GenStatem behaviour (gen_statem v0.1.0)
View SourceA behaviour module for implementing the generic state machine
Documentation for GenStatem
.
Features
- 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 machine which requires the state to be an atom and uses that state as the name of the current callback function, arity 3.handle_event_function
- that allows the state to be any term and that useshandle_event/4
as callback function for all states.
Callback module
A GenStatem
assumes all specific parts to be located
in a callback module exporting a predefined set of functions.
The relationship between the behavior functions
and the callback functions is as follows:
GenStatem module Callback module
----------------- ---------------
GenStatem.start
GenStatem.start_monitor
GenStatem.start_link -----> Module.init/1
Server start or code change
-----> Module.callback_mode/0
selects callback mode
GenStatem.stop
Supervisor exit
Callback failure -----> Module.terminate/3
GenStatem.call
GenStatem.cast
GenStatem.send_request
erlang.send
erlang.'!' -----> Module.state_name/3
or -----> Module.handle_event/4
depending on callback mode
Release upgrade/downgrade
(code change)
-----> Module.code_change/4
State callback
The state callback for a specific state in a GenStatem
is the callback function that is called for all events in this state.
It is selected depending on which callback mode
that the callback module defines with the callback function
Module.callback_mode/0
.
When the callback mode is state_functions
,
the state must be an atom and is used as the state callback name;
see Module.state_name/3
. This co-locates all code
for a specific state in one function as the GenStatem
engine branches
depending on state name. Note the fact that the callback function
Module.terminate/3
makes the state name terminate
unusable in this mode.
When the callback mode is handle_event_function
,
the state can be any term and the state callback name is
Module.handle_event/4
. This makes it easy
to branch depending on state or event as you desire. Be careful about
which events you handle in which states so that you do not accidentally
postpone an event forever creating an infinite busy loop.
Event types
Events are of different types,
therefore the callback functions can know the origin of an event
when handling it. External events are
:call
, :cast
, and :info
. Internal events are
timeout
and :internal
.
Event handling
When GenStatem
receives a process message it is transformed
into an event and the state callback
is called with the event as two arguments: type and content. When the
state callback has processed the event
it returns to GenStatem
which does a state transition. If this
state transition is to a different state, that is: next_state !== state
,
it is a state change.
Transition actions
The state callback may return
transition actions for GenStatem
to execute
during the state transition, for example to set a time-out
or reply to a call.
Reply to a call
See GenStatem:call/2,3
about how to reply
to a call. A reply can be sent from any state callback,
not just the one that got the request event.
Event postponing
One of the possible transition actions is to postpone the current event.
Then it will not be handled in the current state. The GenStatem
engine
keeps a queue of events divided into postponed events and
events still to process (not presented yet). After a state change
the queue restarts with the postponed events.
The GenStatem
event queue model is sufficient to emulate
the normal process message queue with selective receive.
Postponing an event corresponds to not matching it
in a receive statement, and changing states corresponds to
entering a new receive statement.
Event insertion
The state callback can insert
events using the transition action :next_event
,
and such an event is inserted in the event queue as the next to call the
state callback with. That is,
as if it is the oldest incoming event. A dedicated event_type/0
:internal
can be used for such events making it possible to
safely distinguish them from external events.
Inserting an event replaces the trick of calling your own state handling
functions that you often would have to resort to in, for example,
m:gen_fsm
to force processing an inserted event before others.
Note
If you postpone an event and (against good practice) directly call a different state callback, the postponed event is not retried, since there was no state change.
Instead of directly calling a state callback, do a state change.
This makes the GenStatem
engine retry postponed events.
Inserting an event in a state change also triggers the new state callback to be called with that event before receiving any external events.
State enter calls
The GenStatem
engine can automatically make a special call to the
state callback whenever a new state is
entered; see state_enter/0
. This is for writing code common
to all state entries. Another way to do it is to explicitly insert
an event at the state transition, and/or to use a dedicated
state transition function, but that is something you will have to
remember at every state transition to the state(s) that need it.
For the details of a state transition, see type transition_option/0
.
Hibernation
The GenStatem
process can go into hibernation;
see proc_lib:hibernate/3
. It is done when
a state callback or
Module.init/1
specifies hibernate
in the returned Actions
list. This feature
can be useful to reclaim process heap memory while the server
is expected to be idle for a long time. However, use it with care,
as hibernation can be too costly to use after every event;
see :erlang.hibernate/3
.
There is also a server start option
{hibernate_after, Timeout}
for start/3,4
, start_link/3,4
,
start_monitor/3,4
,
that may be used to automatically hibernate the server.
Callback failure
If a callback function fails or returns a bad value,
the GenStatem
terminates. However, an exception of class
throw
is not regarded as an error
but as a valid return, from all callback functions.
System messages and the m:sys
module
A GenStatem
handles system messages as described in m:sys
.
The m:sys
module can be used for debugging a GenStatem
.
Replies sent through transition actions
gets logged, but not replies sent through reply/1,2
.
Trapping exit
A GenStatem
process, like all gen_
* behaviours,
does not trap exit signals automatically;
this must be explicitly initiated in the callback module
(by calling process_flag(trap_exit, true)
preferably from init/1
.
Server termination
If the GenStatem
process terminates, e.g. as a result
of a callback function returning {stop, Reason}
, an exit signal
with this Reason
is sent to linked processes and ports.
See Processes
in the Reference Manual for details regarding error handling
using exit signals.
Note
For some important information about distributed signals, see the
Blocking Signaling Over Distribution
section in the Processes chapter of the Erlang Reference Manual.
Blocking signaling can, for example, cause call time-outs in GenStatem
to be significantly delayed.
Bad argument
Unless otherwise stated, all functions in this module fail if the specified
GenStatem
does not exist or if bad arguments are specified.
Example
The following example shows a simple pushbutton model
for a toggling pushbutton implemented with
callback mode state_functions
.
You can push the button and it replies if it went on or off,
and you can ask for a count of how many times it has been pushed
to switch on.
Pushbutton State Diagram
---
title: Pushbutton State Diagram
---
stateDiagram-v2
[*] --> off
off --> on : push<br/><ul><li>Increment count</li><li>Reply 'on'</li></ul>
on --> off : push<br/><ul><li>Reply 'off'</li></ul>
Not shown in the state diagram:
- The API function
push()
generates an eventpush
of typecall
. - The API function
get_count()
generates an eventget_count
of typecall
that is handled in all states by replying with the current count value. - Unknown events are ignored and discarded.
