gen_statem(3erl) Erlang Module Definition gen_statem(3erl)
NAME
gen_statem - Generic state machine behavior.
DESCRIPTION
gen_statem provides a generic state machine behaviour that for new code
replaces its predecessor gen_fsm since Erlang/OTP 20.0. The gen_fsm be-
haviour remains in OTP "as is".
Note:
If you are new to gen_statem and want an overview of concepts and oper-
ation the section gen_statem Behaviour located in the User's Guide
OTP Design Principles is recommended to read before this reference
manual, possibly after the Description section you are reading here.
This reference manual contains type descriptions generated from types
in the gen_statem source code, so they are correct. However, the gener-
ated descriptions also reflect the type hierarchy, which sometimes
makes it hard to get a good overview. If so, see the section gen_statem
Behaviour in the OTP Design Principles User's Guide.
Note:
* This behavior appeared in Erlang/OTP 19.0.
*
In OTP 19.1 a backwards incompatible change of the return tuple
from Module:init/1 was made and the mandatory callback function
Module:callback_mode/0 was introduced.
*
In OTP 20.0 generic time-outs were added.
*
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 ac-
tions change_callback_module, push_callback_module and pop_call-
back_module, was added.
gen_statem has got the same features that gen_fsm had and adds some re-
ally 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, sys traceable
* Multiple sys traceable replies
* Changing the callback module
Two callback modes are supported:
* One 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.
* One that allows the state to be any term and that uses one callback
function for all states.
The callback model(s) for gen_statem differs from the one for gen_fsm,
but it is still fairly easy to rewrite from gen_fsm to gen_statem.
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 OTP Design Principles.
A gen_statem assumes all specific parts to be located in a callback
module exporting a predefined set of functions. The relationship be-
tween 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
gen_statem:stop -----> Module:terminate/3
gen_statem:call
gen_statem:cast
gen_statem:send_request
erlang:send
erlang:'!' -----> Module:StateName/3
Module:handle_event/4
- -----> Module:terminate/3
- -----> Module:code_change/4
Events are of different types, so the callback functions can know the
origin of an event and how to respond.
If a callback function fails or returns a bad value, the gen_statem
terminates, unless otherwise stated. However, an exception of class
throw is not regarded as an error but as a valid return from all call-
back functions.
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 acci-
dentally postpone an event forever creating an infinite busy loop.
When gen_statem receives a process message it is converted 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.
The state callback may return transition actions for gen_statem to exe-
cute during the state transition, for example to reply to a
gen_statem:call/2,3.
One of the possible transition actions is to postpone the current
event. Then it is not retried in the current state. The gen_statem en-
gine keeps a queue of events divided into the postponed events and the
events still to process. 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 cor-
responds to not matching it in a receive statement, and changing states
corresponds to entering a new receive statement.
The state callback can insert events using the transition actions
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 incom-
ing event. A dedicated event_type() internal can be used for such
events making them impossible to mistake for external events.
Inserting an event replaces the trick of calling your own state han-
dling functions that you often would have to resort to in, for example,
gen_fsm to force processing an inserted event before others.
The gen_statem engine can automatically make a specialized call to the
state callback whenever a new state is entered; see state_enter(). 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.
Note:
If you in gen_statem, for example, postpone an event in one state and
then call another state callback of yours, you have not done a state
change and hence the postponed event is not retried, which is logical
but can be confusing.
For the details of a state transition, see type transition_option().
A gen_statem handles system messages as described in sys. The sys mod-
ule can be used for debugging a gen_statem.
Notice that a gen_statem does not trap exit signals automatically, this
must be explicitly initiated in the callback module (by calling
process_flag(trap_exit, true).
Unless otherwise stated, all functions in this module fail if the spec-
ified gen_statem does not exist or if bad arguments are specified.
