    {:noreply, new_state}
  end
end

In practice, it is common to have both server and client functions in the same module. If the server and/or client implementations are growing complex, you may want to have them in different modules.

The following diagram summarizes the interactions between client and server. Both Client and Server are processes and communication happens via messages (continuous line). The Server <-> Module interaction happens when the GenServer process calls your code (dotted lines):

sequenceDiagram
    participant C as Client (Process)
    participant S as Server (Process)
    participant M as Module (Code)

    note right of C: Typically started by a supervisor
    C->>+S: GenServer.start_link(module, arg, options)
    S-->>+M: init(arg)
    M-->>-S: {:ok, state} | :ignore | {:error, reason}
    S->>-C: {:ok, pid} | :ignore | {:error, reason}

    note right of C: call is synchronous
    C->>+S: GenServer.call(pid, message)
    S-->>+M: handle_call(message, from, state)
    M-->>-S: {:reply, reply, state} | {:stop, reason, reply, state}
    S->>-C: reply

    note right of C: cast is asynchronous
    C-)S: GenServer.cast(pid, message)
    S-->>+M: handle_cast(message, state)
    M-->>-S: {:noreply, state} | {:stop, reason, state}

    note right of C: send is asynchronous
    C-)S: Kernel.send(pid, message)
    S-->>+M: handle_info(message, state)
    M-->>-S: {:noreply, state} | {:stop, reason, state}

How to supervise

A GenServer is most commonly started under a supervision tree. When we invoke use GenServer, it automatically defines a child_spec/1 function that allows us to start the Stack directly under a supervisor. To start a default stack of ["hello", "world"] under a supervisor, we can do:

children = [
  {Stack, "hello,world"}
]

Supervisor.start_link(children, strategy: :one_for_all)

Note that specifying a module MyServer would be the same as specifying the tuple {MyServer, []}.

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

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

When stopping the GenServer, for example by returning a {:stop, reason, new_state} tuple from a callback, the exit reason is used by the supervisor to determine whether the GenServer needs to be restarted. See the "Exit reasons and restarts" section in the Supervisor module.

Name registration

Both start_link/3 and start/3 support the GenServer to register a name on start via the :name option. Registered names are also automatically cleaned up on termination. The supported values are:

  • an atom - the GenServer is registered locally (to the current node) with the given name using Process.register/2.

  • {:global, term} - the GenServer is registered globally with the given term using the functions in the :global module.

  • {:via, module, term} - the GenServer is registered with the given mechanism and name. The :via option expects a module that exports register_name/2, unregister_name/1, whereis_name/1 and send/2. One such example is the :global module which uses these functions for keeping the list of names of processes and their associated PIDs that are available globally for a network of Elixir nodes. Elixir also ships with a local, decentralized and scalable registry called Registry for locally storing names that are generated dynamically.

For example, we could start and register our Stack server locally as follows:

# Start the server and register it locally with name MyStack
{:ok, _} = GenServer.start_link(Stack, "hello", name: MyStack)

# Now messages can be sent directly to MyStack
GenServer.call(MyStack, :pop)
#=> "hello"

Once the server is started, the remaining functions in this module (call/3, cast/2, and friends) will also accept an atom, or any {:global, ...} or {:via, ...} tuples. In general, the following formats are supported:

  • a PID
  • an atom if the server is locally registered
  • {atom, node} if the server is locally registered at another node
  • {:global, term} if the server is globally registered
  • {:via, module, name} if the server is registered through an alternative registry

If there is an interest to register dynamic names locally, do not use atoms, as atoms are never garbage-collected and therefore dynamically generated atoms won't be garbage-collected. For such cases, you can set up your own local registry by using the Registry module.

Receiving "regular" messages

The goal of a GenServer is to abstract the "receive" loop for developers, automatically handling system messages, supporting code change, synchronous calls and more. Therefore, you should never call your own "receive" inside the GenServer callbacks as doing so will cause the GenServer to misbehave.

