gen_statem(3)

NAME
gen_statem - Generic state machine behavior.

DESCRIPTION

gen_statem provides a generic state machine behaviour that since Erlang/OTP 20.0 replaces its predecessor gen_fsm, and should be used for new code. The gen_fsm behaviour remains in OTP "as is" to not break old code using it.

A generic state machine server process (gen_statem) implemented using this module has a standard set of interface functions and includes functionality for tracing and error reporting. It also fits into an OTP supervision tree. For more information, see gen_statem section in OTP Design Principles.

NOTE: If you are new to gen_statem and want an overview of concepts and operation the section gen_statem Behaviour located in the User's Guide OTP Design Principles is recommended to read. This reference manual focuses on being correct and complete, which might make it hard to see the forest for all the trees.

Features

gen_statem has got the same features that gen_fsm had and adds some really useful:

    • Co-located state code

    • Arbitrary term state

    • Event postponing

    • Self-generated events

    • State time-out

    • Multiple generic named time-outs

    • Absolute time-out time

    • Automatic state enter calls

    • Reply from other state than the request, traceable with sys

    • Multiple replies, traceable with sys

    • Changing the callback module

Two callback modes are supported: state_functions
    for finite-state machines ( gen_fsm like), 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.

The callback modes for gen_statem differs from the one for gen_fsm, but it is still fairly easy to rewrite from gen_fsm to gen_statem. See the rewrite guide at the start of the gen_fsm documentation.

Callback module

A gen_statem 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:

    gen_statem module            Callback module
    -----------------            ---------------
    gen_statem:start
    gen_statem:start_monitor
    gen_statem:start_link -----> Module:init/1
    Server start or code change
                          -----> Module:callback_mode/0
                          selects callback mode
    gen_statem:stop
    Supervisor exit
    Callback failure      -----> Module:terminate/3
    gen_statem:call
    gen_statem:cast
    gen_statem:send_request
    erlang:send
    erlang:'!'            -----> Module:StateName/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 gen_statem 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:StateName/3. This co-locates all code for a specific state in one function as the gen_statem 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 gen_statem 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 gen_statem which does a state transition. If this state transition is to a different state, that is: NextState =/= State, it is a state change.

Transition actions

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

Reply to a call

See gen_statem: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 gen_statem 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 gen_statem 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, 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 gen_statem 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 gen_statem 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 gen_statem 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, or enter_loop/4,5,6, that may be used to automatically hibernate the server.

Callback failure

If a callback function fails or returns a bad value, the gen_statem 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 sys module

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

Trapping exit

A gen_statem 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 gen_statem 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 gen_statem to be significantly delayed.

Bad argument

Unless otherwise stated, all functions in this module fail if the specified gen_statem 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\n* Increment count\n* Reply 'on'
        on   --> off : push\n* Reply 'off'

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:

    -module(pushbutton).
    -behaviour(gen_statem).
    -export([start/0,push/0,get_count/0,stop/0]).
    -export([terminate/3,code_change/4,init/1,callback_mode/0]).
    -export([on/3,off/3]).
    name() -> pushbutton_statem. % The registered server name
    %% API.  This example uses a registered name name()
    %% and does not link to the caller.
    start() ->
        gen_statem:start({local,name()}, ?MODULE, [], []).
    push() ->
        gen_statem:call(name(), push).
    get_count() ->
        gen_statem:call(name(), get_count).
    stop() ->
        gen_statem:stop(name()).
    %% Mandatory callback functions
    terminate(_Reason, _State, _Data) ->
        void.
    code_change(_Vsn, State, Data, _Extra) ->
        {ok,State,Data}.
    init([]) ->
        %% Set the initial state + data.  Data is used only as a counter.
        State = off, Data = 0,
        {ok,State,Data}.
    callback_mode() -> state_functions.
    %%% state callback(s)
    off({call,From}, push, Data) ->
        %% Go to 'on', increment count and reply
        %% that the resulting status is 'on'
        {next_state,on,Data+1,[{reply,From,on}]};
    off(EventType, EventContent, Data) ->
        handle_event(EventType, EventContent, Data).
    on({call,From}, push, Data) ->
        %% Go to 'off' and reply that the resulting status is 'off'
        {next_state,off,Data,[{reply,From,off}]};
    on(EventType, EventContent, Data) ->
        handle_event(EventType, EventContent, Data).
    %% Handle events common to all states
    handle_event({call,From}, get_count, Data) ->
        %% Reply with the current count
        {keep_state,Data,[{reply,From,Data}]};
    handle_event(_, _, Data) ->
        %% Ignore all other events
        {keep_state,Data}.