The gen_statem process can go into hibernation; see proc_lib:hiber-
nate/3. It is done when a state callback or Module:init/1 specifies hi-
bernate in the returned Actions list. This feature can be useful to re-
claim process heap memory while the server is expected to be idle for a
long time. However, use this feature 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_monitor/3,4, start_link/3,4 or enter_loop/4,5,6, that
may be used to automatically hibernate the server.
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.
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}.
DATA TYPES
server_name() =
{global, GlobalName :: term()} |
{via, RegMod :: module(), Name :: term()} |
{local, atom()}
Name specification to use when starting a gen_statem server. See
start_link/3 and server_ref() below.
server_ref() =
pid() |
(LocalName :: atom()) |
{Name :: atom(), Node :: atom()} |
{global, GlobalName :: term()} |
{via, RegMod :: module(), ViaName :: term()}
Server specification to use when addressing a gen_statem server.
See call/2 and server_name() above.
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 reg-
istry. 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,Global-
Name} is the same as {global,GlobalName}.
start_opt() =
{timeout, Time :: timeout()} |
{spawn_opt, [proc_lib:start_spawn_option()]} |
enter_loop_opt()
Options that can be used when starting a gen_statem server
through, for example, start_link/3.
start_ret() = {ok, pid()} | ignore | {error, term()}
Return value from the start() and start_link() functions, for
example, start_link/3.
start_mon_ret() =
{ok, {pid(), reference()}} | ignore | {error, term()}
Return value from the start_monitor() functions.
enter_loop_opt() =
{hibernate_after, HibernateAfterTimeout :: timeout()} |
{debug, Dbgs :: [sys:debug_option()]}
Options that can be used when starting a gen_statem server
through, enter_loop/4-6.
hibernate_after:
HibernateAfterTimeout specifies that the gen_statem process
awaits any message for HibernateAfterTimeout milliseconds
and if no message is received, the process goes into hiber-
nation automatically (by calling proc_lib:hibernate/3).
debug:
For every entry in Dbgs, the corresponding function in sys
is called.
from() = {To :: pid(), Tag :: term()}
Destination to use when replying through, for example, the ac-
tion() {reply,From,Reply} to a process that has called the
gen_statem server using call/2.
state() = state_name() | term()
If the callback mode is handle_event_function, the state can be
any term. After a state change (NextState =/= State), all post-
poned events are retried.
state_name() = atom()
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 possi-
ble to use since it would collide with the optional callback
function Module:terminate/3.
data() = term()
A term in which the state machine implementation is to store any
server data it needs. The difference between this and the
state() 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 is to be made a part of the state.
event_type() =
external_event_type() | timeout_event_type() | internal
There are 3 categories of events: external, timeout, and inter-
nal.
internal events can only be generated by the state machine it-
self through the transition action next_event.
external_event_type() = {call, From :: from()} | cast | info
External events are of 3 types: {call,From}, cast, or info. Type
call originates from the API functions call/2 and send_re-
quest/2. For calls, the event contains whom to reply to. Type
cast originates from the API function cast/2. Type info origi-
nates from regular process messages sent to the gen_statem.
timeout_event_type() =
timeout | {timeout, Name :: term()} | state_timeout
There are 3 types of time-out events that the state machine can
generate for itself with the corresponding timeout_action()s.
callback_mode_result() =
callback_mode() | [callback_mode() | state_enter()]
This is the return type from Module:callback_mode/0 and selects
callback mode and whether to do state enter calls, or not.
callback_mode() = state_functions | handle_event_function
The callback mode is selected with the return value from Mod-
ule:callback_mode/0:
state_functions:
The state must be of type state_name() 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 Mod-
ule: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.
state_enter() = state_enter
Whether the state machine should use state enter calls or not is
selected when starting the gen_statem and after code change us-
ing the 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, 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 pre-
vious state callback returned and before any event is delivered
to the new state callback. See Module:StateName/3 and Mod-
ule:handle_event/4. Such a call can be repeated by returning a
repeat_state or repeat_state_and_data tuple from the state call-
back.
If Module:callback_mode/0 does not return such a list, no state
enter calls are done.
