GenStatem behaviour (gen_statem v0.1.0)

View Source

A behaviour module for implementing the generic state machine Documentation for GenStatem.

Features

Two callback modes are supported:

  • state_functions - for finite-state machine which requires the state to be an atom and uses that state as the name of the current callback function, arity 3.
  • handle_event_function - that allows the state to be any term and that uses handle_event/4 as callback function for all states.

Callback module

A GenStatem assumes all specific parts to be located in a callback module exporting a predefined set of functions. The relationship between the behavior functions and the callback functions is as follows:

GenStatem module            Callback module
-----------------            ---------------
GenStatem.start
GenStatem.start_monitor
GenStatem.start_link -----> Module.init/1

Server start or code change
                      -----> Module.callback_mode/0
                      selects callback mode

GenStatem.stop
Supervisor exit
Callback failure      -----> Module.terminate/3

GenStatem.call
GenStatem.cast
GenStatem.send_request
erlang.send
erlang.'!'            -----> Module.state_name/3
                   or -----> Module.handle_event/4
                   depending on callback mode

Release upgrade/downgrade
(code change)
                      -----> Module.code_change/4

State callback

The state callback for a specific state in a GenStatem is the callback function that is called for all events in this state. It is selected depending on which callback mode that the callback module defines with the callback function Module.callback_mode/0.

When the callback mode is state_functions, the state must be an atom and is used as the state callback name; see Module.state_name/3. This co-locates all code for a specific state in one function as the GenStatem engine branches depending on state name. Note the fact that the callback function Module.terminate/3 makes the state name terminate unusable in this mode.

When the callback mode is handle_event_function, the state can be any term and the state callback name is Module.handle_event/4. This makes it easy to branch depending on state or event as you desire. Be careful about which events you handle in which states so that you do not accidentally postpone an event forever creating an infinite busy loop.

Event types

Events are of different types, therefore the callback functions can know the origin of an event when handling it. External events are :call, :cast, and :info. Internal events are timeout and :internal.

Event handling

When GenStatem receives a process message it is transformed into an event and the state callback is called with the event as two arguments: type and content. When the state callback has processed the event it returns to GenStatem which does a state transition. If this state transition is to a different state, that is: next_state !== state, it is a state change.

Transition actions

The state callback may return transition actions for GenStatem to execute during the state transition, for example to set a time-out or reply to a call.

Reply to a call

See GenStatem:call/2,3 about how to reply to a call. A reply can be sent from any state callback, not just the one that got the request event.

Event postponing

One of the possible transition actions is to postpone the current event. Then it will not be handled in the current state. The GenStatem engine keeps a queue of events divided into postponed events and events still to process (not presented yet). After a state change the queue restarts with the postponed events.

The GenStatem event queue model is sufficient to emulate the normal process message queue with selective receive. Postponing an event corresponds to not matching it in a receive statement, and changing states corresponds to entering a new receive statement.

Event insertion

The state callback can insert events using the transition action :next_event, and such an event is inserted in the event queue as the next to call the state callback with. That is, as if it is the oldest incoming event. A dedicated event_type/0 :internal can be used for such events making it possible to safely distinguish them from external events.

Inserting an event replaces the trick of calling your own state handling functions that you often would have to resort to in, for example, m:gen_fsm to force processing an inserted event before others.

Note

If you postpone an event and (against good practice) directly call a different state callback, the postponed event is not retried, since there was no state change.

Instead of directly calling a state callback, do a state change. This makes the GenStatem engine retry postponed events.

Inserting an event in a state change also triggers the new state callback to be called with that event before receiving any external events.

State enter calls

The GenStatem engine can automatically make a special call to the state callback whenever a new state is entered; see state_enter/0. This is for writing code common to all state entries. Another way to do it is to explicitly insert an event at the state transition, and/or to use a dedicated state transition function, but that is something you will have to remember at every state transition to the state(s) that need it.

For the details of a state transition, see type transition_option/0.

Hibernation

The GenStatem process can go into hibernation; see proc_lib:hibernate/3. It is done when a state callback or Module.init/1 specifies hibernate in the returned Actions list. This feature can be useful to reclaim process heap memory while the server is expected to be idle for a long time. However, use it with care, as hibernation can be too costly to use after every event; see :erlang.hibernate/3.

There is also a server start option {hibernate_after, Timeout} for start/3,4, start_link/3,4, start_monitor/3,4, that may be used to automatically hibernate the server.

Callback failure

If a callback function fails or returns a bad value, the GenStatem terminates. However, an exception of class throw is not regarded as an error but as a valid return, from all callback functions.

System messages and the m:sys module

A GenStatem handles system messages as described in m:sys. The m:sys module can be used for debugging a GenStatem. Replies sent through transition actions gets logged, but not replies sent through reply/1,2.

Trapping exit

A GenStatem process, like all gen_* behaviours, does not trap exit signals automatically; this must be explicitly initiated in the callback module (by calling process_flag(trap_exit, true) preferably from init/1.

Server termination

If the GenStatem process terminates, e.g. as a result of a callback function returning {stop, Reason}, an exit signal with this Reason is sent to linked processes and ports. See Processes in the Reference Manual for details regarding error handling using exit signals.

Note

For some important information about distributed signals, see the Blocking Signaling Over Distribution section in the Processes chapter of the Erlang Reference Manual. Blocking signaling can, for example, cause call time-outs in GenStatem to be significantly delayed.

Bad argument

Unless otherwise stated, all functions in this module fail if the specified GenStatem does not exist or if bad arguments are specified.

Example

The following example shows a simple pushbutton model for a toggling pushbutton implemented with callback mode state_functions. You can push the button and it replies if it went on or off, and you can ask for a count of how many times it has been pushed to switch on.

Pushbutton State Diagram

---
title: Pushbutton State Diagram
---
stateDiagram-v2
    [*]  --> off
    off  --> on  : push<br/><ul><li>Increment count</li><li>Reply 'on'</li></ul>
    on   --> off : push<br/><ul><li>Reply 'off'</li></ul>

Not shown in the state diagram:

  • The API function push() generates an event push of type call.
  • The API function get_count() generates an event get_count of type call that is handled in all states by replying with the current count value.
  • Unknown events are ignored and discarded.
  • There is boilerplate code for start, stop, terminate, code change, init, to set the callback mode to state_functions, etc...

