gen_fsm
Generic Finite State Machine Behaviour
A behaviour module for implementing a finite state machine. A generic finite state machine process (gen_fsm) implemented using this module will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into an OTP supervision tree. Refer to OTP Design Principles for more information.
A gen_fsm assumes all specific parts to be located in a callback module exporting a pre-defined set of functions. The relationship between the behaviour functions and the callback functions can be illustrated as follows:
gen_fsm module Callback module -------------- --------------- gen_fsm:start_link -----> Module:init/1 gen_fsm:send_event -----> Module:StateName/2 gen_fsm:send_all_state_event -----> Module:handle_event/3 gen_fsm:sync_send_event -----> Module:StateName/3 gen_fsm:sync_send_all_state_event -----> Module:handle_sync_event/4 - -----> Module:handle_info/3 - -----> Module:terminate/3 - -----> Module:code_change/4
If a callback function fails or returns a bad value, the gen_fsm will terminate.
A gen_fsm handles system messages as documented in
sys(3). The sys
module
can be used for debugging a gen_fsm.
Note that a gen_fsm does not trap exit signals automatically, this must be explicitly initiated in the callback module.
Unless otherwise stated, all functions in this module fail if the specified gen_fsm does not exist or if bad arguments are given.
The gen_fsm process can go into hibernation
(see erlang(3)) if a callback
function specifies 'hibernate'
instead of a timeout value. This
might be useful if the server is expected to be idle for a long
time. However this feature should be used with care as hibernation
implies at least two garbage collections (when hibernating and
shortly after waking up) and is not something you'd want to do
between each call to a busy state machine.
Functions
start_link(Module, Args, Options) -> Result
start_link(FsmName, Module, Args, Options) -> Result
FsmName = {local,Name} | {global,GlobalName}
Name = atom()
GlobalName = term()
Module = atom()
Args = term()
Options = [Option]
Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}
Dbgs = [Dbg]
Dbg = trace | log | statistics
| {log_to_file,FileName} | {install,{Func,FuncState}}
SOpts = [SOpt]
SOpt - see erlang:spawn_opt/2,3,4,5
Result = {ok,Pid} | ignore | {error,Error}
Pid = pid()
Error = {already_started,Pid} | term()
Creates a gen_fsm process as part of a supervision tree. The function should be called, directly or indirectly, by the supervisor. It will, among other things, ensure that the gen_fsm is linked to the supervisor.
The gen_fsm process calls Module:init/1
to
initialize. To ensure a synchronized start-up procedure,
start_link/3,4
does not return until
Module:init/1
has returned.
If FsmName={local,Name}
, the gen_fsm is registered
locally as Name
using register/2
.
If FsmName={global,GlobalName}
, the gen_fsm is
registered globally as GlobalName
using
global:register_name/2
. If no name is provided,
the gen_fsm is not registered.
Module
is the name of the callback module.
Args
is an arbitrary term which is passed as
the argument to Module:init/1
.
If the option {timeout,Time}
is present, the gen_fsm
is allowed to spend Time
milliseconds initializing
or it will be terminated and the start function will return
{error,timeout}
.
If the option {debug,Dbgs}
is present,
the corresponding sys
function will be called for each
item in Dbgs
. See
sys(3).
If the option {spawn_opt,SOpts}
is present,
SOpts
will be passed as option list to
the spawn_opt
BIF which is used to spawn the gen_fsm
process. See
erlang(3).
Note!
Using the spawn option monitor
is currently not
allowed, but will cause the function to fail with reason
badarg
.
If the gen_fsm is successfully created and initialized
the function returns {ok,Pid}
, where Pid
is
the pid of the gen_fsm. If there already exists a process with
the specified FsmName
, the function returns
{error,{already_started,Pid}}
where Pid
is
the pid of that process.