Besides the synchronous and asynchronous communication provided by call/3 and cast/2, "regular" messages sent by functions such as send/2, Process.send_after/4 and similar, can be handled inside the handle_info/2 callback.

handle_info/2 can be used in many situations, such as handling monitor DOWN messages sent by Process.monitor/1. Another use case for handle_info/2 is to perform periodic work, with the help of Process.send_after/4:

defmodule MyApp.Periodically do
  use GenServer

  def start_link(_) do
    GenServer.start_link(__MODULE__, %{})
  end

  @impl true
  def init(state) do
    # Schedule work to be performed on start
    schedule_work()

    {:ok, state}
  end

  @impl true
  def handle_info(:work, state) do
    # Do the desired work here
    # ...

    # Reschedule once more
    schedule_work()

    {:noreply, state}
  end

  defp schedule_work do
    # We schedule the work to happen in 2 hours (written in milliseconds).
    # Alternatively, one might write :timer.hours(2)
    Process.send_after(self(), :work, 2 * 60 * 60 * 1000)
  end
end

Timeouts

The return value of init/1 or any of the handle_* callbacks may include a timeout value in milliseconds; if not, :infinity is assumed. The timeout can be used to detect a lull in incoming messages.

The timeout() value is used as follows:

  • If the process has any message already waiting when the timeout() value is returned, the timeout is ignored and the waiting message is handled as usual. This means that even a timeout of 0 milliseconds is not guaranteed to execute (if you want to take another action immediately and unconditionally, use a :continue instruction instead).

  • If any message arrives before the specified number of milliseconds elapse, the timeout is cleared and that message is handled as usual.

  • Otherwise, when the specified number of milliseconds have elapsed with no message arriving, handle_info/2 is called with :timeout as the first argument.

When (not) to use a GenServer

So far, we have learned that a GenServer can be used as a supervised process that handles sync and async calls. It can also handle system messages, such as periodic messages and monitoring events. GenServer processes may also be named.

A GenServer, or a process in general, must be used to model runtime characteristics of your system. A GenServer must never be used for code organization purposes.

In Elixir, code organization is done by modules and functions, processes are not necessary. For example, imagine you are implementing a calculator and you decide to put all the calculator operations behind a GenServer:

def add(a, b) do
  GenServer.call(__MODULE__, {:add, a, b})
end

def subtract(a, b) do
  GenServer.call(__MODULE__, {:subtract, a, b})
end

def handle_call({:add, a, b}, _from, state) do
  {:reply, a + b, state}
end

def handle_call({:subtract, a, b}, _from, state) do
  {:reply, a - b, state}
end

This is an anti-pattern not only because it convolutes the calculator logic but also because you put the calculator logic behind a single process that will potentially become a bottleneck in your system, especially as the number of calls grow. Instead just define the functions directly:

def add(a, b) do
  a + b
end

def subtract(a, b) do
  a - b
end

If you don't need a process, then you don't need a process. Use processes only to model runtime properties, such as mutable state, concurrency and failures, never for code organization.

Debugging with the :sys module

GenServers, as special processes, can be debugged using the :sys module. Through various hooks, this module allows developers to introspect the state of the process and trace system events that happen during its execution, such as received messages, sent replies and state changes.

Let's explore the basic functions from the :sys module used for debugging:

  • :sys.get_state/2 - allows retrieval of the state of the process. In the case of a GenServer process, it will be the callback module state, as passed into the callback functions as last argument.
  • :sys.get_status/2 - allows retrieval of the status of the process. This status includes the process dictionary, if the process is running or is suspended, the parent PID, the debugger state, and the state of the behaviour module, which includes the callback module state (as returned by :sys.get_state/2). It's possible to change how this status is represented by defining the optional GenServer.format_status/1 callback.
  • :sys.trace/3 - prints all the system events to :stdio.
  • :sys.statistics/3 - manages collection of process statistics.
  • :sys.no_debug/2 - turns off all debug handlers for the given process. It is very important to switch off debugging once we're done. Excessive debug handlers or those that should be turned off, but weren't, can seriously damage the performance of the system.
  • :sys.suspend/2 - allows to suspend a process so that it only replies to system messages but no other messages. A suspended process can be reactivated via :sys.resume/2.