Pushbutton Code

The following is the complete callback module file pushbutton.erl:

defmodule PushButton do
  use GenStatem

  defp name, do: :pushbutton_statem

  ## API. This example uses a registered name name()
  ## and does not link to the caller
  def start, do: GenStatem.start(__MODULE__, [], name: name())

  def push, do: GenStatem.call(name(), :push)

  def get_count, do: GenStatem.call(name(), :get_count)

  def stop, do: GenStatem.stop(name())

  ## Mandatory callback functions
  def init([]) do
    ## Set the initial state + data.  Data is used only as a counter.
    state = :off
    data = 0
    {:ok, state, data}
  end

  def callback_mode(), do: :state_functions

  def code_change(_old_vsn, old_state, old_data, _extra) do
	{:ok, old_state, old_data}
  end

  ## Mandatory callback functions
  def terminate, do: :void

  ### state callback(s)

  def off({:call, from}, :push, data) do
    ## Go to 'on', increment count and reply
    ## that the resulting status is 'on'
    {:next_state, :on, data + 1,[{:reply, from, :on}]}
  end

  def off(event_type, event_content, data) do
    handle_event(event_type, event_content, data)
  end

  def on({:call, from}, :push, data) do
    ## Go to 'off' and reply that the resulting status is 'off'
    {:next_state, :off, data, [{:reply, from, :off}]}
  end

  def on(event_type, event_content, data) do
    handle_event(event_type, event_content, data)
  end

  ## Handle events common to all states
  defp handle_event({:call, from}, :get_count, data) do
    ## Reply with the current count
    {:keep_state, data,[{:reply, from, data}]}
  end
  defp handle_event(_event_type, _event_content, data) do
    ## Ignore all other events
    {:keep_state, data}
  end
end

The following is a shell session when running it:

iex(1)> PushButton.start
{:ok, #PID<0.203.0>}
iex(2)> PushButton.get_count
0
iex(3)> PushButton.push
:on
iex(4)> PushButton.get_count
1
iex(5)> PushButton.push
:off
iex(6)> PushButton.get_count
1
iex(7)> PushButton.stop
:ok
iex(8)> PushButton.push
** (GenStatem.GenError)
Class: exit
Reason: noproc
MFArgs: GenStatem.call(:pushbutton_statem, :push, :infinity)
Stacktrace:
    (stdlib 6.2.2) gen.erl:580: :gen.do_for_proc/2
    (GenStatem 0.1.0) lib/GenStatem.ex:1997: GenStatem.call/3
    (elixir 1.18.3) src/elixir.erl:386: :elixir.eval_external_handler/3
    (stdlib 6.2.2) erl_eval.erl:919: :erl_eval.do_apply/7
    (elixir 1.18.3) src/elixir.erl:364: :elixir.eval_forms/4
    (elixir 1.18.3) lib/module/parallel_checker.ex:120: Module.ParallelChecker.verify/1
    (iex 1.18.3) lib/iex/evaluator.ex:336: IEx.Evaluator.eval_and_inspect/3
    (iex 1.18.3) lib/iex/evaluator.ex:310: IEx.Evaluator.eval_and_inspect_parsed/3

    (GenStatem 0.1.0) lib/GenStatem.ex:2001: GenStatem.call/3
    iex:8: (file)

To compare styles, here follows the same example using callback mode handle_event_function, or rather, the code to replace after function init/1 of the pushbutton.erl example file above:

### state callback(s)

def handle_event({:call, from}, :push, :off, data) do
  ## Go to 'on', increment count and reply
  ## that the resulting status is 'on'
  {:next_state, :on, data + 1,[{:reply, from, :on}]}
end

def handle_event({:call, from}, :push, :on, data) do
  ## Go to 'off' and reply that the resulting status is 'off'
  {:next_state, :off, data, [{:reply, from, :off}]}
end

##
## Event handling common to all states
def handle_event({:call, from}, :get_count, state, data) do
  ## Reply with the current count
  {:next_state, state, data, [{:reply, from, data}]};
end

def handle_event(_event_type, _old_state, current_state, data) do
  ## Ignore all other events
  {:next_state, current_state, data}
end

See Also

GenEvent, GenServer, Supervisor.

Summary

Types

Actions for a state transition, or when starting the server.

One function per state or one common event handler.

Generic state data for the server.

Actions for any callback: hibernate, time-outs or replies.

Event payload from the event's origin, delivered to the state callback.

Return value from a state callback after handling an event.

How long to wait for an event.

All event types: external, time-out, or internal.

Event from a call, cast, or regular process message; "info".

A map that describes the server's status.

A call event's reply destination.

How long to wait for a named time-out event.

Hibernate the server process.

Postpone an event to handle it later.

Reply to a call/2,3.

A handle that associates a reply to the corresponding request.

An opaque request identifier. See send_request/2 for details.

An opaque collection of request identifiers (request_id/0).

Response time-out for an asynchronous call.

Server name specification: :local, :global, or :via registered.

Server specification: pid/0 or registered server_name/0.

Return value from the start_monitor/3,4 functions.

State name or state term.

Callback mode modifier for state enter calls: the atom :state_enter.

Return value from a state callback after a state enter call.

State name in callback mode state_functions.

How long to wait in the current state.

Event time-out, generic time-outs or state time-out.

Clearer way to cancel a time-out than the original setting it to ':infinity'.

Time-out timer start option, to select absolute time of expiry.

Update the event_content without affecting the time of expiry.

State transition options set by actions.

Callbacks

Select the callback mode and possibly state enter calls.

Update the state and data after code change.

Format/limit the status value.

Initialize the state machine.

Handle state machine termination.

Functions

Call a server: send request and wait for response.

Cast an event to a server.

Check if a received message is a request response.

Send one or multiple call replies.

Send a call reply to from.

Store a request identifier in a colletion.

Return the number of request identifiers in req_id_collection.

Send one or multiple call replies.

Send an asynchronous call request.

Send an asynchronous call request and add it to a request identifier collection.

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

Start a GenStatem process linked to the current process.

Starts a GenStatem process monitored and registered, but not linked.

Synchronously stops the state machine with the given reason.

Wait for a request response.

Wait for any request response in a collection.

Types

action()

@type action() ::
  :postpone
  | {:postpone, postpone :: postpone()}
  | {:next_event, event_type :: event_type(), event_content :: event_content()}
  | {:change_callback_module, new_module :: module()}
  | {:push_callback_module, new_module :: module()}
  | :pop_callback_module
  | enter_action()

Actions for a state transition, or when starting the server.

These transition actions can be invoked by returning them from the state callback when it is called with an event, from Module.init/1. They are not allowed from state enter calls.

Actions are executed in the containing list order.

Actions that set transition options override any previous of the same type, so the last in the containing list wins. For example, the last postpone/0 overrides any previous postpone/0 in the list.

  • {:postpone, value} - Sets the transition_option() postpone/0 for this state transition. This action is ignored when returned from Module.init/1, as there is no event to postpone in those cases.

    :postpone is equivalent to {:postpone, true}.

  • {:next_event, event_type, event_content} - This action does not set any transition_option() but instead stores the specified event_type and event_content for insertion after all actions have been executed.

    The stored events are inserted in the queue as the next to process before any already queued events. The order of these stored events is preserved, so the first :next_event in the containing list becomes the first to process.

    An event of type internal should be used when you want to reliably distinguish an event inserted this way from any external event.

  • {:change_callback_module, new_module} - Changes the callback module to new_module which will be used when calling all subsequent state callbacks. The GenStatem engine will find out the callback mode of new_module by calling new_module.callback_mode/0 before the next state callback.

    Changing the callback module does not affect the state transition in any way, it only changes which module that handles the events. Be aware that all relevant callback functions in new_module such as the state callback, new_module.code_change/4, new_module.format_status/1 and new_module.terminate/3 must be able to handle the state and data from the old module.