The following is a shell session when running it:

    1> pushbutton:start().
    {ok,<0.36.0>}
    2> pushbutton:get_count().
    0
    3> pushbutton:push().
    on
    4> pushbutton:get_count().
    1
    5> pushbutton:push().
    off
    6> pushbutton:get_count().
    1
    7> pushbutton:stop().
    ok
    8> pushbutton:push().
    ** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
         in function  gen:do_for_proc/2 (gen.erl, line 261)
         in call from gen_statem:call/3 (gen_statem.erl, line 386)

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:

    callback_mode() -> handle_event_function.
    %%% state callback(s)
    handle_event({call,From}, push, off, Data) ->
        %% Go to 'on', increment count and reply
        %% that the resulting status is 'on'
        {next_state,on,Data+1,[{reply,From,on}]};
    handle_event({call,From}, push, on, Data) ->
        %% Go to 'off' and reply that the resulting status is 'off'
        {next_state,off,Data,[{reply,From,off}]};
    %%
    %% Event handling common to all states
    handle_event({call,From}, get_count, State, Data) ->
        %% Reply with the current count
        {next_state,State,Data,[{reply,From,Data}]};
    handle_event(_, _, State, Data) ->
        %% Ignore all other events
        {next_state,State,Data}.

    Note

  API changes

    • This behavior appeared in Erlang/OTP 19.0 as experimental.

    • In OTP 19.1 a backwards incompatible change of the return tuple from Module:init/1 was made, the mandatory callback function Module:callback_mode/0 was introduced, and enter_loop/4 was added.

    • In OTP 19.2 state enter calls were added.

    • In OTP 19.3 state time-outs were added.

    • In OTP 20.0 generic time-outs were added and gen_statem was stated to be no longer experimental and preferred over gen_fsm.

    • In OTP 22.1 time-out content update and explicit time-out cancel were added.

    • In OTP 22.3 the possibility to change the callback module with actions change_callback_module, push_callback_module and pop_callback_module, was added.

    • In OTP 23.0 start_monitor/3,4 were added, as well as functions for asynchronous calls: send_request/2, wait_response/1,2, and check_response/2.

    • In OTP 24.0 receive_response/1,2 were added.

    • In OTP 25.0 Module:format_status/1 was added to replace Module:format_status/2, as well as functions for collections of asynchronous calls: send_request/4, wait_response/3, receive_response/3, check_response/3, reqids_new/0, reqids_size/1, reqids_add/3, reqids_to_list/1.

    • In OTP 26.0 the possibility to return {error, Reason} from Module:init/1 was added.

    • In OTP 27.0 Module:format_status/2 was deprecated.

See Also

gen_event, gen_fsm, gen_server, proc_lib, supervisor, sys.

DATA TYPES

action() :: postpone | {postpone, Postpone :: postpone()} | {next_event, EventType :: event_type(), EventContent :: event_content()} | {change_callback_module, NewModule :: module()} | {push_callback_module, NewModule :: 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 or by passing them to enter_loop/4,5,6. 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 or passed to enter_loop/4,5,6, as there is no event to postpone in those cases.

        postpone is equivalent to {postpone, true}.
    {next_event, EventType, EventContent}
        This action does not set any transition_option() but instead stores the specified EventType and EventContent 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, NewModule} - Changes the callback module to NewModule which will be used when calling all subsequent state callbacks.\ Since OTP 22.3.

        The gen_statem engine will find out the callback mode of NewModule by calling NewModule: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 NewModule such as the state callback, NewModule:code_change/4, NewModule:format_status/1 and NewModule:terminate/3 must be able to handle the state and data from the old module.

        • {push_callback_module, NewModule} - Pushes the current callback module to the top of an internal stack of callback modules, and changes the callback module to NewModule. Otherwise like {change_callback_module, NewModule} above.\ Since OTP 22.3.

        • 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, NewModule} above.\ Since OTP 22.3.
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: state_functions
        The state must be of type state_name/0 and one callback function per state, that is, Module:StateName/3, is used.
    handle_event_function
        The state can be any term and the callback function Module:handle_event/4 is used for all states.