If Module:code_change/4 should transform the state, it is re-
garded 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 even though this actually is not a state
change. In this case OldState =:= State, which cannot happen for
a subsequent state change, but will happen when repeating the
state enter call.
transition_option() =
postpone() |
hibernate() |
event_timeout() |
generic_timeout() |
state_timeout()
Transition options can be set by actions and modify the state
transition. The state transition takes place when the state
callback has processed an event and returns. Here are the se-
quence 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() are
sent. Other actions set transition_option()s that come into
play in subsequent steps.
* If state enter calls are used, and either it is the initial
state or one of the callback results repeat_state_and_data
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 argu-
ments (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() 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() next_event are inserted to
be processed before previously queued events.
* Time-out timers event_timeout(), generic_timeout() and
state_timeout() 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 en-
queued as the newest received event; that is after already
queued events such as inserted and postponed events.
Any event cancels an event_timeout() so a zero time event
time-out is only generated if the event queue is empty.
A state change cancels a state_timeout() and any new transi-
tion option of this type belongs to the new state, that is;
a state_timeout() applies to the state the state machine en-
ters.
* If there are enqueued events the state callback for the pos-
sibly 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() 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 call-
back is called with the corresponding event, and we start
again from the top of this sequence.
postpone() = boolean()
If true, postpones the current event and retries it after a
state change (NextState =/= State).
hibernate() = boolean()
If true, hibernates the gen_statem by calling proc_lib:hiber-
nate/3 before going into receive to wait for a new external
event.
Note:
If there are enqueued events to process when hibrnation is re-
quested, this is optimized by not hibernating but instead call-
ing erlang:garbage_collect/0 to simulate that the gen_statem en-
tered hibernation and immediately got awakened by an enqueued
event.
event_timeout() = timeout() | integer()
Starts a timer set by enter_action() timeout. When the timer ex-
pires an event of event_type() 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 sup-
ported.
Any event that arrives cancels this time-out. Note that a re-
tried or inserted event counts as arrived. So does a state time-
out zero event, if it was generated before this time-out is re-
quested.
If Time is infinity, no timer is started, as it never would ex-
pire 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 pro-
cessed before any not yet received external event, but after al-
ready queued events.
Note that it is not possible nor needed to cancel this time-out,
as it is cancelled automatically by any other event.
generic_timeout() = timeout() | integer()
Starts a timer set by enter_action() {timeout,Name}. When the
timer expires an event of event_type() {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 neces-
sarily be supported.
If Time is infinity, no timer is started, as it never would ex-
pire 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 pro-
cessed 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.
state_timeout() = timeout() | integer()
Starts a timer set by enter_action() state_timeout. When the
timer expires an event of event_type() 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 neces-
sarily be supported.
If Time is infinity, no timer is started, as it never would ex-
pire 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 pro-
cessed 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.
timeout_option() = {abs, Abs :: boolean()}
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.
action() =
postpone |
{postpone, Postpone :: postpone()} |
{next_event,
EventType :: event_type(),
EventContent :: term()} |
{change_callback_module, NewModule :: module()} |
{push_callback_module, NewModule :: module()} |
pop_callback_module |
enter_action()
These transition actions can be invoked by returning them from
the state callback when it is called with an event, from Mod-
ule:init/1 or by giving them to enter_loop/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 ex-
ample, the last postpone() overrides any previous postpone() in
the list.
postpone:
Sets the transition_option() postpone() for this state tran-
sition. This action is ignored when returned from Mod-
ule:init/1 or given to enter_loop/5,6, as there is no event
to postpone in those cases.
next_event:
This action does not set any transition_option() but instead
stores the specified EventType and EventContent for inser-
tion 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 is to be used when you want to re-
liably distinguish an event inserted this way from any ex-
ternal event.
change_callback_module:
Changes the callback module to NewModule which will be used
when calling all subsequent state callbacks.