- There is boilerplate code for start, stop, terminate, code change,
init, to set the callback mode to
state_functions
, etc...
Pushbutton Code
The following is the complete callback module file pushbutton.erl
:
defmodule PushButton do
use GenStatem
defp name, do: :pushbutton_statem
## API. This example uses a registered name name()
## and does not link to the caller
def start, do: GenStatem.start(__MODULE__, [], name: name())
def push, do: GenStatem.call(name(), :push)
def get_count, do: GenStatem.call(name(), :get_count)
def stop, do: GenStatem.stop(name())
## Mandatory callback functions
def init([]) do
## Set the initial state + data. Data is used only as a counter.
state = :off
data = 0
{:ok, state, data}
end
def callback_mode(), do: :state_functions
def code_change(_old_vsn, old_state, old_data, _extra) do
{:ok, old_state, old_data}
end
## Mandatory callback functions
def terminate, do: :void
### state callback(s)
def off({:call, from}, :push, data) do
## Go to 'on', increment count and reply
## that the resulting status is 'on'
{:next_state, :on, data + 1,[{:reply, from, :on}]}
end
def off(event_type, event_content, data) do
handle_event(event_type, event_content, data)
end
def on({:call, from}, :push, data) do
## Go to 'off' and reply that the resulting status is 'off'
{:next_state, :off, data, [{:reply, from, :off}]}
end
def on(event_type, event_content, data) do
handle_event(event_type, event_content, data)
end
## Handle events common to all states
defp handle_event({:call, from}, :get_count, data) do
## Reply with the current count
{:keep_state, data,[{:reply, from, data}]}
end
defp handle_event(_event_type, _event_content, data) do
## Ignore all other events
{:keep_state, data}
end
end
The following is a shell session when running it:
iex(1)> PushButton.start
{:ok, #PID<0.203.0>}
iex(2)> PushButton.get_count
0
iex(3)> PushButton.push
:on
iex(4)> PushButton.get_count
1
iex(5)> PushButton.push
:off
iex(6)> PushButton.get_count
1
iex(7)> PushButton.stop
:ok
iex(8)> PushButton.push
** (GenStatem.GenError)
Class: exit
Reason: noproc
MFArgs: GenStatem.call(:pushbutton_statem, :push, :infinity)
Stacktrace:
(stdlib 6.2.2) gen.erl:580: :gen.do_for_proc/2
(GenStatem 0.1.0) lib/GenStatem.ex:1997: GenStatem.call/3
(elixir 1.18.3) src/elixir.erl:386: :elixir.eval_external_handler/3
(stdlib 6.2.2) erl_eval.erl:919: :erl_eval.do_apply/7
(elixir 1.18.3) src/elixir.erl:364: :elixir.eval_forms/4
(elixir 1.18.3) lib/module/parallel_checker.ex:120: Module.ParallelChecker.verify/1
(iex 1.18.3) lib/iex/evaluator.ex:336: IEx.Evaluator.eval_and_inspect/3
(iex 1.18.3) lib/iex/evaluator.ex:310: IEx.Evaluator.eval_and_inspect_parsed/3
(GenStatem 0.1.0) lib/GenStatem.ex:2001: GenStatem.call/3
iex:8: (file)
To compare styles, here follows the same example using
callback mode handle_event_function
,
or rather, the code to replace after function init/1
of the pushbutton.erl
example file above:
### state callback(s)
def handle_event({:call, from}, :push, :off, data) do
## Go to 'on', increment count and reply
## that the resulting status is 'on'
{:next_state, :on, data + 1,[{:reply, from, :on}]}
end
def handle_event({:call, from}, :push, :on, data) do
## Go to 'off' and reply that the resulting status is 'off'
{:next_state, :off, data, [{:reply, from, :off}]}
end
##
## Event handling common to all states
def handle_event({:call, from}, :get_count, state, data) do
## Reply with the current count
{:next_state, state, data, [{:reply, from, data}]};
end
def handle_event(_event_type, _old_state, current_state, data) do
## Ignore all other events
{:next_state, current_state, data}
end
See Also
Summary
Types
Actions for a state transition, or when starting the server.
One function per state or one common event handler.
Return value from Module.callback_mode/0
.
Generic state data for the server.
Actions for any callback: hibernate, time-outs or replies.
Server start options for the
start/3,4
, start_link/3,4
,
and start_monitor/3,4
, functions.
Event payload from the event's origin, delivered to the state callback.
Return value from a state callback after handling an event.
How long to wait for an event.
A map that describes the server's status.
How long to wait for a named time-out event.
Hibernate the server process.
The return value from Module.init/1
.
Postpone an event to handle it later.
Reply to a call/2,3
.
A handle that associates a reply to the corresponding request.
An opaque request identifier. See send_request/2
for details.
An opaque collection of request identifiers (request_id/0
).
Response time-out for an asynchronous call.
Server name specification: :local
, :global
, or :via
registered.
Server specification: pid/0
or registered server_name/0
.
Return value from the start_monitor/3,4
functions.
Server start options for the
start/3,4
, start_link/3,4
,
and start_monitor/3,4
functions.
Return value from the start/3,4
and start_link/3,4
functions.
State name or state term.
Return value from any state callback.
Callback mode modifier
for state enter calls: the atom :state_enter
.
Return value from a state callback after a state enter call.
State name in callback mode state_functions
.
How long to wait in the current state.
Event time-out, generic time-outs or state time-out.
Clearer way to cancel a time-out than the original setting it to ':infinity'.
Time-out timer start option, to select absolute time of expiry.
Update the event_content
without affecting the time of expiry.
State transition options set by actions.
Callbacks
Select the callback mode and possibly state enter calls.
Format/limit the status value.
state callback in
callback mode handle_event_function
.
Initialize the state machine.
State callback in
callback mode :state_functions
.
Handle state machine termination.
Functions
Call a server: send request and wait for response.
Cast an event to a server.
Check if a received message is a request response.
Receive a request response.
Send one or multiple call
replies.
Send a call
reply
to from
.
Store a request identifier in a colletion.
Return the number of request identifiers in req_id_collection
.
Send one or multiple call
replies.
Send an asynchronous call
request.
Send an asynchronous call
request and add it to a request identifier collection.
Starts a GenStatem
process without links (outside of a supervision tree).
Start a GenStatem
process linked to the current process.
Starts a GenStatem
process monitored and registered, but not linked.
Synchronously stops the state machine with the given reason
.