Let's see how we could use those functions for debugging the stack server we defined earlier.

iex> {:ok, pid} = Stack.start_link([])
iex> :sys.statistics(pid, true) # turn on collecting process statistics
iex> :sys.trace(pid, true) # turn on event printing
iex> Stack.push(pid, 1)
*DBG* <0.122.0> got cast {push,1}
*DBG* <0.122.0> new state [1]
:ok

iex> :sys.get_state(pid)
[1]

iex> Stack.pop(pid)
*DBG* <0.122.0> got call pop from <0.80.0>
*DBG* <0.122.0> sent 1 to <0.80.0>, new state []
1

iex> :sys.statistics(pid, :get)
{:ok,
 [
   start_time: {{2016, 7, 16}, {12, 29, 41}},
   current_time: {{2016, 7, 16}, {12, 29, 50}},
   reductions: 117,
   messages_in: 2,
   messages_out: 0
 ]}

iex> :sys.no_debug(pid) # turn off all debug handlers
:ok

iex> :sys.get_status(pid)
{:status, #PID<0.122.0>, {:module, :gen_server},
 [
   [
     "$initial_call": {Stack, :init, 1},            # process dictionary
     "$ancestors": [#PID<0.80.0>, #PID<0.51.0>]
   ],
   :running,                                        # :running | :suspended
   #PID<0.80.0>,                                    # parent
   [],                                              # debugger state
   [
     header: 'Status for generic server <0.122.0>', # module status
     data: [
       {'Status', :running},
       {'Parent', #PID<0.80.0>},
       {'Logged events', []}
     ],
     data: [{'State', [1]}]
   ]
 ]}

Learn more

If you wish to find out more about GenServers, the Elixir Getting Started guide provides a tutorial-like introduction. The documentation and links in Erlang can also provide extra insight.

Summary

Types

Debug options supported by the start* functions

Tuple describing the client of a call request.

The GenServer name

Return values of start* functions

Option values used by the start* functions

Options used by the start* functions

The server reference.

Callbacks

Invoked to change the state of the GenServer when a different version of a module is loaded (hot code swapping) and the state's term structure should be changed.

This function is called by a GenServer process in the following situations

Invoked to handle synchronous call/3 messages. call/3 will block until a reply is received (unless the call times out or nodes are disconnected).

Invoked to handle asynchronous cast/2 messages.

Invoked to handle continue instructions.

Invoked to handle all other messages.

Invoked when the server is started. start_link/3 or start/3 will block until it returns.

Invoked when the server is about to exit. It should do any cleanup required.

Functions

Casts all servers locally registered as name at the specified nodes.

Makes a synchronous call to the server and waits for its reply.

Casts a request to the server without waiting for a response.

Calls all servers locally registered as name at the specified nodes.

Replies to a client.

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

Starts a GenServer process linked to the current process.

Synchronously stops the server with the given reason.

Returns the pid or {name, node} of a GenServer process, nil otherwise.

Types

@type debug() :: [:trace | :log | :statistics | {:log_to_file, Path.t()}]

Debug options supported by the start* functions

@type from() :: {pid(), tag :: term()}

Tuple describing the client of a call request.

pid is the PID of the caller and tag is a unique term used to identify the call.

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

The GenServer name

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

Return values of start* functions

@type option() ::
  {:debug, debug()}
  | {:name, name()}
  | {:timeout, timeout()}
  | {:spawn_opt, [Process.spawn_opt()]}
  | {:hibernate_after, timeout()}

Option values used by the start* functions

@type options() :: [option()]

Options used by the start* functions

@type server() :: pid() | name() | {atom(), node()}

The server reference.

This is either a plain PID or a value representing a registered name. See the "Name registration" section of this document for more information.