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 tran-
sition 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, NewMod-
ule:code_change/4, NewModule:format_status/2 and NewMod-
ule:terminate/3 must be able to handle the state and data
from the old module.
push_callback_module:
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, NewMod-
ule} 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, NewModule} above.
enter_action() =
hibernate |
{hibernate, Hibernate :: hibernate()} |
timeout_action() |
reply_action()
These transition actions can be invoked by returning them from
the state callback, from Module:init/1 or by giving them to en-
ter_loop/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() overrides any previous event_timeout()
in the list.
hibernate:
Sets the transition_option() hibernate() for this state
transition.
timeout_action() =
(Time :: event_timeout()) |
{timeout, Time :: event_timeout(), EventContent :: term()} |
{timeout,
Time :: event_timeout(),
EventContent :: term(),
Options :: timeout_option() | [timeout_option()]} |
{{timeout, Name :: term()},
Time :: generic_timeout(),
EventContent :: term()} |
{{timeout, Name :: term()},
Time :: generic_timeout(),
EventContent :: term(),
Options :: timeout_option() | [timeout_option()]} |
{state_timeout,
Time :: state_timeout(),
EventContent :: term()} |
{state_timeout,
Time :: state_timeout(),
EventContent :: term(),
Options :: timeout_option() | [timeout_option()]} |
timeout_cancel_action() |
timeout_update_action()
These transition actions can be invoked by returning them from
the state callback, from Module:init/1 or by giving them to en-
ter_loop/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 make the state
callback return value {next_state,NextState,NewData,Time}
allowed like for gen_fsm.
timeout:
Sets the transition_option() event_timeout() to Time with
EventContent and time-out options Options.
{timeout,Name}:
Sets the transition_option() generic_timeout() to Time for
Name with EventContent and time-out options Options.
state_timeout:
Sets the transition_option() state_timeout() to Time with
EventContent and time-out options Options.
timeout_cancel_action() =
{timeout, cancel} |
{{timeout, Name :: term()}, cancel} |
{state_timeout, cancel}
This is a shorter and clearer form of timeout_action() with
Time = infinity which cancels a time-out.
timeout_update_action() =
{timeout, update, EventContent :: term()} |
{{timeout, Name :: term()}, update, EventContent :: term()} |
{state_timeout, update, EventContent :: term()}
Updates a time-out with a new EventContent. See timeout_ac-
tion() for how to start a time-out.
If no time-out of the same type is active instead insert the
time-out event just like when starting a time-out with relative
Time = 0.
reply_action() = {reply, From :: from(), Reply :: term()}
This transition action can be invoked by returning it from the
state callback, from Module:init/1 or by giving it to en-
ter_loop/5,6.
It does not set any transition_option() but instead replies to a
caller waiting for a reply in call/2. 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/5,6
would be weird on the border of witchcraft since there has been
no earlier call to a state callback in this server.
init_result(StateType) =
{ok, State :: StateType, Data :: data()} |
{ok,
State :: StateType,
Data :: data(),
Actions :: [action()] | action()} |
ignore |
{stop, Reason :: term()}
For a succesful initialization, State is the initial state() and
Data the initial server data() 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} or ignore
should be used; see start_link/3,4.
state_enter_result(State) =
{next_state, State, NewData :: data()} |
{next_state, State,
NewData :: data(),
Actions :: [enter_action()] | enter_action()} |
state_callback_result(enter_action())
State is the current state and it cannot be changed since the
state callback was called with a state enter call.
next_state:
The gen_statem does a state transition to State, which has
to be the current state, sets NewData, and executes all Ac-
tions.
event_handler_result(StateType) =
{next_state, NextState :: StateType, NewData :: data()} |
{next_state,
NextState :: StateType,
NewData :: data(),
Actions :: [action()] | action()} |
state_callback_result(action())
StateType is state_name() if callback mode is state_functions,
or state() if callback mode is handle_event_function.
next_state:
The gen_statem does a state transition to NextState (which
can be the same as the current state), sets NewData, and ex-
ecutes all Actions. If NextState =/= CurrentState the state
transition is a state change.