Wait for a request response.
Wait for any request response in a collection.
Types
@type action() :: :postpone | {:postpone, postpone :: postpone()} | {:next_event, event_type :: event_type(), event_content :: event_content()} | {:change_callback_module, new_module :: module()} | {:push_callback_module, new_module :: module()} | :pop_callback_module | enter_action()
Actions for a state transition, or when starting the server.
These transition actions can be invoked by returning them from the
state callback when it is called
with an event, from Module.init/1
.
They are not allowed from state enter calls.
Actions are executed in the containing list order.
Actions that set transition options
override any previous of the same type, so the last
in the containing list wins. For example, the last postpone/0
overrides any previous postpone/0
in the list.
{:postpone, value}
- Sets thetransition_option()
postpone/0
for this state transition. This action is ignored when returned fromModule.init/1
, as there is no event to postpone in those cases.:postpone
is equivalent to{:postpone, true}
.{:next_event, event_type, event_content}
- This action does not set anytransition_option()
but instead stores the specifiedevent_type
andevent_content
for insertion after all actions have been executed.The stored events are inserted in the queue as the next to process before any already queued events. The order of these stored events is preserved, so the first
:next_event
in the containing list becomes the first to process.An event of type
internal
should be used when you want to reliably distinguish an event inserted this way from any external event.{:change_callback_module, new_module}
- Changes the callback module tonew_module
which will be used when calling all subsequent state callbacks. TheGenStatem
engine will find out the callback mode ofnew_module
by callingnew_module.callback_mode/0
before the next state callback.Changing the callback module does not affect the state transition in any way, it only changes which module that handles the events. Be aware that all relevant callback functions in
new_module
such as the state callback,new_module.code_change/4
,new_module.format_status/1
andnew_module.terminate/3
must be able to handle the state and data from the old module.{:push_callback_module, new_module}
- Pushes the current callback module to the top of an internal stack of callback modules, and changes the callback module tonew_module
. Otherwise like{:change_callback_module, new_module}
above.pop_callback_module
- Pops the top module from the internal stack of callback modules and changes the callback module to be the popped module. If the stack is empty the server fails. Otherwise like{:change_callback_module, new_module}
above.
@type callback_mode() :: :state_functions | :handle_event_function
One function per state or one common event handler.
The callback mode is selected with the return value from
Module.callback_mode/0
:
state_functions
- The state must be of typestate_name/0
and one callback function per state, that is,Module.state_name/3
, is used.handle_event_function
- The state can be any term and the callback functionModule.handle_event/4
is used for all states.
The function Module.callback_mode/0
is called
when starting the GenStatem
, after code change and after changing
the callback module with any of the actions
change_callback_module
,
push_callback_module
,
or pop_callback_module
.
The result is cached for subsequent calls to
state callbacks.
@type callback_mode_result() :: callback_mode() | [callback_mode() | state_enter()]
Return value from Module.callback_mode/0
.
This is the return type from
Module.callback_mode/0
which selects callback mode
and whether to do state enter calls,
or not.
@type data() :: term()
Generic state data for the server.
A term in which the state machine implementation is to store
any server data it needs. The difference between this and the state/0
itself is that a change in this data does not cause postponed events
to be retried. Hence, if a change in this data would change
the set of events that are handled, then that data item
should be part of the state/0
instead.
@type enter_action() :: :hibernate | {:hibernate, hibernate :: hibernate()} | timeout_action() | reply_action()
Actions for any callback: hibernate, time-outs or replies.
These transition actions are allowed when a action/0
is allowed,
and also from a state enter call, and can be invoked
by returning them from the state callback, from
Module.init/1
.
Actions are executed in the containing list order.
Actions that set transition options
override any previous of the same type, so the last in the containing
list wins. For example, the last event_timeout/0
overrides any previous
event_timeout/0
in the list.
{:hibernate, value}
- Sets thetransition_option/0
hibernate/0
for this state transition.hibernate
is equivalent to{:hibernate, true}
.
@type enter_loop_opt() :: {:hibernate_after, hibernate_after_timeout :: timeout()} | {:debug, dbgs :: [:sys.debug_option()]}
Server start options for the
start/3,4
, start_link/3,4
,
and start_monitor/3,4
, functions.
See start_link/4
.
@type event_content() :: term()
Event payload from the event's origin, delivered to the state callback.
See event_type
that describes the origins of
the different event types, which is also where the event's content
comes from.
@type event_handler_result(state_type) :: event_handler_result(state_type, term())
@type event_handler_result(state_type, data_type) :: {:next_state, next_state :: state_type, new_data :: data_type} | {:next_state, next_state :: state_type, new_data :: data_type, actions :: [action()] | action()} | state_callback_result(action(), data_type)
Return value from a state callback after handling an event.
state_type
is state_name/0
if callback mode is :state_functions
,
or state/0
if callback mode is :handle_event_function
.
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.
@type event_type() :: external_event_type() | timeout_event_type() | :internal
All event types: external,
time-out, or internal
.
internal
events can only be generated by the state machine itself
through the transition action next_event
.
@type external_event_type() :: {:call, from :: from()} | :cast | :info
Event from a call, cast, or regular process message; "info".
Type {:call, from}
originates from the API functions
call/2,3
or send_request/2
. The event contains
From
, which is whom to reply to
by a reply_action/0
or reply/2,3
call.
Type cast
originates from the API function cast/2
.
Type info
originates from regular process messages
sent to the GenStatem
process.
@type format_status() :: %{ state: state(), data: data(), reason: term(), queue: [{event_type(), event_content()}], postponed: [{event_type(), event_content()}], timeouts: [{timeout_event_type(), event_content()}], log: [:sys.system_event()] }
A map that describes the server's status.
The keys are:
state
- The current state.data
- The state data.reason
- The reason that caused the process to terminate.queue
- The event queue.postponed
- The queue of postponed events.timeouts
- The active time-outs.log
- The sys log of the server.
New associations may be added to the status map without prior notice.
A call
event's reply destination.
Destination to use when replying through, for example,
the action {:reply, from, reply}
to a process that has called the GenStatem
server
using call/2,3
.
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.
@type hibernate() :: boolean()
Hibernate the server process.
If true
, hibernates the GenStatem
by calling proc_lib:hibernate/3
before going into receive
to wait for a new external event.
There is also a server start option
{hibernate_after, Timeout}
for automatic hibernation.