  • {:push_callback_module, new_module} - Pushes the current callback module to the top of an internal stack of callback modules, and changes the callback module to new_module. Otherwise like {:change_callback_module, new_module} above.

  • pop_callback_module - Pops the top module from the internal stack of callback modules and changes the callback module to be the popped module. If the stack is empty the server fails. Otherwise like {:change_callback_module, new_module} above.

callback_mode()

@type callback_mode() :: :state_functions | :handle_event_function

One function per state or one common event handler.

The callback mode is selected with the return value from Module.callback_mode/0:

The function Module.callback_mode/0 is called when starting the GenStatem, after code change and after changing the callback module with any of the actions change_callback_module, push_callback_module, or pop_callback_module. The result is cached for subsequent calls to state callbacks.

callback_mode_result()

@type callback_mode_result() :: callback_mode() | [callback_mode() | state_enter()]

Return value from Module.callback_mode/0.

This is the return type from Module.callback_mode/0 which selects callback mode and whether to do state enter calls, or not.

data()

@type data() :: term()

Generic state data for the server.

A term in which the state machine implementation is to store any server data it needs. The difference between this and the state/0 itself is that a change in this data does not cause postponed events to be retried. Hence, if a change in this data would change the set of events that are handled, then that data item should be part of the state/0 instead.

enter_action()

@type enter_action() ::
  :hibernate
  | {:hibernate, hibernate :: hibernate()}
  | timeout_action()
  | reply_action()

Actions for any callback: hibernate, time-outs or replies.

These transition actions are allowed when a action/0 is allowed, and also from a state enter call, and can be invoked by returning them from the state callback, from Module.init/1.

Actions are executed in the containing list order.

Actions that set transition options override any previous of the same type, so the last in the containing list wins. For example, the last event_timeout/0 overrides any previous event_timeout/0 in the list.

enter_loop_opt()

@type enter_loop_opt() ::
  {:hibernate_after, hibernate_after_timeout :: timeout()}
  | {:debug, dbgs :: [:sys.debug_option()]}

Server start options for the start/3,4, start_link/3,4, and start_monitor/3,4, functions.

See start_link/4.

event_content()

@type event_content() :: term()

Event payload from the event's origin, delivered to the state callback.

See event_type that describes the origins of the different event types, which is also where the event's content comes from.

event_handler_result(state_type)

@type event_handler_result(state_type) :: event_handler_result(state_type, term())

event_handler_result(state_type, data_type)

@type event_handler_result(state_type, data_type) ::
  {:next_state, next_state :: state_type, new_data :: data_type}
  | {:next_state, next_state :: state_type, new_data :: data_type,
     actions :: [action()] | action()}
  | state_callback_result(action(), data_type)

Return value from a state callback after handling an event.

state_type is state_name/0 if callback mode is :state_functions, or state/0 if callback mode is :handle_event_function.

  • {:next_state, next_state, new_data [, actions]} - The GenStatem does a state transition to next_state (which may be the same as the current state), sets new_data as the current server data/0, and executes all actions. If NextState !== CurrentState the state transition is a state change.

event_timeout()

@type event_timeout() :: time :: timeout() | integer()

How long to wait for an event.

Starts a timer set by timeout_action/0 Time, or {timeout, Time, EventContent [, Options]}.

When the timer expires an event of event_type/0 timeout will be generated. See :erlang.start_timer/4 for how Time and Options are interpreted. Future :erlang.start_timer/4 Options will not necessarily be supported.

Any event that arrives cancels this time-out. Note that a retried or inserted event counts as arrived. So does a state time-out zero event, if it was generated before this time-out is requested.

If Time is infinity, no timer is started, as it never would expire anyway.

If Time is relative and 0 no timer is actually started, instead the the time-out event is enqueued to ensure that it gets processed before any not yet received external event, but after already queued events.

Note that it is not possible nor needed to cancel this time-out, as it is cancelled automatically by any other event, meaning that whenever a callback is invoked that may want to cancel this time-out, the timer is already cancelled or expired.

The timer EventContent can be updated with the {timeout, update, NewEventContent} action without affecting the time of expiry.

event_type()

@type event_type() :: external_event_type() | timeout_event_type() | :internal

All event types: external, time-out, or internal.

internal events can only be generated by the state machine itself through the transition action next_event.

external_event_type()

@type external_event_type() :: {:call, from :: from()} | :cast | :info

Event from a call, cast, or regular process message; "info".

Type {:call, from} originates from the API functions call/2,3 or send_request/2. The event contains From, which is whom to reply to by a reply_action/0 or reply/2,3 call.

Type cast originates from the API function cast/2.

Type info originates from regular process messages sent to the GenStatem process.

format_status()

@type format_status() :: %{
  state: state(),
  data: data(),
  reason: term(),
  queue: [{event_type(), event_content()}],
  postponed: [{event_type(), event_content()}],
  timeouts: [{timeout_event_type(), event_content()}],
  log: [:sys.system_event()]
}

A map that describes the server's status.

The keys are:

  • state - The current state.
  • data - The state data.
  • reason - The reason that caused the process to terminate.
  • queue - The event queue.
  • postponed - The queue of postponed events.
  • timeouts - The active time-outs.
  • log - The sys log of the server.

New associations may be added to the status map without prior notice.

from()

@type from() :: {to :: pid(), tag :: reply_tag()}

A call event's reply destination.

Destination to use when replying through, for example, the action {:reply, from, reply} to a process that has called the GenStatem server using call/2,3.

generic_timeout()

@type generic_timeout() :: time :: timeout() | integer()

How long to wait for a named time-out event.

Starts a timer set by timeout_action/0 {{timeout, Name}, Time, EventContent [, Options]}.

When the timer expires an event of event_type/0 {timeout, Name} will be generated. See :erlang.start_timer/4 for how Time and Options are interpreted. Future :erlang.start_timer/4 Options will not necessarily be supported.

If Time is infinity, no timer is started, as it never would expire anyway.

If Time is relative and 0 no timer is actually started, instead the time-out event is enqueued to ensure that it gets processed before any not yet received external event.

Setting a timer with the same Name while it is running will restart it with the new time-out value. Therefore it is possible to cancel a specific time-out by setting it to infinity. It can also be cancelled more explicitly with the {{timeout, Name}, cancel} action.

The timer EventContent can be updated with the {{timeout, Name}, update, NewEventContent} action without affecting the time of expiry.

hibernate()

@type hibernate() :: boolean()

Hibernate the server process.

If true, hibernates the GenStatem by calling proc_lib:hibernate/3 before going into receive to wait for a new external event.

There is also a server start option {hibernate_after, Timeout} for automatic hibernation.

Note

If there are enqueued events to process when hibernation is requested, this is optimized by not hibernating but instead calling :erlang.garbage_collect/0 to simulate, in a more efficient way, that the GenStatem entered hibernation and immediately got awakened by an enqueued event.

init_result(state_type)

@type init_result(state_type) :: init_result(state_type, term())

init_result(state_type, data_type)

@type init_result(state_type, data_type) ::
  {:ok, state :: state_type, data :: data_type}
  | {:ok, state :: state_type, data :: data_type,
     actions :: [action()] | action()}
  | :ignore
  | {:stop, reason :: term()}
  | {:error, reason :: term()}

The return value from Module.init/1.