state_callback_result(ActionType) =
{keep_state, NewData :: data()} |
{keep_state,
NewData :: data(),
Actions :: [ActionType] | ActionType} |
keep_state_and_data |
{keep_state_and_data, Actions :: [ActionType] | ActionType} |
{repeat_state, NewData :: data()} |
{repeat_state,
NewData :: data(),
Actions :: [ActionType] | ActionType} |
repeat_state_and_data |
{repeat_state_and_data, Actions :: [ActionType] | ActionType} |
stop |
{stop, Reason :: term()} |
{stop, Reason :: term(), NewData :: data()} |
{stop_and_reply,
Reason :: term(),
Replies :: [reply_action()] | reply_action()} |
{stop_and_reply,
Reason :: term(),
Replies :: [reply_action()] | reply_action(),
NewData :: data()}
ActionType is enter_action() if the state callback was called
with a state enter call and action() if the state callback was
called with an event.
keep_state:
The same as {next_state,CurrentState,NewData,Actions}.
keep_state_and_data:
The same as {keep_state,CurrentData,Actions}.
repeat_state:
If the gen_statem runs with state enter calls, the state en-
ter call is repeated, see type transition_option(), other
than that repeat_state is the same as keep_state.
repeat_state_and_data:
The same as {repeat_state,CurrentData,Actions}.
stop:
Terminates the gen_statem by calling Module:terminate/3 with
Reason and NewData, if specified.
stop_and_reply:
Sends all Replies, then terminates the gen_statem by calling
Module:terminate/3 with Reason and NewData, if specified.
All these terms are tuples or atoms and this property will hold
in any future version of gen_statem.
request_id() = term()
A request handle, see send_request/2 for details.
EXPORTS
call(ServerRef :: server_ref(), Request :: term()) ->
Reply :: term()
call(ServerRef :: server_ref(),
Request :: term(),
Timeout ::
timeout() |
{clean_timeout, T :: timeout()} |
{dirty_timeout, T :: timeout()}) ->
Reply :: term()
Makes a synchronous call to the gen_statem ServerRef by sending
a request and waiting until its reply arrives. The gen_statem
calls the state callback with event_type() {call,From} and event
content Request.
A Reply is generated when a state callback returns with {re-
ply,From,Reply} as one action(), and that Reply becomes the re-
turn value of this function.
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 speci-
fied time, the function call fails.
Note:
For Timeout < infinity, to avoid getting a late reply in the
caller's inbox if the caller should catch exceptions, this func-
tion spawns a proxy process that does the call. A late reply
gets delivered to the dead proxy process, hence gets discarded.
This is less efficient than using Timeout == infinity.
Timeout can also be a tuple {clean_timeout,T} or {dirty_time-
out,T}, where T is the time-out time. {clean_timeout,T} works
like just T described in the note above and uses a proxy process
while {dirty_timeout,T} bypasses the proxy process which is more
lightweight.
Note:
If you combine catching exceptions from this function with
{dirty_timeout,T} to avoid that the calling process dies when
the call times out, you will have to be prepared to handle a
late reply. Note that there is an odd chance to get a late reply
even with {dirty_timeout,infinity} or infinity for example in
the event of network problems. So why not just let the calling
process die by not catching the exception?
The call can also fail, for example, if the gen_statem dies be-
fore or during this function call.
cast(ServerRef :: server_ref(), Msg :: term()) -> ok
Sends an asynchronous event to the gen_statem ServerRef and re-
turns ok immediately, ignoring if the destination node or
gen_statem does not exist. The gen_statem calls the state call-
back with event_type() cast and event content Msg.
check_response(Msg :: term(), RequestId :: request_id()) ->
{reply, Reply :: term()} |
no_reply |
{error, {term(), server_ref()}}
This function is used to check if a previously received message,
for example by receive or handle_info/2, is a result of a re-
quest made with send_request/2. If Msg is a reply to the handle
RequestId the result of the request is returned in Reply. Other-
wise returns no_reply and no cleanup is done, and thus the func-
tion shall be invoked repeatedly until a reply is returned.