If Module:init/1
fails with Reason
,
the function returns {error,Reason}
. If
Module:init/1
returns {stop,Reason}
or
ignore
, the process is terminated and the function
returns {error,Reason}
or ignore
, respectively.
start(Module, Args, Options) -> Result
start(FsmName, Module, Args, Options) -> Result
FsmName = {local,Name} | {global,GlobalName}
Name = atom()
GlobalName = term()
Module = atom()
Args = term()
Options = [Option]
Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}
Dbgs = [Dbg]
Dbg = trace | log | statistics
| {log_to_file,FileName} | {install,{Func,FuncState}}
SOpts = [term()]
Result = {ok,Pid} | ignore | {error,Error}
Pid = pid()
Error = {already_started,Pid} | term()
Creates a stand-alone gen_fsm process, i.e. a gen_fsm which is not part of a supervision tree and thus has no supervisor.
See start_link/3,4 for a description of arguments and return values.
send_event(FsmRef, Event) -> ok
FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()
Name = Node = atom()
GlobalName = term()
Event = term()
Sends an event asynchronously to the gen_fsm FsmRef
and returns ok
immediately. The gen_fsm will call
Module:StateName/2
to handle the event, where
StateName
is the name of the current state of
the gen_fsm.
FsmRef
can be:
- the pid,
Name
, if the gen_fsm is locally registered,{Name,Node}
, if the gen_fsm is locally registered at another node, or{global,GlobalName}
, if the gen_fsm is globally registered.
Event
is an arbitrary term which is passed as one of
the arguments to Module:StateName/2
.
send_all_state_event(FsmRef, Event) -> ok
FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()
Name = Node = atom()
GlobalName = term()
Event = term()
Sends an event asynchronously to the gen_fsm FsmRef
and returns ok
immediately. The gen_fsm will call
Module:handle_event/3
to handle the event.
See send_event/2 for a description of the arguments.
The difference between send_event
and
send_all_state_event
is which callback function is
used to handle the event. This function is useful when
sending events that are handled the same way in every state,
as only one handle_event
clause is needed to handle
the event instead of one clause in each state name function.
sync_send_event(FsmRef, Event) -> Reply
sync_send_event(FsmRef, Event, Timeout) -> Reply
FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()
Name = Node = atom()
GlobalName = term()
Event = term()
Timeout = int()>0 | infinity
Reply = term()
Sends an event to the gen_fsm FsmRef
and waits until a
reply arrives or a timeout occurs. The gen_fsm will call
Module:StateName/3
to handle the event, where
StateName
is the name of the current state of
the gen_fsm.
See send_event/2
for a description of FsmRef
and Event
.
Timeout
is an integer greater than zero which
specifies how many milliseconds to wait for a reply, or
the atom infinity
to wait indefinitely. Default value
is 5000. If no reply is received within the specified time,
the function call fails.
The return value Reply
is defined in the return value
of Module:StateName/3
.
The ancient behaviour of sometimes consuming the server exit message if the server died during the call while linked to the client has been removed in OTP R12B/Erlang 5.6.
sync_send_all_state_event(FsmRef, Event) -> Reply
sync_send_all_state_event(FsmRef, Event, Timeout) -> Reply
FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()
Name = Node = atom()
GlobalName = term()
Event = term()
Timeout = int()>0 | infinity
Reply = term()
Sends an event to the gen_fsm FsmRef
and waits until a
reply arrives or a timeout occurs. The gen_fsm will call
Module:handle_sync_event/4
to handle the event.
See send_event/2
for a description of FsmRef
and Event
. See
sync_send_event/3
for a description of Timeout
and Reply
.
See
send_all_state_event/2
for a discussion about the difference between
sync_send_event
and sync_send_all_state_event
.
reply(Caller, Reply) -> true
Caller - see below
Reply = term()
This function can be used by a gen_fsm to explicitly send a
reply to a client process that called
sync_send_event/2,3
or
sync_send_all_state_event/2,3,
when the reply cannot be defined in the return value of
Module:State/3
or Module:handle_sync_event/4
.
Caller
must be the From
argument provided to
the callback function. Reply
is an arbitrary term,
which will be given back to the client as the return value of
sync_send_event/2,3
or
sync_send_all_state_event/2,3
.
send_event_after(Time, Event) -> Ref
Time = integer()
Event = term()
Ref = reference()
Sends a delayed event internally in the gen_fsm that calls
this function after Time
ms. Returns immediately a
reference that can be used to cancel the delayed send using
cancel_timer/1.
The gen_fsm will call Module:StateName/2
to handle
the event, where StateName
is the name of the current
state of the gen_fsm at the time the delayed event is
delivered.