For a successful initialization, state is the initial state/0, and data the initial server data/0 of the GenStatem.

The actions are executed when entering the first state just as for a state callback, except that the action postpone is forced to false since there is no event to postpone.

For an unsuccessful initialization, {:stop, reason}, {:error, reason}, or :ignore should be used; see start_link/3,4.

postpone()

@type postpone() :: boolean()

Postpone an event to handle it later.

If true, postpones the current event. After a state change (next_state !== state), it is retried.

reply_action()

@type reply_action() :: {:reply, from :: from(), reply :: term()}

Reply to a call/2,3.

This transition action can be invoked by returning it from the state callback, from Module.init/1.

It does not set any transition_option() but instead replies to a caller waiting for a reply in call/3. from must be the term from argument {:call, from} in a call to a state callback.

Note that using this action from Module.init/1 would be weird on the border of witchcraft since there has been no earlier call to a state callback in this server.

reply_tag()

@opaque reply_tag()

A handle that associates a reply to the corresponding request.

request_id()

@opaque request_id()

An opaque request identifier. See send_request/2 for details.

request_id_collection()

@opaque request_id_collection()

An opaque collection of request identifiers (request_id/0).

Each request identifier can be associated with a label chosen by the user. For more information see reqids_new/0.

response_timeout()

@type response_timeout() :: timeout() | {:abs, integer()}

Response time-out for an asynchronous call.

Used to set a time limit on how long to wait for a response using either receive_response/2, receive_response/3, wait_response/2, or wait_response/3. The time unit used is millisecond.

Currently valid values:

  • 0..4294967295 - Time-out relative to current time in milliseconds.

  • infinity - Infinite time-out. That is, the operation will never time out.

  • {abs, Timeout} - An absolute Erlang monotonic time time-out in milliseconds. That is, the operation will time out when :erlang.monotonic_time(millisecond) returns a value larger than or equal to Timeout. Timeout is not allowed to identify a time further into the future than 4294967295 milliseconds. Specifying the time-out using an absolute value is especially handy when you have a deadline for responses corresponding to a complete collection of requests (request_id_collection/0), since you do not have to recalculate the relative time until the deadline over and over again.

server_name()

@type server_name() ::
  {:local, atom()}
  | {:global, global_name :: term()}
  | {:via, reg_mod :: module(), name :: term()}

Server name specification: :local, :global, or :via registered.

Name specification to use when starting a GenStatem server. See start_link/3 and server_ref/0 below.

server_ref()

@type server_ref() ::
  pid()
  | (local_name :: atom())
  | {name :: atom(), node :: atom()}
  | {:global, global_name :: term()}
  | {:via, reg_mod :: module(), via_name :: term()}

Server specification: pid/0 or registered server_name/0.

To be used in call/2,3 to specify the server.

It can be:

  • pid() | local_name - The GenStatem is locally registered.

  • {name, node} - The GenStatem is locally registered on another node.

  • {:global, global_name} - The GenStatem is globally registered in m:global.

  • {:via, reg_mod, via_name} - The GenStatem is registered in an alternative process registry. The registry callback module reg_mod is to export functions register_name/2, unregister_name/1, whereis_name/1, and send/2, which are to behave like the corresponding functions in m:global. Thus, {:via, :global, global_name} is the same as {:global, global_name}.

start_mon_ret()

@type start_mon_ret() :: {:ok, {pid(), reference()}} | :ignore | {:error, term()}

Return value from the start_monitor/3,4 functions.

As for start_link/4 but a successful return wraps the process ID and the monitor reference in a {:ok, {pid(),reference()}} tuple.

start_opt()

@type start_opt() ::
  {:name, server_name() | atom()}
  | {:timeout, time :: timeout()}
  | {:spawn_opt, [:proc_lib.start_spawn_option()]}
  | enter_loop_opt()

Server start options for the start/3,4, start_link/3,4, and start_monitor/3,4 functions.

See start_link/4.

start_ret()

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

Return value from the start/3,4 and start_link/3,4 functions.

See start_link/4.

state()

@type state() :: state_name() | term()

State name or state term.

If the callback mode is handle_event_function, the state can be any term. After a state change (next_state !== state), all postponed events are retried.

Comparing two states for strict equality is assumed to be a fast operation, since for every state transition the GenStatem engine has to deduce if it is a state change.

Note

The smaller the state term, in general, the faster the comparison.

Note that if the "same" state term is returned for a state transition (or a return action without a next_state field is used), the comparison for equality is always fast because that can be seen from the term handle.

But if a newly constructed state term is returned, both the old and the new state terms will have to be traversed until an inequality is found, or until both terms have been fully traversed.

So it is possible to use large state terms that are fast to compare, but very easy to accidentally mess up. Using small state terms is the safe choice.

state_callback_result(action_type, data_type)

@type state_callback_result(action_type, data_type) ::
  {:keep_state, new_data :: data_type}
  | {:keep_state, new_data :: data_type, actions :: [action_type] | action_type}
  | :keep_state_and_data
  | {:keep_state_and_data, actions :: [action_type] | action_type}
  | {:repeat_state, new_data :: data_type}
  | {:repeat_state, new_data :: data_type,
     actions :: [action_type] | action_type}
  | :repeat_state_and_data
  | {:repeat_state_and_data, actions :: [action_type] | action_type}
  | :stop
  | {:stop, reason :: term()}
  | {:stop, reason :: term(), new_data :: data_type}
  | {:stop_and_reply, reason :: term(),
     replies :: [reply_action()] | reply_action()}
  | {:stop_and_reply, reason :: term(),
     replies :: [reply_action()] | reply_action(), new_data :: data_type}

Return value from any state callback.

ActionType is enter_action/0 if the state callback was called with a state enter call, and action/0 if the state callback was called with an event.

  • {keep_state, NewData [, Actions]} - The same as {next_state, CurrentState, NewData [, Actions]}.

  • keep_state_and_data | {keep_state_and_data, Actions} - The same as {keep_state, CurrentData [, Actions]}.

  • {repeat_state, NewData [, Actions]} - If the GenStatem runs with state enter calls, the state enter call is repeated, see type transition_option/0. Other than that {repeat_state, NewData [, Actions]} is the same as {keep_state, NewData [, Actions]}.

  • repeat_state_and_data | {repeat_state_and_data, Actions} - The same as {repeat_state, CurrentData [, Actions]}.

  • {stop, Reason [, NewData]} - Terminates the GenStatem by calling Module.terminate/3 with Reason and NewData, if specified. An exit signal with this reason is sent to linked processes and ports.

  • stop - The same as {stop, normal}.

  • {stop_and_reply, Reason, Replies [, NewData]} - Sends all Replies, then terminates the GenStatem like with {stop, Reason [, NewData]}.

All these terms are tuples or atoms and will be so in all future versions of GenStatem.

state_enter()

@type state_enter() :: :state_enter

Callback mode modifier for state enter calls: the atom :state_enter.