The return value Reply is generated when a state callback re-
turns with {reply,From,Reply} as one action(), and that Reply
becomes the return value of this function.
The function returns an error if the gen_statem dies before or
during this request.
enter_loop(Module :: module(),
Opts :: [enter_loop_opt()],
State :: state(),
Data :: data()) ->
no_return()
The same as enter_loop/6 with Actions = [] except that no
server_name() must have been registered. This creates an anony-
mous server.
enter_loop(Module :: module(),
Opts :: [enter_loop_opt()],
State :: state(),
Data :: data(),
Server_or_Actions :: server_name() | pid() | [action()]) ->
no_return()
If Server_or_Actions is a list(), the same as enter_loop/6 ex-
cept that no server_name() must have been registered and Actions
= Server_or_Actions. This creates an anonymous server.
Otherwise the same as enter_loop/6 with Server = Server_or_Ac-
tions and Actions = [].
enter_loop(Module :: module(),
Opts :: [enter_loop_opt()],
State :: state(),
Data :: data(),
Server :: server_name() | pid(),
Actions :: [action()] | action()) ->
no_return()
Makes the calling process become a gen_statem. 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 pro-
cedure is needed than the gen_statem behavior provides.
Module, Opts have the same meaning as when calling
start[_link|_monitor]/3,4.
If Server is self() an anonymous server is created just as when
using start[_link|_monitor]/3. If Server is a server_name() a
named server is created just as when using start[_link|_moni-
tor]/4. However, the server_name() name must have been regis-
tered 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().
reply(Replies :: [reply_action()] | reply_action()) -> ok
reply(From :: from(), Reply :: term()) -> ok
This function can be used by a gen_statem to explicitly send a
reply to a process that waits in call/2 when the reply cannot be
defined in the return value of a state callback.
From must be the term from argument {call,From} to the state
callback. A reply or multiple replies canalso be sent using one
or several reply_action()s from a state callback.
Note:
A reply sent with this function is not visible in sys debug out-
put.
send_request(ServerRef :: server_ref(), Request :: term()) ->
RequestId :: request_id()
Sends a request to the gen_statem ServerRef and returns a handle
RequestId.
The return value RequestId shall later be used with wait_re-
sponse/1,2 or check_response/2 to fetch the actual result of the
request.
The call gen_statem:wait_response(gen_statem:send_re-
quest(ServerRef,Request), Timeout) can be seen as equivalent to
gen_statem:call(Server,Request,Timeout), ignoring the error han-
dling.
The gen_statem calls the state callback with event_type()
{call,From} and event content Request.
A Reply is generated when a state callback returns with {re-
ply,From,Reply} as one action(), and that Reply becomes the re-
turn value of wait_response/1,2 or check_response/2 function.
start(Module :: module(), Args :: term(), Opts :: [start_opt()]) ->
start_ret()
start(ServerName :: server_name(),
Module :: module(),
Args :: term(),
Opts :: [start_opt()]) ->
start_ret()
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/3,4.
start_link(Module :: module(),
Args :: term(),
Opts :: [start_opt()]) ->
start_ret()
start_link(ServerName :: server_name(),
Module :: module(),
Args :: term(),
Opts :: [start_opt()]) ->
start_ret()
Creates a gen_statem process according to OTP design principles
(using proc_lib primitives) that is 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 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.
ServerName specifies the server_name() to register for the
gen_statem. If the gen_statem is started with start_link/3, no
ServerName is provided and the gen_statem is not registered.
Module is the name of the callback module.
Args is an arbitrary term that is passed as the argument to Mod-
ule:init/1.
* If option {timeout,Time} is present in Opts, the gen_statem
is allowed to spend Time milliseconds initializing or it
terminates and the start function returns {error,timeout}.
* If option {hibernate_after,HibernateAfterTimeout} is
present, the gen_statem process awaits any message for Hi-
bernateAfterTimeout milliseconds and if no message is re-
ceived, the process goes into hibernation automatically (by
calling proc_lib:hibernate/3).