    The function Module:callback_mode/0 is called when starting the gen_statem, 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() :: 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() :: 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() :: 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 or by passing them to enter_loop/4,5,6.

    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. {hibernate, Value}
        Sets the transition_option/0 hibernate/0 for this state transition.

        hibernate is equivalent to {hibernate, true}.
enter_loop_opt() :: {hibernate_after, HibernateAfterTimeout :: timeout()} | {debug, Dbgs :: [sys:debug_option()]}.

    Server start options for the enter_loop/4,5,6, start/3,4, start_link/3,4, and start_monitor/3,4, functions.

    See start_link/4.
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(StateType) :: event_handler_result(StateType, term()).
event_handler_result(StateType, DataType) :: {next_state, NextState :: StateType, NewData :: DataType} | {next_state, NextState :: StateType, NewData :: DataType, Actions :: [action()] | action()} | state_callback_result(action(), DataType).

    Return value from a state callback after handling an event.

    StateType is state_name/0 if callback mode is state_functions, or state/0 if callback mode is handle_event_function. {next_state, NextState, NewData [, Actions]}
        The gen_statem does a state transition to NextState (which may be the same as the current state), sets NewData as the current server data/0, and executes all Actions. If NextState =/= CurrentState the state transition is a state change.
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() :: 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() :: {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 gen_statem process.
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() :: {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 gen_statem server using call/2,3.
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() :: boolean().

    Hibernate the server process.

    If true, hibernates the gen_statem 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 gen_statem entered hibernation and immediately got awakened by an enqueued event.
init_result(StateType) :: init_result(StateType, term()).
init_result(StateType, DataType) :: {ok, State :: StateType, Data :: DataType} | {ok, State :: StateType, Data :: DataType, Actions :: [action()] | action()} | ignore | {stop, Reason :: term()} | {error, Reason :: term()}.

    The return value from Module:init/1.

    For a succesful initialization, State is the initial state/0, and Data the initial server data/0 of the gen_statem.

    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 unsuccesful initialization, {stop, Reason}, {error, Reason}, or ignore should be used; see start_link/3,4.

    {error, Reason} has been allowed since OTP 26.0.

    The {ok, ...} tuples have existed since OTP 19.1, before that they were not ok tagged. This was before gen_statem replaced gen_fsm in OTP 20.0.
postpone() :: boolean().

    Postpone an event to handle it later.

    If true, postpones the current event. After a state change (NextState =/= State), it is retried.
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 or by passing it to enter_loop/4,5,6.

    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 or enter_loop/4,5,6 would be weird on the border of witchcraft since there has been no earlier call to a state callback in this server.
-opaque reply_tag()

    A handle that associates a reply to the corresponding request.
-opaque request_id()

    An opaque request identifier. See send_request/2 for details.
-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() :: 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() :: {local, atom()} | {global, GlobalName :: term()} | {via, RegMod :: module(), Name :: term()}.

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

    Name specification to use when starting a gen_statem server. See start_link/3 and server_ref/0 below.
server_ref() :: pid() | (LocalName :: atom()) | {Name :: atom(), Node :: atom()} | {global, GlobalName :: term()} | {via, RegMod :: module(), ViaName :: 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() | LocalName
        The gen_statem is locally registered.
    {Name, Node}
        The gen_statem is locally registered on another node.
    {global, GlobalName}
        The gen_statem is globally registered in global.
    {via, RegMod, ViaName}
        The gen_statem is registered in an alternative process registry. The registry callback module RegMod 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 global. Thus, {via, global, GlobalName} is the same as {global, GlobalName}.
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 succesful return wraps the process ID and the monitor reference in a {ok, {pid(), reference()}} tuple.
start_opt() :: {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() :: {ok, pid()} | ignore | {error, term()}.

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

    See start_link/4.
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 (NextState =/= 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 gen_statem 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 NextState 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(ActionType, DataType) :: {keep_state, NewData :: DataType} | {keep_state, NewData :: DataType, Actions :: [ActionType] | ActionType} | keep_state_and_data | {keep_state_and_data, Actions :: [ActionType] | ActionType} | {repeat_state, NewData :: DataType} | {repeat_state, NewData :: DataType, Actions :: [ActionType] | ActionType} | repeat_state_and_data | {repeat_state_and_data, Actions :: [ActionType] | ActionType} | stop | {stop, Reason :: term()} | {stop, Reason :: term(), NewData :: DataType} | {stop_and_reply, Reason :: term(), Replies :: [reply_action()] | reply_action()} | {stop_and_reply, Reason :: term(), Replies :: [reply_action()] | reply_action(), NewData :: DataType}.