Both callback modes can use state enter calls, and this is selected by adding this :state_enter flag to the callback mode return value from Module.callback_mode/0.

If Module.callback_mode/0 returns a list containing :state_enter, the GenStatem engine will, at every state change, that is; next_state !== current_state, call the state callback with arguments (enter, OldState, Data) or (enter, OldState, State, Data), depending on the callback mode.

This may look like an event but is really a call performed after the previous state callback returned, and before any event is delivered to the new state callback. See Module.state_name/3 and Module.handle_event/4. A state enter call may be repeated without doing a state change by returning a repeat_state or repeat_state_and_data action from the state callback.

If Module.callback_mode/0 does not return a list containing :state_enter, no state enter calls are done.

If Module.code_change/4 should transform the state, it is regarded as a state rename and not a state change, which will not cause a state enter call.

Note that a state enter call will be done right before entering the initial state, which may be seen as a state change from no state to the initial state. In this case OldState =:= State, which cannot happen for a subsequent state change, but will happen when repeating the state enter call.

state_enter_result(state)

@type state_enter_result(state) :: state_enter_result(state, term())

state_enter_result(state, data_type)

@type state_enter_result(state, data_type) ::
  {:next_state, state, new_data :: data_type}
  | {:next_state, state, new_data :: data_type,
     actions :: [enter_action()] | enter_action()}
  | state_callback_result(enter_action(), data_type)

Return value from a state callback after a state enter call.

state is the current state and it cannot be changed since the state callback was called with a state enter call.

  • {:next_state, state, new_data [, actions]} - The GenStatem does a state transition to state, which has to be equal to the current state, sets new_data, and executes all actions.

state_name()

@type state_name() :: atom()

State name in callback mode state_functions.

If the callback mode is state_functions, the state must be an atom. After a state change (next_state !== state), all postponed events are retried. Note that the state terminate is not possible to use since it would collide with the optional callback function Module.terminate/3.

state_timeout()

@type state_timeout() :: time :: timeout() | integer()

How long to wait in the current state.

Starts a timer set by timeout_action/0, or {:state_timeout, time, event_content [, options]}.

When the timer expires an event of event_type/0 :state_timeout will be generated. See :erlang.start_timer/4 for how time and options are interpreted. Future :erlang.start_timer/4 options will not necessarily be supported.

A state change cancels this timer, if it is running. That is, if the timeout_action/0 that starts this timer is part of a list of action/0s for a state change, next_state !== current_state, the timer runs in the next_state.

If the state machine stays in that new state, now the current state, the timer will run until it expires, which creates the time-out event. If the state machine changes states from the now current state, the timer is cancelled. During the state change from the now current state, a new state time-out may be started for the next next_state.

If the timeout_action/0 that starts this timer is part of a list of action/0s for a state transition that is not a state change, the timer runs in the current state.

If Time is infinity, no timer is started, as it never would expire anyway.

If Time is relative and 0 no timer is actually started, instead the the time-out event is enqueued to ensure that it gets processed before any not yet received external event.

Setting this timer while it is running will restart it with the new time-out value. Therefore it is possible to cancel this time-out by setting it to infinity. It can also be cancelled more explicitly with {state_timeout, cancel}.

The timer EventContent can be updated with the {state_timeout, update, NewEventContent} action without affecting the time of expiry.

timeout_action()

@type timeout_action() ::
  (time :: event_timeout())
  | {:timeout, time :: event_timeout(), event_content :: event_content()}
  | {:timeout, time :: event_timeout(), event_content :: event_content(),
     options :: timeout_option() | [timeout_option()]}
  | {{:timeout, name :: term()}, time :: generic_timeout(),
     event_content :: event_content()}
  | {{:timeout, name :: term()}, time :: generic_timeout(),
     event_content :: event_content(),
     options :: timeout_option() | [timeout_option()]}
  | {:state_timeout, time :: state_timeout(), event_content :: event_content()}
  | {:state_timeout, time :: state_timeout(), event_content :: event_content(),
     options :: timeout_option() | [timeout_option()]}
  | timeout_cancel_action()
  | timeout_update_action()

Event time-out, generic time-outs or state time-out.

These transition actions can be invoked by returning them from the state callback, from Module.init/1.

These time-out actions sets time-out transition options.

timeout_cancel_action()

@type timeout_cancel_action() ::
  {:timeout, :cancel}
  | {{:timeout, name :: term()}, :cancel}
  | {:state_timeout, :cancel}

Clearer way to cancel a time-out than the original setting it to ':infinity'.

It has always been possible to cancel a time-out using timeout_action/0 with time = :infinity, since setting a new time-out time overrides a running timer, and since setting the time to :infinity is optimized to not setting a timer (that never will expire). Using this action shows the intention more clearly.

timeout_event_type()

@type timeout_event_type() :: :timeout | {:timeout, name :: term()} | :state_timeout

Event time-out, generic time-out, or state time-out.

The time-out event types that the state machine can generate for itself with the corresponding timeout_action/0s:

Time-out typeActionEvent type
Event time-out{:timeout, time, ...}:timeout
Generic time-out{{:timeout, name}, time, ...}{:timeout, name}
State time-out{:state_timeout, time, ...}:state_timeout

In short; the action to set a time-out with event_type is {event_type, time, ...}.

timeout_option()

@type timeout_option() :: {:abs, abs :: boolean()}

Time-out timer start option, to select absolute time of expiry.

If Abs is true an absolute timer is started, and if it is false a relative, which is the default. See :erlang.start_timer/4 for details.

timeout_update_action()

@type timeout_update_action() ::
  {:timeout, :update, event_content :: event_content()}
  | {{:timeout, name :: term()}, :update, event_content :: event_content()}
  | {:state_timeout, :update, event_content :: event_content()}

Update the event_content without affecting the time of expiry.

Sets a new event_content for a running time-out timer. See timeout_action() for how to start a time-out.

If no time-out of this type is active, instead inserts the time-out event just like when starting a time-out with relative time = 0. This is a time-out autostart with immediate expiry, so there will be noise for example if a generic time-out name was misspelled.

transition_option()

@type transition_option() ::
  postpone()
  | hibernate()
  | event_timeout()
  | generic_timeout()
  | state_timeout()

State transition options set by actions.

These determine what happens during the state transition. The state transition takes place when the state callback has processed an event and returns. Here are the sequence of steps for a state transition:

  1. All returned actions are processed in order of appearance. In this step all replies generated by any reply_action/0 are sent. Other actions set transition_option/0s that come into play in subsequent steps.

  2. If state enter calls are used, it is either the initial state or one of the callback results repeat_state or repeat_state_and_data is used the GenStatem engine calls the current state callback with arguments (enter, State, Data) or (enter, State, State, Data) (depending on callback mode) and when it returns starts again from the top of this sequence.

    If state enter calls are used, and the state changes, the GenStatem engine calls the new state callback with arguments (enter, OldState, Data) or (enter, OldState, State, Data) (depending on callback mode) and when it returns starts again from the top of this sequence.