* If option {debug,Dbgs} is present in Opts, debugging through
sys is activated.
* If option {spawn_opt,SpawnOpts} is present in Opts,
SpawnOpts is passed as option list to erlang:spawn_opt/2,
which is used to spawn the gen_statem process.
Note:
Using spawn option monitor is not allowed, it causes this func-
tion to fail with reason badarg.
If the gen_statem is successfully created and initialized, this
function returns {ok,Pid}, where Pid is the pid() of the
gen_statem. If a process with the specified ServerName exists
already, this function returns {error,{already_started,Pid}},
where Pid is the pid() of that process.
If Module:init/1 fails with Reason, this function returns {er-
ror,Reason}. If Module:init/1 returns {stop,Reason} or ignore,
the process is terminated and this function returns {error,Rea-
son} or ignore, respectively.
start_monitor(Module :: module(),
Args :: term(),
Opts :: [start_opt()]) ->
start_mon_ret()
start_monitor(ServerName :: server_name(),
Module :: module(),
Args :: term(),
Opts :: [start_opt()]) ->
start_mon_ret()
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 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/3,4. Note that the return value on successful start
differs from start_link/3,4. start_monitor/3,4 will return
{ok,{Pid,Mon}} where Pid is the process identifier of the
process, and Mon is a reference to the monitor set up to monitor
the process. If the start is not successful, the caller will be
blocked until the DOWN message has been received and removed
from the message queue.
stop(ServerRef :: server_ref()) -> ok
The same as stop(ServerRef, normal, infinity).
stop(ServerRef :: server_ref(),
Reason :: term(),
Timeout :: timeout()) ->
ok
Orders the gen_statem ServerRef to exit with the specified Rea-
son and waits for it to terminate. The gen_statem calls Mod-
ule:terminate/3 before exiting.
This function returns ok if the server terminates with the ex-
pected reason. Any other reason than normal, shutdown, or {shut-
down,Term} causes an error report to be issued through log-
ger(3erl). 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, a timeout exception is
raised.
If the process does not exist, a noproc exception is raised.
wait_response(RequestId :: request_id()) ->
{reply, Reply :: term()} |
{error, {term(), server_ref()}}
wait_response(RequestId :: request_id(), Timeout :: timeout()) ->
{reply, Reply :: term()} |
timeout |
{error, {term(), server_ref()}}
This function is used to wait for a reply of a request made with
send_request/2 from the gen_statem process. This function must
be called from the same process from which send_request/2 was
made.
Timeout is an integer greater then or equal to zero that speci-
fies how many milliseconds to wait for an reply, or the atom in-
finity to wait indefinitely. Defaults to infinity. If no reply
is received within the specified time, the function returns
timeout and no cleanup is done, and thus the function can be in-
voked repeatedly until a reply is returned.
The return value Reply is generated when a state callback re-
turns with {reply,From,Reply} as one action(), and that Reply
becomes the return value of this function.
The function returns an error if the gen_statem dies before or
during this function call.
CALLBACK FUNCTIONS
The following functions are to be exported from a gen_statem callback
module.
EXPORTS
Module:callback_mode() -> CallbackMode
Types:
CallbackMode = callback_mode() | [ callback_mode() |
state_enter() ]
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 af-
ter changing the callback module, but before the first state
callback in the current callback module's code version 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/4-6 is called. Code change happens when Mod-
ule:code_change/4 returns. A change of the callback module hap-
pens when a state callback returns any of the actions
change_callback_module, push_callback_module or pop_call-
back_module.
The CallbackMode is either just callback_mode() or a list con-
taining callback_mode() 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.