Callbacks

Link to this callback

code_change(old_vsn, state, extra)

View Source (optional)
@callback code_change(old_vsn, state :: term(), extra :: term()) ::
  {:ok, new_state :: term()} | {:error, reason :: term()}
when old_vsn: term() | {:down, term()}

Invoked to change the state of the GenServer when a different version of a module is loaded (hot code swapping) and the state's term structure should be changed.

old_vsn is the previous version of the module (defined by the @vsn attribute) when upgrading. When downgrading the previous version is wrapped in a 2-tuple with first element :down. state is the current state of the GenServer and extra is any extra data required to change the state.

Returning {:ok, new_state} changes the state to new_state and the code change is successful.

Returning {:error, reason} fails the code change with reason reason and the state remains as the previous state.

If code_change/3 raises the code change fails and the loop will continue with its previous state. Therefore this callback does not usually contain side effects.

This callback is optional.

Link to this callback

format_status(status)

View Source (optional) (since 1.17.0)
@callback format_status(status :: :gen_server.format_status()) ::
  new_status :: :gen_server.format_status()

This function is called by a GenServer process in the following situations:

This callback is used to limit the status of the process returned by :sys.get_status/1,2 or sent to logger.

The callback gets a map status describing the current status and shall return a map new_status with the same keys, but it may transform some values.

Two possible use cases for this callback is to remove sensitive information from the state to prevent it from being printed in log files, or to compact large irrelevant status items that would only clutter the logs.

Example

@impl GenServer
def format_status(status) do
  Map.new(status, fn
    {:state, state} -> {:state, Map.delete(state, :private_key)}
    {:message, {:password, _}} -> {:message, {:password, "redacted"}}
    key_value -> key_value
  end)
end
Link to this callback

format_status(reason, pdict_and_state)

View Source (optional)
This callback is deprecated. Use format_status/1 callback instead.
@callback format_status(reason, pdict_and_state :: list()) :: term()
when reason: :normal | :terminate
Link to this callback

handle_call(request, from, state)

View Source (optional)
@callback handle_call(request :: term(), from(), state :: term()) ::
  {:reply, reply, new_state}
  | {:reply, reply, new_state,
     timeout() | :hibernate | {:continue, continue_arg :: term()}}
  | {:noreply, new_state}
  | {:noreply, new_state,
     timeout() | :hibernate | {:continue, continue_arg :: term()}}
  | {:stop, reason, reply, new_state}
  | {:stop, reason, new_state}
when reply: term(), new_state: term(), reason: term()

Invoked to handle synchronous call/3 messages. call/3 will block until a reply is received (unless the call times out or nodes are disconnected).

request is the request message sent by a call/3, from is a 2-tuple containing the caller's PID and a term that uniquely identifies the call, and state is the current state of the GenServer.

Returning {:reply, reply, new_state} sends the response reply to the caller and continues the loop with new state new_state.

Returning {:reply, reply, new_state, timeout} is similar to {:reply, reply, new_state} except that it also sets a timeout. See the "Timeouts" section in the module documentation for more information.

Returning {:reply, reply, new_state, :hibernate} is similar to {:reply, reply, new_state} except the process is hibernated and will continue the loop once a message is in its message queue. However, if a message is already in the message queue, the process will continue the loop immediately. Hibernating a GenServer causes garbage collection and leaves a continuous heap that minimises the memory used by the process.

Hibernating should not be used aggressively as too much time could be spent garbage collecting, which would delay the processing of incoming messages. Normally it should only be used when you are not expecting new messages to immediately arrive and minimising the memory of the process is shown to be beneficial.

Returning {:reply, reply, new_state, {:continue, continue_arg}} is similar to {:reply, reply, new_state} except that handle_continue/2 will be invoked immediately after with continue_arg as the first argument and state as the second one.

Returning {:noreply, new_state} does not send a response to the caller and continues the loop with new state new_state. The response must be sent with reply/2.

There are three main use cases for not replying using the return value:

  • To reply before returning from the callback because the response is known before calling a slow function.
  • To reply after returning from the callback because the response is not yet available.
  • To reply from another process, such as a task.

When replying from another process the GenServer should exit if the other process exits without replying as the caller will be blocking awaiting a reply.

Returning {:noreply, new_state, timeout | :hibernate | {:continue, continue_arg}} is similar to {:noreply, new_state} except a timeout, hibernation or continue occurs as with a :reply tuple.