  3. If postpone/0 is true, the current event is postponed.

  4. If this is a state change, the queue of incoming events is reset to start with the oldest postponed.

  5. All events stored with action/0 next_event are inserted to be processed before previously queued events.

  6. Time-out timers event_timeout/0, generic_timeout/0 and state_timeout/0 are handled. Time-outs with zero time are guaranteed to be delivered to the state machine before any external not yet received event so if there is such a time-out requested, the corresponding time-out zero event is enqueued as the newest received event; that is after already queued events such as inserted and postponed events.

    Any event cancels an event_timeout/0 so a zero time event time-out is only generated if the event queue is empty.

    A state change cancels a state_timeout/0 and any new transition option of this type belongs to the new state, that is; a state_timeout/0 applies to the state the state machine enters.

  7. If there are enqueued events the state callback for the possibly new state is called with the oldest enqueued event, and we start again from the top of this sequence.

  8. Otherwise the GenStatem goes into receive or hibernation (if hibernate/0 is true) to wait for the next message. In hibernation the next non-system event awakens the GenStatem, or rather the next incoming message awakens the GenStatem, but if it is a system event it goes right back into hibernation. When a new message arrives the state callback is called with the corresponding event, and we start again from the top of this sequence.

Note

The behaviour of a time-out zero (a time-out with time 0) differs subtly from Erlang's receive ... after 0 ... end.

The latter receives one message if there is one, while using the timeout_action/0 {timeout, 0} does not receive any external event.

m:gen_server's time-out works like Erlang's receive ... after 0 ... end, in contrast to GenStatem.

Callbacks

callback_mode()

@callback callback_mode() :: callback_mode_result()

Select the callback mode and possibly state enter calls.

This function is called by a GenStatem when it needs to find out the callback mode of the callback module.

The value is cached by GenStatem for efficiency reasons, so this function is only called once after server start, after code change, and after changing the callback module, but before the first state callback in the current callback module's code is called. More occasions may be added in future versions of GenStatem.

Server start happens either when Module.init/1 returns. Code change happens when Module.code_change/4 returns. A change of the callback module happens when a state callback returns any of the actions change_callback_module, push_callback_module or pop_callback_module.

The callback_mode is either just callback_mode/0 or a list containing callback_mode/0 and possibly the atom :state_enter.

Note

If this function's body does not return an inline constant value the callback module is doing something strange.

code_change(old_vsn, old_state, old_data, extra)

(optional)
@callback code_change(
  old_vsn :: term() | {:down, term()},
  old_state :: state(),
  old_data :: data(),
  extra :: term()
) :: {:ok, new_state :: state(), new_data :: data()} | (reason :: term())

Update the state and data after code change.

This function is called by a GenStatem when it is to update its internal state during a release upgrade/downgrade that is, when the instruction {:update, module, change, ...}, where change = {:advanced, extra}, is specified in the appup file. For more information, see Release Handling Instructions in OTP Design Principles .

For an upgrade, old_vsn is vsn, and for a downgrade, old_vsn is {:down, vsn}. vsn is defined by the vsn attribute(s) of the old version of the callback module Module. If no such attribute is defined, the version is the checksum of the Beam file.

old_state and old_data are the internal state of the GenStatem.

extra is passed "as is" from the {:advanced, extra} part of the update instruction.

If successful, the function must return the updated internal state in an {ok, new_state, new_data} tuple.

If the function returns a failure reason, the ongoing upgrade fails and rolls back to the old release. Note that reason cannot be an {:ok, _, _} tuple since that will be regarded as a {:ok, new_state, new_data} tuple, and that a tuple matching {:ok, _} is an also invalid failure reason. It is recommended to use an atom as reason since it will be wrapped in an {:error, reason} tuple.

Also note when upgrading a GenStatem, this function and hence the change = {:advanced, extra} parameter in the appup file is not only needed to update the internal state or to act on the extra argument. It is also needed if an upgrade or downgrade should change callback mode, or else the callback mode after the code change will not be honoured, most probably causing a server crash.

If the server changes callback module using any of the actions change_callback_module, push_callback_module, or pop_callback_module, be aware that it is always the current callback module that will get this callback call. That the current callback module handles the current state and data update should be no surprise, but it must be able to handle even parts of the state and data that it is not familiar with, somehow.

In the supervisor child specification there is a list of modules which is recommended to contain only the callback module. For a GenStatem with multiple callback modules there is no real need to list all of them, it may not even be possible since the list could change after code upgrade. If this list would contain only the start callback module, as recommended, what is important is to upgrade that module whenever a synchronized code replacement is done. Then the release handler concludes that an upgrade that upgrades that module needs to suspend, code change, and resume any server whose child specification declares that it is using that module. And again; the current callback module will get the Module.code_change/4 call.

Note

If a release upgrade/downgrade with change = {:advanced, extra} specified in the .appup file is made when Module.code_change/4 is not implemented the process will crash with exit reason undef.

format_status(status)

(optional)
@callback format_status(status :: format_status()) :: new_status :: format_status()

Format/limit the status value.

This function is called by a GenStatem process in order to format/limit the server status for debugging and logging purposes.

It is called in the following situations:

This function is useful for changing the form and appearance of the GenStatem status for these cases. A callback module wishing to change the :sys.get_status/1,2 return value and how its status appears in termination error logs, exports an instance of Module.format_status/1, which will get a map Status that describes the current state of the GenStatem, and shall return a map NewStatus containing the same keys as the input map, but it may transform some values.

One use case for this function is to return compact alternative state representations to avoid having large state terms printed in log files. Another is to hide sensitive data from being written to the error log.

Example:

def format_status(status) when is_map(status) do
  status
  |> Enum.map(fn
    {:state, state} when is_map(state) ->
      {:state, Map.delete(state, :private_key)}
    {:message, {:password, _pass}} ->
      {:message, {:password, :removed}}
    pair ->
      pair
  end)
  |> Map.new()
end

Note

This callback is optional, so a callback module does not need to export it. The GenStatem module provides a default implementation of this function that returns {state, data}.

handle_event(atom, old_state, current_state, data)

(optional)
@callback handle_event(
  :enter,
  old_state :: state(),
  current_state :: state(),
  data :: data()
) :: state_enter_result(current_state :: state())
@callback handle_event(
  event_type :: event_type(),
  event_content :: event_content(),
  current_state :: state(),
  data :: data()
) :: event_handler_result(state())

state callback in callback mode handle_event_function.

Whenever a GenStatem receives an event from call/2,3, cast/2, or as a normal process message, this function is called.

If event_type is {:call, from}, the caller waits for a reply. The reply can be sent from this or from any other state callback by returning with {:reply, from, reply} in actions, in replies, or by calling reply(from, reply).

If this function returns with a next state that does not match equal (!==) to the current state, all postponed events are retried in the next state.

For options that can be set and actions that can be done by GenStatem after returning from this function, see action/0.