    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 gen_statem 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 gen_statem 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 gen_statem like with {stop, Reason [, NewData]}.

    All these terms are tuples or atoms and will be so in all future versions of gen_statem.
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 gen_statem engine will, at every state change, that is; NextState =/= CurrentState, 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:StateName/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) :: state_enter_result(State, term()).
state_enter_result(State, DataType) :: {next_state, State, NewData :: DataType} | {next_state, State, NewData :: DataType, Actions :: [enter_action()] | enter_action()} | state_callback_result(enter_action(), DataType).

    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, NewData [, Actions]}
        The gen_statem does a state transition to State, which has to be equal to the current state, sets NewData, and executes all Actions.
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 (NextState =/= 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() :: Time :: timeout() | integer().

    How long to wait in the current state.

    Starts a timer set by timeout_action/0, or {state_timeout, Time, EventContent [, 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, NextState =/= CurrentState, the timer runs in the NextState.

    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 NextState.

    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() :: (Time :: event_timeout()) | {timeout, Time :: event_timeout(), EventContent :: event_content()} | {timeout, Time :: event_timeout(), EventContent :: event_content(), Options :: timeout_option() | [timeout_option()]} | {{timeout, Name :: term()}, Time :: generic_timeout(), EventContent :: event_content()} | {{timeout, Name :: term()}, Time :: generic_timeout(), EventContent :: event_content(), Options :: timeout_option() | [timeout_option()]} | {state_timeout, Time :: state_timeout(), EventContent :: event_content()} | {state_timeout, Time :: state_timeout(), EventContent :: 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 or by passing them to enter_loop/4,5,6.

    These time-out actions sets time-out transition options. Time
        Short for {timeout, Time, Time}, that is, the time-out message is the time-out time. This form exists to allow the state callback return value {next_state, NextState, NewData, Time} like in gen_fsm.
    {timeout, Time, EventContent [, Options]}
        Sets the transition_option/0 event_timeout/0 to Time with EventContent, and time-out options Options.
    {{timeout,Name}, Time, EventContent [, Options]}
        Sets the transition_option/0 generic_timeout/0 to Time for time-out Name with EventContent, and time-out options Options.\ Since OTP 20.0.
    {state_timeout, Time, EventContent [, Options]}
        Sets the transition_option/0 state_timeout/0 to Time with EventContent, and time-out options Options.\ Since OTP 19.3.
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() :: 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 type | Action | Event 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 EventType is {EventType, Time, ...}.
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() :: {timeout, update, EventContent :: event_content()} | {{timeout, Name :: term()}, update, EventContent :: event_content()} | {state_timeout, update, EventContent :: event_content()}.

    Update the EventContent without affecting the time of expiry.

    Sets a new EventContent 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() :: 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:

        . 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.

        . 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 gen_statem 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 gen_statem 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.

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

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

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

        . 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.

        . 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.

        . Otherwise the gen_statem 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 gen_statem, or rather the next incoming message awakens the gen_statem, 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. gen_server's time-out works like Erlang's receive ... after 0 ... end, in contrast to gen_statem.

FUNCTIONS

call(ServerRef :: server_ref(), Request :: term(), Timeout :: timeout() | {clean_timeout, T :: timeout()} | {dirty_timeout, T :: timeout()}) -> Reply :: term().

    Call a server: send request and wait for response.

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

    The gen_statem 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.

    Previous issue with late replies that could occur when having network issues or using dirty_timeout is now prevented by use of process aliases. {clean_timeout, T} and {dirty_timeout, T} therefore no longer serves any purpose and will work the same as Timeout while all of them also being equally efficient.

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

    When this call fails it exits the calling process. The exit term is on the form {Reason, Location} where Location = {gen_statem, call, ArgList}. See gen_server:call/3 that has a description of relevant values for the Reason in the exit term.
cast(ServerRef :: server_ref(), Msg :: term()) -> ok.

    Cast an event to a server.

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

    The gen_statem calls the state callback with event_type/0 cast and event content Msg.
check_response(Msg, ReqId) -> Result when Msg :: term(), ReqId :: request_id(), Response :: {reply, Reply :: term()} | {error, {Reason :: term(), server_ref()}}, Result :: Response | no_reply.

    Check if a received message is a request response.