Note
If there are enqueued events to process when hibernation is requested,
this is optimized by not hibernating but instead calling
:erlang.garbage_collect/0
to simulate,
in a more efficient way, that the GenStatem
entered hibernation
and immediately got awakened by an enqueued event.
@type init_result(state_type) :: init_result(state_type, term())
@type init_result(state_type, data_type) :: {:ok, state :: state_type, data :: data_type} | {:ok, state :: state_type, data :: data_type, actions :: [action()] | action()} | :ignore | {:stop, reason :: term()} | {:error, reason :: term()}
The return value from Module.init/1
.
For a successful initialization, state
is the initial state/0
,
and data
the initial server data/0
of the GenStatem
.
The actions
are executed when entering the first
state just as for a state callback,
except that the action postpone
is forced to false
since there is
no event to postpone.
For an unsuccessful initialization, {:stop, reason}
, {:error, reason}
,
or :ignore
should be used; see start_link/3,4
.
@type postpone() :: boolean()
Postpone an event to handle it later.
If true
, postpones the current event.
After a state change (next_state !== state
), it is retried.
Reply to a call/2,3
.
This transition action can be invoked by returning it from the
state callback, from Module.init/1
.
It does not set any transition_option()
but instead replies to a caller waiting for a reply in call/3
.
from
must be the term from argument {:call, from}
in a call to a state callback.
Note that using this action from Module.init/1
would be weird
on the border of witchcraft since there has been no earlier call to a
state callback in this server.
@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 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 toTimeout
.Timeout
is not allowed to identify a time further into the future than4294967295
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.
@type server_name() :: {:local, atom()} | {:global, global_name :: term()} | {:via, reg_mod :: module(), name :: term()}
Server name specification: :local
, :global
, or :via
registered.
Name specification to use when starting a GenStatem
server.
See start_link/3
and server_ref/0
below.
@type server_ref() :: pid() | (local_name :: atom()) | {name :: atom(), node :: atom()} | {:global, global_name :: term()} | {:via, reg_mod :: module(), via_name :: term()}
Server specification: pid/0
or registered server_name/0
.
To be used in call/2,3
to specify the server.
It can be:
pid() | local_name
- TheGenStatem
is locally registered.{name, node}
- TheGenStatem
is locally registered on another node.{:global, global_name}
- TheGenStatem
is globally registered inm:global
.{:via, reg_mod, via_name}
- TheGenStatem
is registered in an alternative process registry. The registry callback modulereg_mod
is to export functionsregister_name/2
,unregister_name/1
,whereis_name/1
, andsend/2
, which are to behave like the corresponding functions inm:global
. Thus,{:via, :global, global_name}
is the same as{:global, global_name}
.
Return value from the start_monitor/3,4
functions.
As for start_link/4
but a successful return
wraps the process ID and the monitor reference in a
{:ok, {
pid()
,
reference()
}}
tuple.
@type start_opt() :: {:name, server_name() | atom()} | {:timeout, time :: timeout()} | {:spawn_opt, [:proc_lib.start_spawn_option()]} | enter_loop_opt()
Server start options for the
start/3,4
, start_link/3,4
,
and start_monitor/3,4
functions.
See start_link/4
.
Return value from the start/3,4
and start_link/3,4
functions.
See start_link/4
.
@type state() :: state_name() | term()
State name or state term.
If the callback mode is handle_event_function
,
the state can be any term. After a state change (next_state !== state
),
all postponed events are retried.
Comparing two states for strict equality is assumed to be a fast operation,
since for every state transition the GenStatem
engine has to deduce
if it is a state change.
Note
The smaller the state term, in general, the faster the comparison.
Note that if the "same" state term is returned for a state transition
(or a return action without a next_state
field is used),
the comparison for equality is always fast because that can be seen
from the term handle.
But if a newly constructed state term is returned, both the old and the new state terms will have to be traversed until an inequality is found, or until both terms have been fully traversed.
So it is possible to use large state terms that are fast to compare, but very easy to accidentally mess up. Using small state terms is the safe choice.
@type state_callback_result(action_type, data_type) :: {:keep_state, new_data :: data_type} | {:keep_state, new_data :: data_type, actions :: [action_type] | action_type} | :keep_state_and_data | {:keep_state_and_data, actions :: [action_type] | action_type} | {:repeat_state, new_data :: data_type} | {:repeat_state, new_data :: data_type, actions :: [action_type] | action_type} | :repeat_state_and_data | {:repeat_state_and_data, actions :: [action_type] | action_type} | :stop | {:stop, reason :: term()} | {:stop, reason :: term(), new_data :: data_type} | {:stop_and_reply, reason :: term(), replies :: [reply_action()] | reply_action()} | {:stop_and_reply, reason :: term(), replies :: [reply_action()] | reply_action(), new_data :: data_type}
Return value from any state callback.
ActionType
is enter_action/0
if the state callback
was called with a state enter call,
and action/0
if the state callback was called with an event.
{keep_state, NewData [, Actions]}
- The same as{next_state, CurrentState, NewData [, Actions]}
.keep_state_and_data | {keep_state_and_data, Actions}
- The same as{keep_state, CurrentData [, Actions]}
.{repeat_state, NewData [, Actions]}
- If theGenStatem
runs with state enter calls, the state enter call is repeated, see typetransition_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 theGenStatem
by callingModule.terminate/3
withReason
andNewData
, 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 allReplies
, then terminates theGenStatem
like with{stop, Reason [, NewData]}
.
All these terms are tuples or atoms and will be so
in all future versions of GenStatem
.
@type state_enter() :: :state_enter
Callback mode modifier
for state enter calls: the atom :state_enter
.
Both callback modes can use state enter calls,
and this is selected by adding this :state_enter
flag
to the callback mode return value from
Module.callback_mode/0
.
If Module.callback_mode/0
returns
a list containing :state_enter
, the GenStatem
engine will,
at every state change, that is; next_state !== current_state
,
call the state callback with arguments
(enter, OldState, Data)
or (enter, OldState, State, Data)
,
depending on the callback mode.
This may look like an event but is really a call performed
after the previous state callback returned,
and before any event is delivered to the new
state callback.
See Module.state_name/3
and
Module.handle_event/4
. A state enter call
may be repeated without doing a state change by returning
a repeat_state
or
repeat_state_and_data
action
from the state callback.
If Module.callback_mode/0
does not return
a list containing :state_enter
, no state enter calls are done.
If Module.code_change/4
should transform the state,
it is regarded as a state rename and not a state change,
which will not cause a state enter call.