When the GenStatem runs with state enter calls, this function is also called with arguments (:enter, old_state, ...) during every state change. In this case there are some restrictions on the actions that may be returned:

  • postpone/0 is not allowed since a state enter call is not an event so there is no event to postpone.
  • {:next_event, _, _} is not allowed since using state enter calls should not affect how events are consumed and produced.
  • It is not allowed to change states from this call. Should you return {:next_state, next_state, ...} with next_state !== state the GenStatem crashes.

Note that it is actually allowed to use {:repeat_state, new_data, ...} although it makes little sense since you immediately will be called again with a new state enter call making this just a weird way of looping, and there are better ways to loop.

If you do not update new_data and have some loop termination condition, or if you use {:repeat_state_and_data, _} or :repeat_state_and_data you have an infinite loop!

You are advised to use {:keep_state, ...}, {:keep_state_and_data, _} or :keep_state_and_data since changing states from a state enter call is not possible anyway.

Note the fact that you can use throw to return the result, which can be useful. For example to bail out with throw(:keep_state_and_data) from deep within complex code that cannot return {:next_state, state, data} because state or data is no longer in scope.

init(args)

@callback init(args :: term()) :: init_result(state())

Initialize the state machine.

Whenever a GenStatem is started using start_link/3,4, start_monitor/3,4, or start/3,4, this function is called by the new process to initialize the implementation state and server data.

Args is the Args argument provided to that start function.

Note

Note that if the GenStatem is started through m:proc_lib, this callback will never be called. Since this callback is not optional it can in that case be implemented as:

-spec init(_) -> no_return().
init(Args) -> :erlang.error(not_implemented, [Args]).

state_name(atom, old_state_name, data)

(optional)
@callback state_name(:enter, old_state_name :: state_name(), data()) ::
  state_enter_result(:state_name)
@callback state_name(
  event_type :: event_type(),
  event_content :: event_content(),
  data :: data()
) :: event_handler_result(state_name())

State callback in callback mode :state_functions.

State callback that handles all events in state state_name, where state_name :: state_name() has to be an atom/0.

state_name cannot be terminate since that would collide with the callback function Module.terminate/3.

Besides that when doing a state change the next state always has to be an atom/0, this function is equivalent to Module.handle_event(event_type, event_content, __ENV__.function, data), which is the state callback in callback mode handle_event_function.

terminate(reason, current_state, data)

(optional)
@callback terminate(
  reason :: :normal | :shutdown | {:shutdown, term()} | term(),
  current_state :: state(),
  data()
) :: any()

Handle state machine termination.

This function is called by a GenStatem when it is about to terminate. It is to be the opposite of Module.init/1 and do any necessary cleaning up. When it returns, the GenStatem terminates with reason. The return value is ignored.

reason is a term denoting the stop reason and state is the internal state of the GenStatem.

reason depends on why the GenStatem is terminating. If it is because another callback function has returned, a stop tuple {:stop, reason} in actions, reason has the value specified in that tuple. If it is because of a failure, reason is the error reason.

If the GenStatem is part of a supervision tree and is ordered by its supervisor to terminate, this function is called with reason = :shutdown if both the following conditions apply:

  • The GenStatem process has been set to trap exit signals.
  • The shutdown strategy as defined in the supervisor's child specification is an integer time-out value, not :brutal_kill.

Even if the GenStatem is not part of a supervision tree, this function is called if it receives an :EXIT message from its parent. reason is the same as in the :EXIT message.

If the GenStatem process is not set up to trap exit signals it is immediately terminated, just like any process, and this function is not called.

Notice that for any other reason than :normal, :shutdown, or {:shutdown, term}, the GenStatem is assumed to terminate because of an error and an error report is issued using m:logger.

When the GenStatem process exits, an exit signal with the same reason is sent to linked processes and ports, just as for any process.

Functions

call(server_ref, request, timeout \\ :infinity)

@spec call(
  server_ref :: server_ref(),
  request :: term(),
  timeout :: timeout()
) :: reply :: term()

Call a server: send request and wait for response.

Makes a synchronous call to the GenStatem ServerRef by sending a request and waiting until the response arrives.

The GenStatem calls the state callback with event_type/0 {:call, from} and event content request.

The server's reply is sent from a state callback, by returning a transition action {:reply, from, reply}, calling reply(replies) with such a reply action in the replies list, or calling reply(from, reply).

timeout is an integer > 0, which specifies how many milliseconds to wait for a reply, or the atom :infinity to wait indefinitely, which is the default. If no reply is received within the specified time, the function call fails.

The call can also fail, for example, if the GenStatem dies before or during this function call.

When this call fails it raises the GenError Exception and exit the calling process.

cast(server_ref, msg)

@spec cast(server_ref :: server_ref(), msg :: term()) :: :ok

Cast an event to a server.

Sends an asynchronous cast event to the GenStatem ServerRef and returns ok immediately, ignoring if the destination node or GenStatem does not exist.

The GenStatem calls the state callback with event_type/0 cast and event content Msg.

check_response(msg, req_id)

@spec check_response(msg :: term(), req_id :: request_id()) ::
  {:reply, reply :: term()}
  | {:error, {reason :: term(), server_ref()}}
  | :no_reply

Check if a received message is a request response.

Checks if msg is a response corresponding to the request identifier req_id. The request must have been made by send_request/2 and by the same process calling this function.

If msg is a reply to the handle req_id the result of the request is returned in reply. Otherwise this function returns :no_reply and no cleanup is done, and thus the function shall be invoked repeatedly until the response is returned.

See call/3 about how the request is handled and the reply is sent by the GenStatem server.

If the GenStatem server process has died when this function is called, that is; msg reports the server's death, this function returns an error return with the exit reason.

check_response(msg, req_id_col, delete)

@spec check_response(
  msg :: term(),
  req_id_collection :: request_id_collection(),
  delete :: boolean()
) ::
  {{:reply, reply :: term()} | {:error, {reason :: term(), server_ref()}},
   label :: term(), new_req_id_collection :: request_id_collection()}
  | :no_request
  | :no_reply

receive_response(req_id, timeout \\ :infinity)

@spec receive_response(
  req_id :: request_id(),
  response_timeout :: response_timeout()
) ::
  {:reply, reply :: term()}
  | {:error, {reason :: term(), server_ref()}}
  | timeout()

Receive a request response.

Receive a response corresponding to the request identifier req_id. The request must have been made by send_request/2 to the gen_statem process. This function must be called from the same process from which send_request/2 was made.

timeout specifies how long to wait for a response. If no response is received within the specified time, this function returns timeout. Assuming that the server executes on a node supporting aliases (introduced in OTP 24) the request will also be abandoned. That is, no response will be received after a time-out. Otherwise, a stray response might be received at a later time.

See call/3 about how the request is handled and the reply is sent by the gen_statem server.

If the gen_statem server process is dead or dies while this function waits for the reply, it returns an error return with the exit reason.

The difference between wait_response/2 and receive_response/2 is that receive_response/2 abandons the request at time-out so that a potential future response is ignored, while wait_response/2 does not.

receive_response(req_id_col, timeout, delete)

@spec receive_response(
  req_id_col :: request_id_collection(),
  timeout :: response_timeout(),
  delete :: boolean()
) ::
  result ::
  {response ::
     {:reply, reply :: term()} | {:error, {reason :: term(), server_ref()}},
   label :: term(), new_req_id_col :: request_id_collection()}
  | :no_request
  | :timeout

reply(replies)

@spec reply(replies :: [reply_action()] | reply_action()) :: :ok

Send one or multiple call replies.