Event
is an arbitrary term which is passed as one of
the arguments to Module:StateName/2
.
start_timer(Time, Msg) -> Ref
Time = integer()
Msg = term()
Ref = reference()
Sends a timeout event internally in the gen_fsm that calls
this function after Time
ms. Returns immediately a
reference that can be used to cancel the timer using
cancel_timer/1.
The gen_fsm will call Module:StateName/2
to handle
the event, where StateName
is the name of the current
state of the gen_fsm at the time the timeout message is
delivered.
Msg
is an arbitrary term which is passed in the
timeout message, {timeout, Ref, Msg}
, as one of
the arguments to Module:StateName/2
.
cancel_timer(Ref) -> RemainingTime | false
Ref = reference()
RemainingTime = integer()
Cancels an internal timer referred by Ref
in the
gen_fsm that calls this function.
Ref
is a reference returned from
send_event_after/2
or
start_timer/2.
If the timer has already timed out, but the event not yet been delivered, it is cancelled as if it had not timed out, so there will be no false timer event after returning from this function.
Returns the remaining time in ms until the timer would
have expired if Ref
referred to an active timer,
false
otherwise.
enter_loop(Module, Options, StateName, StateData)
enter_loop(Module, Options, StateName, StateData, FsmName)
enter_loop(Module, Options, StateName, StateData, Timeout)
enter_loop(Module, Options, StateName, StateData, FsmName, Timeout)
Module = atom()
Options = [Option]
Option = {debug,Dbgs}
Dbgs = [Dbg]
Dbg = trace | log | statistics
| {log_to_file,FileName} | {install,{Func,FuncState}}
StateName = atom()
StateData = term()
FsmName = {local,Name} | {global,GlobalName}
Name = atom()
GlobalName = term()
Timeout = int() | infinity
Makes an existing process into a gen_fsm. Does not return,
instead the calling process will enter the gen_fsm receive
loop and become a gen_fsm process. The process must
have been started using one of the start functions in
proc_lib
, see
proc_lib(3). 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_fsm behaviour provides.
Module
, Options
and FsmName
have
the same meanings as when calling
start[_link]/3,4.
However, if FsmName
is specified, the process must have
been registered accordingly before this function is
called.
StateName
, StateData
and Timeout
have
the same meanings as in the return value of
Module:init/1.
Also, the callback module Module
does not need to
export an init/1
function.
Failure: If the calling process was not started by a
proc_lib
start function, or if it is not registered
according to FsmName
.
CALLBACK FUNCTIONS
The following functions should be exported from a gen_fsm
callback module.
In the description, the expression state name is used to denote a state of the state machine. state data is used to denote the internal state of the Erlang process which implements the state machine.
Functions
Module:init(Args) -> Result
Args = term()
Return = {ok,StateName,StateData} | {ok,StateName,StateData,Timeout}
| {ok,StateName,StateData,hibernate}
| {stop,Reason} | ignore
StateName = atom()
StateData = term()
Timeout = int()>0 | infinity
Reason = term()
Whenever a gen_fsm is started using gen_fsm:start/3,4 or gen_fsm:start_link/3,4, this function is called by the new process to initialize.
Args
is the Args
argument provided to the start
function.
If initialization is successful, the function should return
{ok,StateName,StateData}
,
{ok,StateName,StateData,Timeout}
or {ok,StateName,StateData,hibernate}
,
where StateName
is the initial state name and StateData
the initial
state data of the gen_fsm.
If an integer timeout value is provided, a timeout will occur
unless an event or a message is received within Timeout
milliseconds. A timeout is represented by the atom
timeout
and should be handled by
the Module:StateName/2
callback functions. The atom
infinity
can be used to wait indefinitely, this is
the default value.
If hibernate
is specified instead of a timeout value, the process will go
into hibernation when waiting for the next message to arrive (by calling
proc_lib:hibernate/3).
If something goes wrong during the initialization
the function should return {stop,Reason}
, where
Reason
is any term, or ignore
.
Module:StateName(Event, StateData) -> Result
Event = timeout | term()
StateData = term()
Result = {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,NewStateData}
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = term()
There should be one instance of this function for each
possible state name. Whenever a gen_fsm receives an event
sent using
gen_fsm:send_event/2,
the instance of this function with the same name as
the current state name StateName
is called to handle
the event. It is also called if a timeout occurs.