Returning {:stop, reason, reply, new_state} stops the loop and terminate/2 is called with reason reason and state new_state. Then, the reply is sent as the response to call and the process exits with reason reason.

Returning {:stop, reason, new_state} is similar to {:stop, reason, reply, new_state} except a reply is not sent.

This callback is optional. If one is not implemented, the server will fail if a call is performed against it.

Link to this callback

handle_cast(request, state)

View Source (optional)
@callback handle_cast(request :: term(), state :: term()) ::
  {:noreply, new_state}
  | {:noreply, new_state,
     timeout() | :hibernate | {:continue, continue_arg :: term()}}
  | {:stop, reason :: term(), new_state}
when new_state: term()

Invoked to handle asynchronous cast/2 messages.

request is the request message sent by a cast/2 and state is the current state of the GenServer.

Returning {:noreply, new_state} continues the loop with new state new_state.

Returning {:noreply, new_state, timeout} is similar to {:noreply, new_state} except that it also sets a timeout. See the "Timeouts" section in the module documentation for more information.

Returning {:noreply, new_state, :hibernate} is similar to {:noreply, new_state} except the process is hibernated before continuing the loop. See handle_call/3 for more information.

Returning {:noreply, new_state, {:continue, continue_arg}} is similar to {:noreply, new_state} except handle_continue/2 will be invoked immediately after with continue_arg as the first argument and state as the second one.

Returning {:stop, reason, new_state} stops the loop and terminate/2 is called with the reason reason and state new_state. The process exits with reason reason.

This callback is optional. If one is not implemented, the server will fail if a cast is performed against it.

Link to this callback

handle_continue(continue_arg, state)

View Source (optional)
@callback handle_continue(continue_arg, state :: term()) ::
  {:noreply, new_state}
  | {:noreply, new_state, timeout() | :hibernate | {:continue, continue_arg}}
  | {:stop, reason :: term(), new_state}
when new_state: term(), continue_arg: term()

Invoked to handle continue instructions.

It is useful for performing work after initialization or for splitting the work in a callback in multiple steps, updating the process state along the way.

Return values are the same as handle_cast/2.

This callback is optional. If one is not implemented, the server will fail if a continue instruction is used.

Link to this callback

handle_info(msg, state)

View Source (optional)
@callback handle_info(msg :: :timeout | term(), state :: term()) ::
  {:noreply, new_state}
  | {:noreply, new_state,
     timeout() | :hibernate | {:continue, continue_arg :: term()}}
  | {:stop, reason :: term(), new_state}
when new_state: term()

Invoked to handle all other messages.

msg is the message and state is the current state of the GenServer. When a timeout occurs the message is :timeout.

Return values are the same as handle_cast/2.

This callback is optional. If one is not implemented, the received message will be logged.

@callback init(init_arg :: term()) ::
  {:ok, state}
  | {:ok, state, timeout() | :hibernate | {:continue, continue_arg :: term()}}
  | :ignore
  | {:stop, reason :: any()}
when state: any()

Invoked when the server is started. start_link/3 or start/3 will block until it returns.

init_arg is the argument term (second argument) passed to start_link/3.

Returning {:ok, state} will cause start_link/3 to return {:ok, pid} and the process to enter its loop.

Returning {:ok, state, timeout} is similar to {:ok, state}, except that it also sets a timeout. See the "Timeouts" section in the module documentation for more information.

Returning {:ok, state, :hibernate} is similar to {:ok, state} except the process is hibernated before entering the loop. See handle_call/3 for more information on hibernation.

Returning {:ok, state, {:continue, continue_arg}} is similar to {:ok, state} except that immediately after entering the loop, the handle_continue/2 callback will be invoked with continue_arg as the first argument and state as the second one.