This funcion can be used by a GenStatem callback to explicitly send one or multiple replies to processes waiting for call requests' replies, when it is impractical or impossible to return reply_action/0s from a state callback.

Note

A reply sent with this function is not visible in m:sys debug output.

reply(from, reply)

@spec reply(from :: from(), reply :: term()) :: :ok

Send a call reply to from.

This funcion can be used by a GenStatem callback to explicitly send a reply to a process waiting for a call requests' reply, when it is impractical or impossible to return a reply_action/0 from a state callback.

Note

A reply sent with this function is not visible in m:sys debug output.

reqids_add(req_id, label, req_id_collection)

@spec reqids_add(
  req_id :: request_id(),
  label :: term(),
  req_id_collection :: request_id_collection()
) :: new_req_id_collection :: request_id_collection()

Store a request identifier in a colletion.

Stores req_id and associates a label with the request identifier by adding this information to req_id_collection and returning the resulting request identifier collection.

reqids_new()

@spec reqids_new() :: new_req_id_collection :: request_id_collection()

reqids_size(req_id_collection)

@spec reqids_size(req_id_collection :: request_id_collection()) :: non_neg_integer()

Return the number of request identifiers in req_id_collection.

reqids_to_list(req_id_collection)

Send one or multiple call replies.

This funcion can be used by a GenStatem callback to explicitly send one or multiple replies to processes waiting for call requests' replies, when it is impractical or impossible to return reply_action/0s from a state callback.

Note

A reply sent with this function is not visible in m:sys debug output.

send_request(server_ref, request)

@spec send_request(server_ref :: server_ref(), request :: term()) ::
  req_id :: request_id()

Send an asynchronous call request.

Sends request to the GenStatem process identified by server_ref and returns a request identifier req_id.

The return value req_id shall later be used with receive_response/2, wait_response/2, or check_response/2 to fetch the actual result of the request. Besides passing the request identifier directly to these functions, it can also be stored in a request identifier collection using reqids_add/3. Such a collection of request identifiers can later be used in order to get one response corresponding to a request in the collection by passing the collection as argument to receive_response/3, wait_response/3, or check_response/3. If you are about to store the request identifier in a collection, you may want to consider using send_request/4 instead.

The call GenStatem.send_request(server_ref, request) |> GenStatem.wait_response(timeout) can be seen as equivalent to GenStatem.call(server, request, timeout), ignoring the error handling.

See call/3 about how the request is handled and the reply is sent by the GenStatem server.

The server's reply is returned by one of the receive_response/1,2, wait_response/1,2, or check_response/2 functions.

send_request(server_ref, request, label, req_id_col)

@spec send_request(
  server_ref :: server_ref(),
  request :: term(),
  label :: term(),
  req_id_col :: request_id_collection()
) :: new_req_id_collection :: request_id_collection()

Send an asynchronous call request and add it to a request identifier collection.

Sends request to the GenStatem process identified by server_ref. The label will be associated with the request identifier of the operation and added to the returned request identifier collection new_req_id_collection. The collection can later be used in order to get one response corresponding to a request in the collection by passing the collection as argument to receive_response/3, wait_response/3, or check_response/3.

The same as calling reqids_add(​send_request(server_ref, request),label, req_id_collection), but slightly more efficient.

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

@spec start(module(), term(), [start_opt()]) :: start_ret()

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

See start_link/3 for more information.

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

@spec start_link(module(), term(), [start_opt()]) :: start_ret()

Start a GenStatem process linked to the current process.

This is often used to start the GenStatem 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 GenStatem started with start_link/3 is linked to the parent process and will exit in case of crashes from the parent. The GenStatem 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 GenStatem

  • :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 GenStatem 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.

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

@spec start_monitor(module(), term(), [start_opt()]) :: start_mon_ret()

Starts a GenStatem process monitored and registered, but not linked.

See start_link/3 for more information.

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

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

Synchronously stops the state machine with the given reason.

The terminate/3 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.

wait_response(req_id, wait_time \\ :infinity)

@spec wait_response(
  req_id :: request_id(),
  wait_time :: response_timeout()
) ::
  result ::
  {:reply, reply :: term()}
  | {:error, {reason :: term(), server_ref()}}
  | timeout()

Wait for a request response.

Waits for the response to the request identifier req_id. The request must have been made by send_request/2 to the GenStatem process. This function must be called from the same process from which send_request/2 was called.

wait_time specifies how long to wait for a reply. If no reply is received within the specified time, the function returns timeout and no cleanup is done, Thus the function can be invoked repeatedly until a reply is returned.

See call/3 about how the request is handled and the Reply is sent by the GenStatem server.

If the GenStatem server process is dead or dies while this function waits for the reply, it returns an error return with the exit reason.

The difference between receive_response/2 and wait_response/2 is that receive_response/2 abandons the request at time-out so that a potential future response is ignored, while wait_response/2 does not.

wait_response(req_id_col, wait_time, delete)

@spec wait_response(
  req_id_collection :: request_id_collection(),
  wait_time :: response_timeout(),
  delete :: boolean()
) ::
  result ::
  {response ::
     {:reply, reply :: term()} | {:error, {reason :: term(), server_ref()}},
   label :: term(), new_req_id_collection :: request_id_collection()}
  | :no_request
  | :timeout

Wait for any request response in a collection.

Waits for a response in req_id_collection. All request identifiers of req_id_collection must correspond to requests that have been made using send_request/2 or send_request/4, and all requests must have been made by the process calling this function.

The label in the response is the label associated with the request identifier that the response corresponds to. The label of a request identifier is associated when adding the request id to a collection, or when sending the request using send_request/4.

Compared to wait_response/2, the returned result or exception associated with a specific request identifier will be wrapped in a 3-tuple {response, label, new_req_id_collection}. response is the value that would have been produced by wait_response/2, label is the value associated with the specific request identifier and new_req_id_collection is a possibly modified request identifier collection.

If req_id_collection is empty, no_request is returned.

If no response is received before wait_time has expired, timeout is returned. It is valid to continue waiting for a response as many times as needed up until a response has been received and completed by check_response(), receive_response(), or wait_response().

The difference between receive_response/3 and wait_response/3 is that receive_response/3 abandons requests at time-out so that potential future responses are ignored, while wait_response/3 does not.

If delete is true, the association with label has been deleted from req_id_collection in the resulting new_req_id_collection. If delete is false, new_req_id_collection will equal req_id_collection. Note that deleting an association is not for free and that a collection containing already handled requests can still be used by subsequent calls to wait_response/3, check_response/3, and receive_response/3.

However, without deleting handled associations, the above calls will not be able to detect when there are no more outstanding requests to handle, so you will have to keep track of this some other way than relying on a :no_request return. Note that if you pass a collection only containing associations of already handled or abandoned requests to this function, it will always block until wait_time expires and then return :timeout.