Note that a state enter call will be done right before entering
the initial state, which may be seen as a state change from no state
to the initial state. In this case OldState =:= State
,
which cannot happen for a subsequent state change,
but will happen when repeating the state enter call.
@type state_enter_result(state) :: state_enter_result(state, term())
@type state_enter_result(state, data_type) :: {:next_state, state, new_data :: data_type} | {:next_state, state, new_data :: data_type, actions :: [enter_action()] | enter_action()} | state_callback_result(enter_action(), data_type)
Return value from a state callback after a state enter call.
state
is the current state and it cannot be changed since the state callback
was called with a state enter call.
{:next_state, state, new_data [, actions]}
- TheGenStatem
does a state transition tostate
, which has to be equal to the current state, setsnew_data
, and executes allactions
.
@type state_name() :: atom()
State name in callback mode state_functions
.
If the callback mode is state_functions
,
the state must be an atom. After a state change (next_state !== state
),
all postponed events are retried. Note that the state terminate
is not possible to use since it would collide with the optional
callback function Module.terminate/3
.
How long to wait in the current state.
Starts a timer set by timeout_action/0
, or
{:state_timeout, time, event_content [, options]}
.
When the timer expires an event of event_type/0
:state_timeout
will be generated. See :erlang.start_timer/4
for how time
and options
are interpreted. Future
:erlang.start_timer/4
options
will not necessarily be supported.
A state change cancels this timer, if it is running. That is, if the
timeout_action/0
that starts this timer is part of a list of
action/0
s for a state change, next_state !== current_state
,
the timer runs in the next_state
.
If the state machine stays in that new state, now the current state,
the timer will run until it expires, which creates the time-out event.
If the state machine changes states from the now current state,
the timer is cancelled. During the state change from
the now current state, a new state time-out may be started
for the next next_state
.
If the timeout_action/0
that starts this timer
is part of a list of action/0
s 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.
@type timeout_action() :: (time :: event_timeout()) | {:timeout, time :: event_timeout(), event_content :: event_content()} | {:timeout, time :: event_timeout(), event_content :: event_content(), options :: timeout_option() | [timeout_option()]} | {{:timeout, name :: term()}, time :: generic_timeout(), event_content :: event_content()} | {{:timeout, name :: term()}, time :: generic_timeout(), event_content :: event_content(), options :: timeout_option() | [timeout_option()]} | {:state_timeout, time :: state_timeout(), event_content :: event_content()} | {:state_timeout, time :: state_timeout(), event_content :: event_content(), options :: timeout_option() | [timeout_option()]} | timeout_cancel_action() | timeout_update_action()
Event time-out, generic time-outs or state time-out.
These transition actions can be invoked by returning them from the
state callback, from
Module.init/1
.
These time-out actions sets time-out transition options.
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, next_state, new_data, time}
{:timeout, time, event_content, [options]}
- Sets thetransition_option/0
event_timeout/0
totime
withevent_content
, and time-out optionsoptions
.{{:timeout, name}, time, event_content, [options]}
- Sets thetransition_option/0
generic_timeout/0
totime
for time-outname
withevent_content
, and time-out optionsoptions
.{:state_timeout, time, event_content, [options]}
- Sets thetransition_option/0
state_timeout/0
totime
withevent_content
, and time-out optionsOptions
.
@type timeout_cancel_action() :: {:timeout, :cancel} | {{:timeout, name :: term()}, :cancel} | {:state_timeout, :cancel}
Clearer way to cancel a time-out than the original setting it to ':infinity'.
It has always been possible to cancel a time-out using
timeout_action/0
with time = :infinity
, since setting a new
time-out time overrides a running timer, and since setting the time
to :infinity
is optimized to not setting a timer (that never
will expire). Using this action shows the intention more clearly.
@type timeout_event_type() :: :timeout | {:timeout, name :: term()} | :state_timeout
Event time-out, generic time-out, or state time-out.
The time-out event types that the state machine can generate
for itself with the corresponding timeout_action/0
s:
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
event_type
is {event_type, time, ...}
.
@type timeout_option() :: {:abs, abs :: boolean()}
Time-out timer start option, to select absolute time of expiry.
If Abs
is true
an absolute timer is started,
and if it is false
a relative, which is the default.
See :erlang.start_timer/4
for details.
@type timeout_update_action() :: {:timeout, :update, event_content :: event_content()} | {{:timeout, name :: term()}, :update, event_content :: event_content()} | {:state_timeout, :update, event_content :: event_content()}
Update the event_content
without affecting the time of expiry.
Sets a new event_content
for a running time-out timer. See
timeout_action() for how to start a time-out.
If no time-out of this type is active, instead inserts the time-out event
just like when starting a time-out with relative time = 0
. This is a
time-out autostart with immediate expiry, so there will be noise for example
if a generic time-out name was misspelled.
@type transition_option() :: postpone() | hibernate() | event_timeout() | generic_timeout() | state_timeout()
State transition options set by actions.
These determine what happens during the state transition. The state transition takes place when the state callback has processed an event and returns. Here are the sequence of steps for a state transition:
All returned actions are processed in order of appearance. In this step all replies generated by any
reply_action/0
are sent. Other actions settransition_option/0
s 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
orrepeat_state_and_data
is used theGenStatem
engine calls the current state callback with arguments(enter, State, Data)
or(enter, State, State, Data)
(depending on callback mode) and when it returns starts again from the top of this sequence.If state enter calls are used, and the state changes, the
GenStatem
engine calls the new state callback with arguments(enter, OldState, Data)
or(enter, OldState, State, Data)
(depending on callback mode) and when it returns starts again from the top of this sequence.If
postpone/0
istrue
, 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
andstate_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; astate_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
GenStatem
goes intoreceive
or hibernation (ifhibernate/0
istrue
) to wait for the next message. In hibernation the next non-system event awakens theGenStatem
, or rather the next incoming message awakens theGenStatem
, but if it is a system event it goes right back into hibernation. When a new message arrives the state callback is called with the corresponding event, and we start again from the top of this sequence.
Note
The behaviour of a time-out zero (a time-out with time 0
)
differs subtly from Erlang's receive ... after 0 ... end
.
The latter receives one message if there is one,
while using the timeout_action/0
{timeout, 0}
does not
receive any external event.
m:gen_server
's time-out works like Erlang's
receive ... after 0 ... end
, in contrast to GenStatem
.
Callbacks
@callback callback_mode() :: callback_mode_result()
Select the callback mode and possibly state enter calls.