    Checks if Msg is a response corresponding to the request identifier ReqId. 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 ReqId 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 gen_statem server.

    If the gen_statem 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, ReqIdCollection, Delete) -> Result when Msg :: term(), ReqIdCollection :: request_id_collection(), Delete :: boolean(), Response :: {reply, Reply :: term()} | {error, {Reason :: term(), server_ref()}}, Result :: {Response, Label :: term(), NewReqIdCollection :: request_id_collection()} | no_request | no_reply.

    Check if a received message is a request response in a collection.

    Check if Msg is a response corresponding to a request identifier stored in ReqIdCollection. All request identifiers of ReqIdCollection must correspond to requests that have been made using send_request/2 or send_request/4, by the process calling this function.

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

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

    If ReqIdCollection is empty, no_request is returned.

    If Msg does not correspond to any of the request identifiers in ReqIdCollection, no_reply is returned.

    If Delete equals true, the association with Label has been deleted from ReqIdCollection in the resulting NewReqIdCollection. If Delete is false, NewReqIdCollection will equal ReqIdCollection. 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 return no_reply.
enter_loop(Module :: term(), Opts :: term(), State :: term(), Data :: term(), Actions) -> no_return() when Actions :: list(); (Module :: term(), Opts :: term(), State :: term(), Data :: term(), Server) -> no_return() when Server :: server_name() | pid().

    Make the calling process become a gen_statem server.

    With argument Actions, equivalent to enter_loop(Module, Opts, State, Data, self(), Actions).

    Otherwise equivalent to enter_loop(Module, Opts, State, Data, Server, []).
enter_loop(Module :: module(), Opts :: [enter_loop_opt()], State :: state(), Data :: data(), Server :: server_name() | pid(), Actions :: [action()] | action()) -> no_return().

    Make the calling process become a gen_statem server.

    Does not return, instead the calling process enters the gen_statem receive loop and becomes a gen_statem server. The process must have been started using one of the start functions in proc_lib. The user is responsible for any initialization of the process, including registering a name for it.

    This function is useful when a more complex initialization procedure is needed than the gen_statem Module:init/1 callback offers.

    Module and Opts have the same meanings as when calling start[link | monitor]/3,4.

    If Server is self/0 an anonymous server is created just as when using start[link |_monitor]/3](start_link/3). If Server is a server_name/0 a named server is created just as when using [start[link |_monitor]/4. However, the server_name/0 name must have been registered accordingly before this function is called.

    State, Data, and Actions have the same meanings as in the return value of Module:init/1. Also, the callback module does not need to export a Module:init/1 function.

    The function fails if the calling process was not started by a proc_lib start function, or if it is not registered according to server_name/0.
receive_response(ReqId, Timeout) -> Result when ReqId :: request_id(), Timeout :: response_timeout(), Response :: {reply, Reply :: term()} | {error, {Reason :: term(), server_ref()}}, Result :: Response | timeout.

    Receive a request response.

    Receive a response corresponding to the request identifier ReqId. 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(ReqIdCollection, Timeout, Delete) -> Result when ReqIdCollection :: request_id_collection(), Timeout :: response_timeout(), Delete :: boolean(), Response :: {reply, Reply :: term()} | {error, {Reason :: term(), server_ref()}}, Result :: {Response, Label :: term(), NewReqIdCollection :: request_id_collection()} | no_request | timeout.

    Receive a request response in a collection.

    Receive a response in ReqIdCollection. All request identifiers of ReqIdCollection 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 receive_response/2, the returned result or exception associated with a specific request identifier will be wrapped in a 3-tuple {Response, Label, NewReqIdCollection}. Response is the value that would have been produced by receive_response/2, Label is the value associated with the specific request identifier and NewReqIdCollection is a possibly modified request identifier collection.

    If ReqIdCollection is empty, no_request will be returned.

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

    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 is deleted from ReqIdCollection in the resulting NewReqIdCollection. If Delete is false, NewReqIdCollection will equalReqIdCollection. 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 Timeout expires and then return timeout.
reply(Replies :: [reply_action()] | reply_action()) -> ok.

    Send one or multiple call replies.

    This funcion can be used by a gen_statem 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 sys debug output.
reply(From :: from(), Reply :: term()) -> ok.

    Send a call Reply to From.

    This funcion can be used by a gen_statem 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 sys debug output.
reqids_add(ReqId :: request_id(), Label :: term(), ReqIdCollection :: request_id_collection()) -> NewReqIdCollection :: request_id_collection().