Returning :ignore will cause start_link/3 to return :ignore and the process will exit normally without entering the loop or calling terminate/2. If used when part of a supervision tree the parent supervisor will not fail to start nor immediately try to restart the GenServer. The remainder of the supervision tree will be started and so the GenServer should not be required by other processes. It can be started later with Supervisor.restart_child/2 as the child specification is saved in the parent supervisor. The main use cases for this are:

  • The GenServer is disabled by configuration but might be enabled later.
  • An error occurred and it will be handled by a different mechanism than the Supervisor. Likely this approach involves calling Supervisor.restart_child/2 after a delay to attempt a restart.

Returning {:stop, reason} will cause start_link/3 to return {:error, reason} and the process to exit with reason reason without entering the loop or calling terminate/2.

Link to this callback

terminate(reason, state)

View Source (optional)
@callback terminate(reason, state :: term()) :: term()
when reason: :normal | :shutdown | {:shutdown, term()} | term()

Invoked when the server is about to exit. It should do any cleanup required.

reason is exit reason and state is the current state of the GenServer. The return value is ignored.

terminate/2 is useful for cleanup that requires access to the GenServer's state. However, it is not guaranteed that terminate/2 is called when a GenServer exits. Therefore, important cleanup should be done using process links and/or monitors. A monitoring process will receive the same exit reason that would be passed to terminate/2.

terminate/2 is called if:

  • the GenServer traps exits (using Process.flag/2) and the parent process (the one which called start_link/1) sends an exit signal

  • a callback (except init/1) does one of the following:

    • returns a :stop tuple

    • raises (via raise/2) or exits (via exit/1)

    • returns an invalid value

If part of a supervision tree, a GenServer will receive an exit signal from its parent process (its supervisor) when the tree is shutting down. The exit signal is based on the shutdown strategy in the child's specification, where this value can be:

  • :brutal_kill: the GenServer is killed and so terminate/2 is not called.

  • a timeout value, where the supervisor will send the exit signal :shutdown and the GenServer will have the duration of the timeout to terminate. If after duration of this timeout the process is still alive, it will be killed immediately.

For a more in-depth explanation, please read the "Shutdown values (:shutdown)" section in the Supervisor module.

If the GenServer receives an exit signal (that is not :normal) from any process when it is not trapping exits it will exit abruptly with the same reason and so not call terminate/2. Note that a process does NOT trap exits by default and an exit signal is sent when a linked process exits or its node is disconnected.

terminate/2 is only called after the GenServer finishes processing all messages which arrived in its mailbox prior to the exit signal. If it receives a :kill signal before it finishes processing those, terminate/2 will not be called. If terminate/2 is called, any messages received after the exit signal will still be in the mailbox.

There is no cleanup needed when the GenServer controls a port (for example, :gen_tcp.socket) or File.io_device/0, because these will be closed on receiving a GenServer's exit signal and do not need to be closed manually in terminate/2.

If reason is neither :normal, :shutdown, nor {:shutdown, term} an error is logged.

This callback is optional.

Functions

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abcast(nodes \\ [node() | Node.list()], name, request)

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@spec abcast([node()], name :: atom(), term()) :: :abcast

Casts all servers locally registered as name at the specified nodes.

This function returns immediately and ignores nodes that do not exist, or where the server name does not exist.

See multi_call/4 for more information.

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call(server, request, timeout \\ 5000)

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@spec call(server(), term(), timeout()) :: term()

Makes a synchronous call to the server and waits for its reply.

The client sends the given request to the server and waits until a reply arrives or a timeout occurs. handle_call/3 will be called on the server to handle the request.

server can be any of the values described in the "Name registration" section of the documentation for this module.

Timeouts

timeout is an integer greater than zero which specifies how many milliseconds to wait for a reply, or the atom :infinity to wait indefinitely. The default value is 5000. If no reply is received within the specified time, the function call fails and the caller exits. If the caller catches the failure and continues running, and the server is just late with the reply, it may arrive at any time later into the caller's message queue. The caller must in this case be prepared for this and discard any such garbage messages that are two-element tuples with a reference as the first element.

@spec cast(server(), term()) :: :ok

Casts a request to the server without waiting for a response.

This function always returns :ok regardless of whether the destination server (or node) exists. Therefore it is unknown whether the destination server successfully handled the request.

server can be any of the values described in the "Name registration" section of the documentation for this module.