This function is called by a GenStatem
when it needs to find out the
callback mode of the callback module.
The value is cached by GenStatem
for efficiency reasons,
so this function is only called once after server start,
after code change, and after changing the callback module,
but before the first state callback
in the current callback module's code is called. More occasions may be
added in future versions of GenStatem
.
Server start happens either when Module.init/1
returns.
Code change happens when Module.code_change/4
returns. A change of the callback module happens when
a state callback returns
any of the actions change_callback_module
,
push_callback_module
or
pop_callback_module
.
The callback_mode
is either just callback_mode/0
or a list containing callback_mode/0
and possibly
the atom :state_enter
.
Note
If this function's body does not return an inline constant value the callback module is doing something strange.
@callback code_change( old_vsn :: term() | {:down, term()}, old_state :: state(), old_data :: data(), extra :: term() ) :: {:ok, new_state :: state(), new_data :: data()} | (reason :: term())
Update the state and data after code change.
This function is called by a GenStatem
when it is to update
its internal state during a release upgrade/downgrade that is,
when the instruction {:update, module, change, ...}
,
where change = {:advanced, extra}
, is specified in
the appup
file.
For more information, see
Release Handling Instructions in OTP Design Principles
.
For an upgrade, old_vsn
is vsn
, and for a downgrade, old_vsn
is
{:down, vsn}
. vsn
is defined by the vsn
attribute(s)
of the old version of the callback module Module
.
If no such attribute is defined, the version is the checksum
of the Beam file.
old_state
and old_data
are the internal state of the GenStatem
.
extra
is passed "as is" from the {:advanced, extra}
part
of the update instruction.
If successful, the function must return the updated internal state
in an {ok, new_state, new_data}
tuple.
If the function returns a failure reason
, the ongoing upgrade fails
and rolls back to the old release. Note that reason
cannot be
an {:ok, _, _}
tuple since that will be regarded
as a {:ok, new_state, new_data}
tuple, and that a tuple matching {:ok, _}
is an also invalid failure reason
. It is recommended to use
an atom as reason
since it will be wrapped in an {:error, reason}
tuple.
Also note when upgrading a GenStatem
, this function and hence the
change = {:advanced, extra}
parameter
in the appup
file is not only
needed to update the internal state or to act on the extra
argument. It is also needed if an upgrade or downgrade should change
callback mode, or else the callback mode
after the code change will not be honoured, most probably causing
a server crash.
If the server changes callback module using any of the actions
change_callback_module
,
push_callback_module
, or
pop_callback_module
, be aware that it is always
the current callback module that will get this callback call.
That the current callback module handles the current
state and data update should be no surprise, but it
must be able to handle even parts of the state and data
that it is not familiar with, somehow.
In the supervisor child specification
there is a list of modules which is recommended to contain
only the callback module. For a GenStatem
with multiple callback modules
there is no real need to list all of them, it may not even be possible since
the list could change after code upgrade. If this list would contain only
the start callback module, as recommended, what is important
is to upgrade that module whenever a synchronized code replacement is done.
Then the release handler concludes that an upgrade that upgrades that module
needs to suspend, code change, and resume any server whose child specification
declares that it is using that module.
And again; the current callback module will get the
Module.code_change/4
call.
Note
If a release upgrade/downgrade with change = {:advanced, extra}
specified in the .appup
file is made
when Module.code_change/4
is not implemented
the process will crash with exit reason undef
.
@callback format_status(status :: format_status()) :: new_status :: format_status()
Format/limit the status value.
This function is called by a GenStatem
process
in order to format/limit the server status
for debugging and logging purposes.
It is called in the following situations:
:sys.get_status/1,2
is invoked to get theGenStatem
status.- The
GenStatem
process terminates abnormally and logs an error.
This function is useful for changing the form and appearance
of the GenStatem
status for these cases. A callback module
wishing to change the :sys.get_status/1,2
return value and how its status appears in termination error logs,
exports an instance of Module.format_status/1
,
which will get a map Status
that describes the current state
of the GenStatem
, and shall return a map NewStatus
containing the same keys as the input map,
but it may transform some values.
One use case for this function is to return compact alternative state representations to avoid having large state terms printed in log files. Another is to hide sensitive data from being written to the error log.
Example:
def format_status(status) when is_map(status) do
status
|> Enum.map(fn
{:state, state} when is_map(state) ->
{:state, Map.delete(state, :private_key)}
{:message, {:password, _pass}} ->
{:message, {:password, :removed}}
pair ->
pair
end)
|> Map.new()
end
Note
This callback is optional, so a callback module does not need
to export it. The GenStatem
module provides
a default implementation of this function that returns {state, data}
.
@callback handle_event( :enter, old_state :: state(), current_state :: state(), data :: data() ) :: state_enter_result(current_state :: state())
@callback handle_event( event_type :: event_type(), event_content :: event_content(), current_state :: state(), data :: data() ) :: event_handler_result(state())
state callback in
callback mode handle_event_function
.
Whenever a GenStatem
receives an event from call/2,3
,
cast/2
, or as a normal process message, this function is called.
If event_type
is {:call, from}
,
the caller waits for a reply. The reply can be sent from this
or from any other state callback
by returning with {:reply, from, reply}
in actions
,
in replies
, or by calling
reply(from, reply)
.
If this function returns with a next state
that does not match equal (!==
) to the current state,
all postponed events are retried in the next state.
For options that can be set and actions that can be done
by GenStatem
after returning from this function, see action/0
.
When the GenStatem
runs with state enter calls,
this function is also called with arguments (:enter, old_state, ...)
during every state change. In this case there are some restrictions
on the actions that may be returned:
postpone/0
is not allowed since a state enter call is not an event so there is no event to postpone.{:next_event, _, _}
is not allowed since using state enter calls should not affect how events are consumed and produced.- It is not allowed to change states from this call.
Should you return
{:next_state, next_state, ...}
withnext_state !== state
theGenStatem
crashes.
Note that it is actually allowed to use {:repeat_state, new_data, ...}
although it makes little sense since you immediately
will be called again with a new state enter call making this
just a weird way of looping, and there are better ways to loop.
If you do not update new_data
and have some loop termination condition,
or if you use {:repeat_state_and_data, _}
or :repeat_state_and_data
you have an infinite loop!
You are advised to use {:keep_state, ...}
, {:keep_state_and_data, _}
or :keep_state_and_data
since changing states
from a state enter call is not possible anyway.