    Store a request identifier in a colletion.

    Stores ReqId and associates a Label with the request identifier by adding this information to ReqIdCollection and returning the resulting request identifier collection.
reqids_new() -> NewReqIdCollection :: request_id_collection().

    Create an empty request identifier collection.

    Returns a new empty request identifier collection. A request identifier collection can be used to handle multiple outstanding requests.

    Request identifiers of requests made by send_request/2 can be stored in a 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.

    reqids_size/1 can be used to determine the number of request identifiers in a collection.
reqids_size(ReqIdCollection :: request_id_collection()) -> non_neg_integer().

    Return the number of request identifiers in ReqIdCollection.
reqids_to_list(ReqIdCollection :: request_id_collection()) -> [{ReqId :: request_id(), Label :: term()}].

    Convert a request identifier collection to a list.

    Returns a list of {ReqId, Label} tuples which corresponds to all request identifiers with their associated labels in ReqIdCollection.
send_request(ServerRef :: server_ref(), Request :: term()) -> ReqId :: request_id().

    Send an asynchronous call request.

    Sends Request to the gen_statem process identified by ServerRef and returns a request identifier ReqId.

    The return value ReqId 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 gen_statem:wait_response(gen_statem:send_request(ServerRef, Request), Timeout) can be seen as equivalent to gen_statem:call(Server, Request, Timeout), ignoring the error handling.

    See call/3 about how the request is handled and the Reply is sent by the gen_statem 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(ServerRef :: server_ref(), Request :: term(), Label :: term(), ReqIdCollection :: request_id_collection()) -> NewReqIdCollection :: request_id_collection().

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

    Sends Request to the gen_statem process identified by ServerRef. The Label will be associated with the request identifier of the operation and added to the returned request identifier collection NewReqIdCollection. 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(ServerRef, Request), Label, ReqIdCollection), but slightly more efficient.
start(Module :: module(), Args :: term(), Opts :: [start_opt()]) -> start_ret().

    Start a server, neither linked nor registered.

    Equivalent to start/4 except that the gen_statem process is not registered with any name service.
start(ServerName :: server_name(), Module :: module(), Args :: term(), Opts :: [start_opt()]) -> start_ret().

    Start a server, registered but not linked.

    Creates a standalone gen_statem process according to OTP design principles (using proc_lib primitives). As it does not get linked to the calling process, this start function cannot be used by a supervisor to start a child.

    For a description of arguments and return values, see start_link/4.
start_link(Module :: module(), Args :: term(), Opts :: [start_opt()]) -> start_ret().

    Start a server, linked but not registered.

    Equivalent to start_link/4 except that the gen_statem process is not registered with any name service.
start_link(ServerName :: server_name(), Module :: module(), Args :: term(), Opts :: [start_opt()]) -> start_ret().

    Start a server, linked and registered.

    Creates a gen_statem process according to OTP design principles (using proc_lib primitives) that is spawned linked to the calling process. This is essential when the gen_statem must be part of a supervision tree so it gets linked to its supervisor.

    The spawned gen_statem process calls Module:init/1 to initialize the server. To ensure a synchronized startup procedure, start_link/3,4 does not return until Module:init/1 has returned or failed.

    ServerName specifies the server_name/0 to register for the gen_statem process. If the gen_statem process is started with start_link/3, no ServerName is provided and the gen_statem process is not registered.

    Module is the name of the callback module.

    Args is an arbitrary term that is passed as the argument to Module:init/1.

    Start options in Opts {timeout, Time}
        The gen_statem process is allowed to spend Time milliseconds before returning from Module:init/1, or it is terminated and this start function returns {error, timeout}.
    {spawn_opt, SpawnOpts}
        SpawnOpts is passed as option list to erlang:spawn_opt/2, which is used to spawn the gen_statem process. See proc_lib:start_spawn_option/0.

        NOTE: Using spawn option monitor is not allowed, it causes a badarg failure.
    {hibernate_after, HibernateAfterTimeout}
        When the gen_statem process waits for a message, if no message is received within HibernateAfterTimeout milliseconds, the process goes into hibernation automatically (by calling proc_lib:hibernate/3). This option is also allowed for the enter_loop functions.