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multi_call(nodes \\ [node() | Node.list()], name, request, timeout \\ :infinity)

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@spec multi_call([node()], name :: atom(), term(), timeout()) ::
  {replies :: [{node(), term()}], bad_nodes :: [node()]}

Calls all servers locally registered as name at the specified nodes.

First, the request is sent to every node in nodes; then, the caller waits for the replies. This function returns a two-element tuple {replies, bad_nodes} where:

  • replies - is a list of {node, reply} tuples where node is the node that replied and reply is its reply
  • bad_nodes - is a list of nodes that either did not exist or where a server with the given name did not exist or did not reply

nodes is a list of node names to which the request is sent. The default value is the list of all known nodes (including this node).

Examples

Assuming the Stack GenServer mentioned in the docs for the GenServer module is registered as Stack in the :"foo@my-machine" and :"bar@my-machine" nodes:

GenServer.multi_call(Stack, :pop)
#=> {[{:"foo@my-machine", :hello}, {:"bar@my-machine", :world}], []}
@spec reply(from(), term()) :: :ok

Replies to a client.

This function can be used to explicitly send a reply to a client that called call/3 or multi_call/4 when the reply cannot be specified in the return value of handle_call/3.

client must be the from argument (the second argument) accepted by handle_call/3 callbacks. reply is an arbitrary term which will be given back to the client as the return value of the call.

Note that reply/2 can be called from any process, not just the GenServer that originally received the call (as long as that GenServer communicated the from argument somehow).

This function always returns :ok.

Examples

def handle_call(:reply_in_one_second, from, state) do
  Process.send_after(self(), {:reply, from}, 1_000)
  {:noreply, state}
end

def handle_info({:reply, from}, state) do
  GenServer.reply(from, :one_second_has_passed)
  {:noreply, state}
end
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start(module, init_arg, options \\ [])

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@spec start(module(), any(), options()) :: on_start()

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

See start_link/3 for more information.

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start_link(module, init_arg, options \\ [])

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@spec start_link(module(), any(), options()) :: on_start()

Starts a GenServer process linked to the current process.

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

Once the server is started, the init/1 function of the given module is called with init_arg as its argument to initialize the server. To ensure a synchronized start-up procedure, this function does not return until init/1 has returned.

Note that a GenServer started with start_link/3 is linked to the parent process and will exit in case of crashes from the parent. The GenServer will also exit due to the :normal reasons in case it is configured to trap exits in the init/1 callback.

Options

  • :name - used for name registration as described in the "Name registration" section in the documentation for GenServer

  • :timeout - if present, the server is allowed to spend the given number of milliseconds initializing or it will be terminated and the start function will return {:error, :timeout}

  • :debug - if present, the corresponding function in the :sys module is invoked

  • :spawn_opt - if present, its value is passed as options to the underlying process as in Process.spawn/4

  • :hibernate_after - if present, the GenServer process awaits any message for the given number of milliseconds and if no message is received, the process goes into hibernation automatically (by calling :proc_lib.hibernate/3).

Return values

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

If the init/1 callback fails with reason, this function returns {:error, reason}. Otherwise, if it returns {:stop, reason} or :ignore, the process is terminated and this function returns {:error, reason} or :ignore, respectively.

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

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@spec stop(server(), reason :: term(), timeout()) :: :ok

Synchronously stops the server with the given reason.

The terminate/2 callback of the given server will be invoked before exiting. This function returns :ok if the server terminates with the given reason; if it terminates with another reason, the call exits.

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

@spec whereis(server()) :: pid() | {atom(), node()} | nil

Returns the pid or {name, node} of a GenServer process, nil otherwise.

To be precise, nil is returned whenever a pid or {name, node} cannot be returned. Note there is no guarantee the returned pid or {name, node} is alive, as a process could terminate immediately after it is looked up.

Examples

For example, to lookup a server process, monitor it and send a cast to it:

process = GenServer.whereis(server)
monitor = Process.monitor(process)
GenServer.cast(process, :hello)