Note the fact that you can use throw
to return the result, which can be useful. For example to bail out with
throw(:keep_state_and_data)
from deep within complex code
that cannot return {:next_state, state, data}
because state
or data
is no longer in scope.
@callback init(args :: term()) :: init_result(state())
Initialize the state machine.
Whenever a GenStatem
is started using
start_link/3,4
,
start_monitor/3,4
, or
start/3,4
, this function is called by the new process
to initialize the implementation state and server data.
Args
is the Args
argument provided to that start function.
Note
Note that if the GenStatem
is started through m:proc_lib
, this
callback will never be called. Since this callback is not optional
it can in that case be implemented as:
-spec init(_) -> no_return().
init(Args) -> :erlang.error(not_implemented, [Args]).
@callback state_name(:enter, old_state_name :: state_name(), data()) :: state_enter_result(:state_name)
@callback state_name( event_type :: event_type(), event_content :: event_content(), data :: data() ) :: event_handler_result(state_name())
State callback in
callback mode :state_functions
.
State callback that handles all events in state state_name
, where
state_name :: state_name()
has to be an atom/0
.
state_name
cannot be terminate
since that would collide
with the callback function Module.terminate/3
.
Besides that when doing a state change
the next state always has to be an atom/0
,
this function is equivalent to
Module.handle_event(event_type, event_content, __ENV__.function, data)
,
which is the state callback in
callback mode handle_event_function
.
@callback terminate( reason :: :normal | :shutdown | {:shutdown, term()} | term(), current_state :: state(), data() ) :: any()
Handle state machine termination.
This function is called by a GenStatem
when it is about to terminate.
It is to be the opposite of Module.init/1
and do any necessary cleaning up. When it returns, the GenStatem
terminates with reason
. The return value is ignored.
reason
is a term denoting the stop reason and state
is the internal state of the GenStatem
.
reason
depends on why the GenStatem
is terminating. If it is because
another callback function has returned, a stop tuple {:stop, reason}
in
actions
, reason
has the value specified in that tuple.
If it is because of a failure, reason
is the error reason.
If the GenStatem
is part of a supervision tree and is ordered by its
supervisor to terminate, this function is called with reason = :shutdown
if both the following conditions apply:
- The
GenStatem
process has been set to trap exit signals. - The shutdown strategy as defined in the supervisor's
child specification is an integer time-out value, not
:brutal_kill
.
Even if the GenStatem
is not part of a supervision tree,
this function is called if it receives an :EXIT
message
from its parent. reason
is the same as in the :EXIT
message.
If the GenStatem
process is not set up to trap exit signals it is
immediately terminated, just like any process, and this function is not called.
Notice that for any other reason than :normal
, :shutdown
, or
{:shutdown, term}
, the GenStatem
is assumed to terminate
because of an error and an error report is issued using m:logger
.
When the GenStatem
process exits, an exit signal
with the same reason is sent to linked processes and ports,
just as for any process.
Functions
@spec call( server_ref :: server_ref(), request :: term(), timeout :: timeout() ) :: reply :: term()
Call a server: send request and wait for response.
Makes a synchronous call to the GenStatem
ServerRef
by sending a request
and waiting until the response arrives.
The GenStatem
calls the
state callback
with event_type/0
{:call, from}
and event content request
.
The server's reply is sent from a state callback,
by returning a transition action {:reply, from, reply}
,
calling reply(replies)
with such a reply action
in the replies
list, or calling reply(from, reply)
.
timeout
is an integer > 0, which specifies how many milliseconds
to wait for a reply, or the atom :infinity
to wait indefinitely,
which is the default. If no reply is received within the specified time,
the function call fails.
The call can also fail, for example, if the GenStatem
dies before or during this function call.
When this call fails it raises the GenError
Exception and exit
the calling process.
@spec cast(server_ref :: server_ref(), msg :: term()) :: :ok
Cast an event to a server.
Sends an asynchronous cast
event to the GenStatem
ServerRef
and returns ok
immediately,
ignoring if the destination node or GenStatem
does not exist.
The GenStatem
calls the
state callback
with event_type/0
cast
and event content Msg
.
@spec check_response(msg :: term(), req_id :: request_id()) :: {:reply, reply :: term()} | {:error, {reason :: term(), server_ref()}} | :no_reply
Check if a received message is a request response.
Checks if msg
is a response corresponding to the request identifier req_id
.
The request must have been made by send_request/2
and by the same process
calling this function.
If msg
is a reply to the handle req_id
the result of the request is returned
in reply
. Otherwise this function returns :no_reply
and no cleanup is done,
and thus the function shall be invoked repeatedly until the response is returned.
See call/3
about how the request is handled and the reply
is
sent by the GenStatem
server.
If the GenStatem
server process has died when this function is called, that
is; msg
reports the server's death, this function returns an error
return
with the exit reason
.
@spec check_response( msg :: term(), req_id_collection :: request_id_collection(), delete :: boolean() ) :: {{:reply, reply :: term()} | {:error, {reason :: term(), server_ref()}}, label :: term(), new_req_id_collection :: request_id_collection()} | :no_request | :no_reply
@spec receive_response( req_id :: request_id(), response_timeout :: response_timeout() ) :: {:reply, reply :: term()} | {:error, {reason :: term(), server_ref()}} | timeout()
Receive a request response.
Receive a response corresponding to the request identifier req_id
.
The request must have been made by send_request/2
to the gen_statem
process. This function must be called
from the same process from which send_request/2
was made.
timeout
specifies how long to wait for a response. If no response is
received within the specified time, this function returns timeout
.
Assuming that the server executes on a node supporting aliases (introduced in OTP 24)
the request will also be abandoned. That is, no response will be received
after a time-out. Otherwise, a stray response might be received at a later time.
See call/3
about how the request is handled and the reply
is sent
by the gen_statem
server.
If the gen_statem
server process is dead or dies while this function waits for the
reply, it returns an error
return with the exit reason
.
The difference between wait_response/2
and receive_response/2
is that
receive_response/2
abandons the request at time-out so that a potential future
response is ignored, while wait_response/2
does not.
@spec receive_response( req_id_col :: request_id_collection(), timeout :: response_timeout(), delete :: boolean() ) :: result :: {response :: {:reply, reply :: term()} | {:error, {reason :: term(), server_ref()}}, label :: term(), new_req_id_col :: request_id_collection()} | :no_request | :timeout
@spec reply(replies :: [reply_action()] | reply_action()) :: :ok
Send one or multiple call
replies.