        Note that there is also a transition_option/0 to explicitly hibernate the server from a state callback.
    {debug, Dbgs}
        Activates debugging through sys. For every entry in Dbgs, the corresponding function in sys is called. This option is also allowed for the enter_loop functions.

    Return values {ok, Pid}
        The gen_statem server process was successfully created and initialized. Pid is the pid/0 of the process.
    ignore
        Module:init/1 returned ignore. The gen_statem process has exited with reason normal.
    {error, {already_started, OtherPid}}
        A process with the specified ServerName already exists. OtherPid is the pid/0 of that process. The gen_statem process exited with reason normal before calling Module:init/1.
    {error, timeout}
        Module:init/1 did not return within the start time-out. The gen_statem process has been killed with exit_signal(_, kill).

        • {error, Reason}

            • Either Module:init/1 returned {stop, Reason} or failed with reason Reason, The gen_statem process exited with reason Reason.

            • Or Module:init/1 returned {error, Reason}. The gen_statem process did a graceful exit with reason normal.

    If the return value is ignore or {error, _}, the started gen_statem process has terminated. If an 'EXIT' message was delivered to the calling process (due to the process link), that message has been consumed.

    WARNING: Before OTP 26.0, if the started gen_statem process returned e.g. {stop, Reason} from Module:init/1, this function could return {error, Reason} before the started gen_statem process had terminated, so starting again might fail because VM resources such as the registered name was not yet unregistered, and an 'EXIT' message could arrive later to the process calling this function. But if the started gen_statem process instead failed during Module:init/1, a process link {'EXIT', Pid, Reason} message caused this function to return {error, Reason}, so the 'EXIT' message had been consumed and the started gen_statem process had terminated. Since it was impossible to tell the difference between these two cases from start_link/3,4's return value, this inconsistency was cleaned up in OTP 26.0.
start_monitor(Module :: module(), Args :: term(), Opts :: [start_opt()]) -> start_mon_ret().

    Start a server, monitored but neither linked nor registered.

    Equivalent to start_monitor/4 except that the gen_statem process is not registered with any name service.
start_monitor(ServerName :: server_name(), Module :: module(), Args :: term(), Opts :: [start_opt()]) -> start_mon_ret().

    Start a server, monitored and registered, but not linked.

    Creates a standalone gen_statem process according to OTP design principles (using proc_lib primitives), and atomically sets up a monitor to the newly created process.

    As the started process does not get linked to the calling process, this start function cannot be used by a supervisor to start a child.

    For a description of arguments and return values, see start_link/4, but note that for a succesful start the return value differs since this function returns {ok, {Pid, Mon}}, where Pid is the process identifier of the process, and Mon is the monitor reference for the process. If the start is not successful, the caller will be blocked until the DOWN message has been received and removed from the caller's message queue.
stop(ServerRef :: server_ref(), Reason :: term(), Timeout :: timeout()) -> ok.

    Stop a server.

    Orders the gen_statem ServerRef to exit with the specified Reason and waits for it to terminate. The gen_statem calls Module:terminate/3 before exiting.

    This function returns ok if the server terminates with the expected reason. Any other reason than normal, shutdown, or {shutdown, Term} causes an error report to be issued through logger. An exit signal with the same reason is sent to linked processes and ports. The default Reason is normal.

    Timeout is an integer > 0, which specifies how many milliseconds to wait for the server to terminate, or the atom infinity to wait indefinitely. Defaults to infinity. If the server does not terminate within the specified time, the call exits the calling process with reason timeout.

    If the process does not exist, the call exits the calling process with reason noproc, or with reason {nodedown, Node} if the connection fails to the remote Node where the server runs.
wait_response(ReqId, WaitTime) -> Result when ReqId :: request_id(), WaitTime :: response_timeout(), Response :: {reply, Reply :: term()} | {error, {Reason :: term(), server_ref()}}, Result :: Response | timeout.

    Wait for a request response.

    Waits for the response to the request identifier ReqId. 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 called.

    WaitTime 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 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 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(ReqIdCollection, WaitTime, Delete) -> Result when ReqIdCollection :: request_id_collection(), WaitTime :: response_timeout(), Delete :: boolean(), Response :: {reply, Reply :: term()} | {error, {Reason :: term(), server_ref()}}, Result :: {Response, Label :: term(), NewReqIdCollection :: request_id_collection()} | no_request | timeout.

    Wait for any request response in a collection.