Module:code_change(OldVsn, OldState, OldData, Extra) -> Result
Types:
OldVsn = Vsn | {down,Vsn}
Vsn = term()
OldState = NewState = term()
Extra = term()
Result = {ok,NewState,NewData} | Reason
OldState = NewState = state()
OldData = NewData = data()
Reason = term()
Note:
This callback is optional, so callback modules need not export
it. If a release upgrade/downgrade with Change = {advanced,Ex-
tra} specified in the .appup file is made when code_change/4 is
not implemented the process will crash with exit reason undef.
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 = {ad-
vanced,Extra}, is specified in the appup file. For more informa-
tion, see 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,New-
State,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_call-
back_module, be aware that it is always the current callback
module that will get this callback call. That the current call-
back 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.
Module:init(Args) -> Result(StateType)
Types:
Args = term()
Result(StateType) = init_result(StateType)
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 en-
ter_loop/4-6, this callback will never be called. Since this
callback is not optional it can in that case be implemented as:
init(Args) -> erlang:error(not_implemented, [Args]).
Module:format_status(Opt, [PDict,State,Data]) -> Status
Types:
Opt = normal | terminate
PDict = [{Key, Value}]
State = state()
Data = data()
Key = term()
Value = term()
Status = term()
Note:
This callback is optional, so a callback module does not need to
export it. The gen_statem module provides a default implementa-
tion of this function that returns {State,Data}.
If this callback is exported but fails, to hide possibly sensi-
tive data, the default function will instead return
{State,Info}, where Info says nothing but the fact that for-
mat_status/2 has crashed.
This function is called by a gen_statem process when any of the
following apply:
*
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 exports an instance of for-
mat_status/2, which 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 ap-
propriate details of the current state and status of the
gen_statem. There are no restrictions on 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.
Module:StateName(enter, OldState, Data) -> StateEnterResult(StateName)
Module:StateName(EventType, EventContent, Data) -> StateFunctionResult
Module:handle_event(enter, OldState, State, Data) -> StateEnterRe-
sult(State)
Module:handle_event(EventType, EventContent, State, Data) -> Han-
dleEventResult
Types:
EventType = event_type()
EventContent = term()
State = state()
Data = NewData = data()
StateEnterResult(StateName) = state_enter_result(StateName)
StateFunctionResult = event_handler_result(state_name())
StateEnterResult(State) = state_enter_result(State)
HandleEventResult = event_handler_result(state())
Whenever a gen_statem receives an event from call/2, cast/2, or
as a normal process message, one of these functions is called.
If callback mode is state_functions, Module:StateName/3 is
called, and if it is handle_event_function, Module:han-
dle_event/4 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 re-
tried in the next state.
The only difference between StateFunctionResult and HandleEven-
tResult is that for StateFunctionResult the next state must be
an atom, but for HandleEventResult there is no restriction on
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().
When the gen_statem runs with state enter calls, these functions
are also called with arguments (enter, OldState, ...) during ev-
ery state change. In this case there are some restrictions on
the actions that may be returned: postpone() is not allowed
since a state enter call is not an event so there is no event to
postpone, and {next_event,_,_} is not allowed since using state
enter calls should not affect how events are consumed and pro-
duced. You may also not 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 {re-
peat_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 {re-
peat_state_and_data, _} or repeat_state_and_data you have an in-
finite 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.
Module:terminate(Reason, State, Data) -> Ignored
Types:
Reason = normal | shutdown | {shutdown,term()} | term()
State = state()
Data = data()
Ignored = term()
Note:
This callback is optional, so callback modules need not export
it. The gen_statem module provides a default implementation
without cleanup.
This function is called by a gen_statem when it is about to ter-
minate. It is to be the opposite of Module:init/1 and do any
necessary cleaning up. When it returns, the gen_statem termi-
nates with Reason. The return value is ignored.
Reason is a term denoting the stop reason and State is the in-
ternal 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 rea-
son.
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 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.
Otherwise, the gen_statem is immediately terminated.
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(3erl).
SEE ALSO
gen_event(3erl), gen_fsm(3erl), gen_server(3erl), proc_lib(3erl), su-
pervisor(3erl), sys(3erl).
Ericsson AB stdlib 3.13 gen_statem(3erl)