Event
is either the atom timeout
, if a timeout
has occurred, or the Event
argument provided to
send_event/2
.
StateData
is the state data of the gen_fsm.
If the function returns
{next_state,NextStateName,NewStateData}
,
{next_state,NextStateName,NewStateData,Timeout}
or
{next_state,NextStateName,NewStateData,hibernate}
,
the gen_fsm will continue executing with the current state
name set to NextStateName
and with the possibly
updated state data NewStateData
. See
Module:init/1
for a description of Timeout
and hibernate
.
If the function returns {stop,Reason,NewStateData}
,
the gen_fsm will call
Module:terminate(Reason,NewStateData)
and terminate.
Module:handle_event(Event, StateName, StateData) -> Result
Event = term()
StateName = atom()
StateData = term()
Result = {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,NewStateData}
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = term()
Whenever a gen_fsm receives an event sent using gen_fsm:send_all_state_event/2, this function is called to handle the event.
StateName
is the current state name of the gen_fsm.
See Module:StateName/2
for a description of the other
arguments and possible return values.
Module:StateName(Event, From, StateData) -> Result
Event = term()
From = {pid(),Tag}
StateData = term()
Result = {reply,Reply,NextStateName,NewStateData}
| {reply,Reply,NextStateName,NewStateData,Timeout}
| {reply,Reply,NextStateName,NewStateData,hibernate}
| {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}
Reply = term()
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = normal | term()
There should be one instance of this function for each
possible state name. Whenever a gen_fsm receives an event
sent using
gen_fsm:sync_send_event/2,3,
the instance of this function with the same name as
the current state name StateName
is called to handle
the event.
Event
is the Event
argument provided to
sync_send_event
.
From
is a tuple {Pid,Tag}
where Pid
is
the pid of the process which called sync_send_event/2,3
and Tag
is a unique tag.
StateData
is the state data of the gen_fsm.
If the function returns
{reply,Reply,NextStateName,NewStateData}
,
{reply,Reply,NextStateName,NewStateData,Timeout}
or
{reply,Reply,NextStateName,NewStateData,hibernate}
,
Reply
will be given back to From
as the return
value of sync_send_event/2,3
. The gen_fsm then
continues executing with the current state name set to
NextStateName
and with the possibly updated state data
NewStateData
. See Module:init/1
for a
description of Timeout
and hibernate
.
If the function returns
{next_state,NextStateName,NewStateData}
,
{next_state,NextStateName,NewStateData,Timeout}
or
{next_state,NextStateName,NewStateData,hibernate}
,
the gen_fsm will continue executing in NextStateName
with NewStateData
. Any reply to From
must be
given explicitly using
gen_fsm:reply/2.
If the function returns
{stop,Reason,Reply,NewStateData}
, Reply
will be
given back to From
. If the function returns
{stop,Reason,NewStateData}
, any reply to From
must be given explicitly using gen_fsm:reply/2
.
The gen_fsm will then call
Module:terminate(Reason,NewStateData)
and terminate.
Module:handle_sync_event(Event, From, StateName, StateData) -> Result
Event = term()
From = {pid(),Tag}
StateName = atom()
StateData = term()
Result = {reply,Reply,NextStateName,NewStateData}
| {reply,Reply,NextStateName,NewStateData,Timeout}
| {reply,Reply,NextStateName,NewStateData,hibernate}
| {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}
Reply = term()
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = term()
Whenever a gen_fsm receives an event sent using gen_fsm:sync_send_all_state_event/2,3, this function is called to handle the event.
StateName
is the current state name of the gen_fsm.
See Module:StateName/3
for a description of the other
arguments and possible return values.
Module:handle_info(Info, StateName, StateData) -> Result
Info = term()
StateName = atom()
StateData = term()
Result = {next_state,NextStateName,NewStateData}
> | {next_state,NextStateName,NewStateData,Timeout}
> | {next_state,NextStateName,NewStateData,hibernate}
> | {stop,Reason,NewStateData}
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = normal | term()
This function is called by a gen_fsm when it receives any other message than a synchronous or asynchronous event (or a system message).