    Waits for a response in ReqIdCollection. All request identifiers of ReqIdCollection 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, NewReqIdCollection}. Response is the value that would have been produced by wait_response/2, Label is the value associated with the specific request identifier and NewReqIdCollection is a possibly modified request identifier collection.

    If ReqIdCollection is empty, no_request is returned.

    If no response is received before WaitTime 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 ReqIdCollection in the resulting NewReqIdCollection. If Delete is false, NewReqIdCollection will equalReqIdCollection. 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 WaitTime expires and then return timeout.

CALLBACKS

'StateName'/3

    State callback in callback mode state_functions.

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

    StateName 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(​EventType, EventContent, ?FUNCTION_NAME, Data), which is the state callback in callback mode handle_event_function.
callback_mode()

    Select the callback mode and possibly state enter calls.

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

    The value is cached by gen_statem 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 gen_statem.

    Server start happens either when Module:init/1 returns or when enter_loop/4,5,6 is called. 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 CallbackMode 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(OldVsn, OldState, OldData, Extra)

    Update the state and data after code change.

    This function is called by a gen_statem 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, OldVsn is Vsn, and for a downgrade, OldVsn 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.

    OldState and OldData is the internal state of the gen_statem.

    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, NewState, NewData} 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, NewState, NewData} 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 gen_statem, 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 gen_statem 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)

    Format/limit the status value.

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

    It is called in the following situations:

        • sys:get_status/1,2 is invoked to get the gen_statem status.

        • The gen_statem process terminates abnormally and logs an error.

    This function is useful for changing the form and appearance of the gen_statem 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 gen_statem, 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:

        format_status(Status) ->
          maps:map(
            fun(state,State) ->
                    maps:remove(private_key, State);
               (message,{password, _Pass}) ->
                    {password, removed};
               (_,Value) ->
                    Value
            end, Status).

    NOTE: This callback is optional, so a callback module does not need to export it. The gen_statem module provides a default implementation of this function that returns {State, Data}. If this callback is exported but fails, to hide possibly sensitive data, the default function will instead return {State, Info}, where Info says nothing but the fact that Module:format_status/2 has crashed.
format_status/2

    Deprecated: the callback gen_statem:format_status(_,_) is deprecated; use format_status/1 instead

    Format/limit the status value.

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

    It is called in the following situations:

        • One of sys:get_status/1,2 is invoked to get the gen_statem status. Opt is set to the atom normal for this case.

        • The gen_statem terminates abnormally and logs an error. Opt is set to the atom terminate for this case.

    This function is useful for changing the form and appearance of the gen_statem 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, should export an instance of Module:format_status/2, that returns a term describing the current status of the gen_statem.

    PDict is the current value of the process dictionary of the gen_statem.

    State is the internal state of the gen_statem.

    Data is the internal server data of the gen_statem.

    The function is to return Status, a term that contains the appropriate details of the current state and status of the gen_statem. There are no restrictions of the form Status can take, but for the sys:get_status/1,2 case (when Opt is normal), the recommended form for the Status value is [{data, [{"State", Term}]}], where Term provides relevant details of the gen_statem state. Following this recommendation is not required, but it makes the callback module status consistent with the rest of the sys:get_status/1,2 return value.

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

    NOTE: This callback is optional, so a callback module does not need to export it. The gen_statem module provides a default implementation of this function that returns {State, Data}. If this callback is exported but fails, to hide possibly sensitive data, the default function will instead return {State, Info}, where Info says nothing but the fact that Module:format_status/2 has crashed.
handle_event/4

    State callback in callback mode handle_event_function.

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

    If EventType 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 gen_statem after returning from this function, see action/0.

    When the gen_statem runs with state enter calls, this function is also called with arguments (enter, OldState, ...) 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, NextState, ...} with NextState =/= State the gen_statem crashes.

        Note that it is actually allowed to use {repeat_state, NewData, ...} 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 in Erlang.

        If you do not update NewData 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)

    Initialize the state machine.

    Whenever a gen_statem 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 gen_statem is started through proc_lib and enter_loop/4,5,6, 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]).
terminate/3

    Handle state machine termination.

    This function is called by a gen_statem 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 gen_statem terminates with Reason. The return value is ignored.

    Reason is a term denoting the stop reason and State is the internal state of the gen_statem.

    Reason depends on why the gen_statem 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 gen_statem 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 gen_statem 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 gen_statem 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 gen_statem 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 gen_statem is assumed to terminate because of an error and an error report is issued using logger.

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