Info
is the received message.
See Module:StateName/2
for a description of the other
arguments and possible return values.
Module:terminate(Reason, StateName, StateData)
Reason = normal | shutdown | {shutdown,term()} | term()
StateName = atom()
StateData = term()
This function is called by a gen_fsm when it is about to
terminate. It should be the opposite of Module:init/1
and do any necessary cleaning up. When it returns, the gen_fsm
terminates with Reason
. The return value is ignored.
Reason
is a term denoting the stop reason,
StateName
is the current state name, and
StateData
is the state data of the gen_fsm.
Reason
depends on why the gen_fsm is terminating. If
it is because another callback function has returned a stop
tuple {stop,..}
, Reason
will have the value
specified in that tuple. If it is due to a failure,
Reason
is the error reason.
If the gen_fsm is part of a supervision tree and is ordered
by its supervisor to terminate, this function will be called
with Reason=shutdown
if the following conditions apply:
- the gen_fsm has been set to trap exit signals, and
- the shutdown strategy as defined in the supervisor's
child specification is an integer timeout value, not
brutal_kill
.
Even if the gen_fsm is not part of a supervision tree,
this function will be called if it receives an 'EXIT'
message from its parent. Reason
will be the same as in
the 'EXIT'
message.
Otherwise, the gen_fsm will be immediately terminated.
Note that for any other reason than normal
,
shutdown
, or {shutdown,Term}
the gen_fsm is
assumed to terminate due to an error and
an error report is issued using
error_logger:format/2.
Module:code_change(OldVsn, StateName, StateData, Extra) -> {ok, NextStateName, NewStateData}
OldVsn = Vsn | {down, Vsn}
Vsn = term()
StateName = NextStateName = atom()
StateData = NewStateData = term()
Extra = term()
This function is called by a gen_fsm when it should update
its internal state data during a release upgrade/downgrade,
i.e. when the instruction {update,Module,Change,...}
where Change={advanced,Extra}
is given in
the appup
file. See
OTP Design Principles.
In the case of an upgrade, OldVsn
is Vsn
, and
in the case of 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.
StateName
is the current state name and
StateData
the internal state data of the gen_fsm.
Extra
is passed as-is from the {advanced,Extra}
part of the update instruction.
The function should return the new current state name and updated internal data.
Module:format_status(Opt, [PDict, StateData]) -> Status
Opt = normal | terminate
PDict = [{Key, Value}]
StateData = term()
Status = term()
Note!
This callback is optional, so callback modules need not export it. The gen_fsm module provides a default implementation of this function that returns the callback module state data.
This function is called by a gen_fsm process when:
- One
of sys:get_status/1,2
is invoked to get the gen_fsm status.
Opt
is set to the atomnormal
for this case. - The gen_fsm terminates abnormally and logs an
error.
Opt
is set to the atomterminate
for this case.
This function is useful for customising the form and
appearance of the gen_fsm status for these cases. A callback
module wishing to customise the sys:get_status/1,2
return value as well as how its status appears in
termination error logs exports an instance
of format_status/2
that returns a term describing the
current status of the gen_fsm.
PDict
is the current value of the gen_fsm's
process dictionary.
StateData
is the internal state data of the
gen_fsm.
The function should return Status
, a term that
customises the details of the current state and status of
the gen_fsm. 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, [{"StateData",
Term}]}]
where Term
provides relevant details of
the gen_fsm state data. Following this recommendation isn't
required, but doing so will make 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 data representations to avoid having large state terms printed in logfiles.
SEE ALSO
gen_event(3), gen_server(3), supervisor(3), proc_lib(3), sys(3)
- start_link/3
- start_link/4
- start/3
- start/4
- send_event/2
- send_all_state_event/2
- sync_send_event/2
- sync_send_event/3
- sync_send_all_state_event/2
- sync_send_all_state_event/3
- reply/2
- send_event_after/2
- start_timer/2
- cancel_timer/1
- enter_loop/4
- enter_loop/5
- enter_loop/5-1
- enter_loop/6
- init/1
- StateName/2
- handle_event/3
- StateName/3
- handle_sync_event/4
- handle_info/3
- terminate/3
- code_change/4
- format_status/3