This funcion can be used by a GenStatem
callback to explicitly send
one or multiple replies to processes waiting for call
requests' replies,
when it is impractical or impossible to return reply_action/0
s
from a state callback.
Note
A reply sent with this function is not visible in m:sys
debug output.
Send a call
reply
to from
.
This funcion can be used by a GenStatem
callback to explicitly send
a reply to a process waiting for a call
requests' reply,
when it is impractical or impossible to return a reply_action/0
from a state callback.
Note
A reply sent with this function is not visible in m:sys
debug output.
@spec reqids_add( req_id :: request_id(), label :: term(), req_id_collection :: request_id_collection() ) :: new_req_id_collection :: request_id_collection()
Store a request identifier in a colletion.
Stores req_id
and associates a label
with the request identifier
by adding this information to req_id_collection
and returning
the resulting request identifier collection.
@spec reqids_new() :: new_req_id_collection :: request_id_collection()
@spec reqids_size(req_id_collection :: request_id_collection()) :: non_neg_integer()
Return the number of request identifiers in req_id_collection
.
Send one or multiple call
replies.
This funcion can be used by a GenStatem
callback to explicitly send
one or multiple replies to processes waiting for call
requests' replies,
when it is impractical or impossible to return reply_action/0
s
from a state callback.
Note
A reply sent with this function is not visible in m:sys
debug output.
@spec send_request(server_ref :: server_ref(), request :: term()) :: req_id :: request_id()
Send an asynchronous call
request.
Sends request
to the GenStatem
process identified by server_ref
and returns a request identifier req_id
.
The return value req_id
shall later be used with receive_response/2
,
wait_response/2
, or check_response/2
to fetch the actual result
of the request. Besides passing the request identifier directly
to these functions, it can also be stored in a request identifier
collection using reqids_add/3
. Such a collection of request identifiers
can later be used in order to get one response corresponding to a
request in the collection by passing the collection as argument to
receive_response/3
, wait_response/3
, or check_response/3
. If you
are about to store the request identifier in a collection, you may want
to consider using send_request/4
instead.
The call
GenStatem.send_request(server_ref, request) |> GenStatem.wait_response(timeout)
can be seen as equivalent to GenStatem.call(server, request, timeout)
,
ignoring the error handling.
See call/3
about how the request is handled
and the reply
is sent by the GenStatem
server.
The server's reply
is returned by one of the receive_response/1,2
,
wait_response/1,2
, or check_response/2
functions.
@spec send_request( server_ref :: server_ref(), request :: term(), label :: term(), req_id_col :: request_id_collection() ) :: new_req_id_collection :: request_id_collection()
Send an asynchronous call
request and add it to a request identifier collection.
Sends request
to the GenStatem
process identified by server_ref
.
The label
will be associated with the request identifier
of the operation and added to the returned request identifier collection
new_req_id_collection
. The collection can later be used in order to
get one response corresponding to a request in the collection
by passing the collection as argument to receive_response/3
,
wait_response/3
, or check_response/3
.
The same as calling
reqids_add(
send_request(server_ref, request),
label, req_id_collection)
,
but slightly more efficient.
Starts a GenStatem
process without links (outside of a supervision tree).
See start_link/3
for more information.
Start a GenStatem
process linked to the current process.
This is often used to start the GenStatem
as part of a supervision tree.
Once the server is started, the init/1
function of the given module
is
called with init_arg
as its argument to initialize the server. To ensure a
synchronized start-up procedure, this function does not return until init/1
has returned.
Note that a GenStatem
started with start_link/3
is linked to the
parent process and will exit in case of crashes from the parent. The GenStatem
will also exit due to the :normal
reasons in case it is configured to trap
exits in the init/1
callback.
Options
:name
- used for name registration as described in the "Name registration" section in the documentation forGenStatem
:timeout
- if present, the server is allowed to spend the given number of milliseconds initializing or it will be terminated and the start function will return{:error, :timeout}
:debug
- if present, the corresponding function in the:sys
module is invoked:spawn_opt
- if present, its value is passed as options to the underlying process as inProcess.spawn/4
:hibernate_after
- if present, the GenStatem process awaits any message for the given number of milliseconds and if no message is received, the process goes into hibernation automatically (by calling:proc_lib.hibernate/3
).
Return values
If the server is successfully created and initialized, this function returns
{:ok, pid}
, where pid
is the PID of the server. If a process with the
specified server name already exists, this function returns
{:error, {:already_started, pid}}
with the PID of that process.
If the init/1
callback fails with reason
, this function returns
{:error, reason}
. Otherwise, if it returns {:stop, reason}
or :ignore
, the process is terminated and this function returns
{:error, reason}
or :ignore
, respectively.
@spec start_monitor(module(), term(), [start_opt()]) :: start_mon_ret()
Starts a GenStatem
process monitored and registered, but not linked.
See start_link/3
for more information.
@spec stop(server_ref(), reason :: term(), timeout()) :: :ok
Synchronously stops the state machine with the given reason
.
The terminate/3
callback of the given server
will be invoked before
exiting. This function returns :ok
if the server terminates with the
given reason; if it terminates with another reason, the call exits.
This function keeps OTP semantics regarding error reporting.
If the reason is any other than :normal
, :shutdown
or
{:shutdown, _}
, an error report is logged.
@spec wait_response( req_id :: request_id(), wait_time :: response_timeout() ) :: result :: {:reply, reply :: term()} | {:error, {reason :: term(), server_ref()}} | timeout()
Wait for a request response.
Waits for the response to the request identifier req_id
. The request
must have been made by send_request/2
to the GenStatem
process.
This function must be called from the same process from which
send_request/2
was called.
wait_time
specifies how long to wait for a reply. If no reply is received
within the specified time, the function returns timeout
and no cleanup is done,
Thus the function can be invoked repeatedly until a reply is returned.
See call/3
about how the request is handled and the Reply
is
sent by the GenStatem
server.
If the GenStatem
server process is dead or dies while this function waits for
the reply, it returns an error
return with the exit reason
.
The difference between receive_response/2
and wait_response/2
is that
receive_response/2
abandons the request at time-out so that a potential future
response is ignored, while wait_response/2
does not.
@spec wait_response( req_id_collection :: request_id_collection(), wait_time :: response_timeout(), delete :: boolean() ) :: result :: {response :: {:reply, reply :: term()} | {:error, {reason :: term(), server_ref()}}, label :: term(), new_req_id_collection :: request_id_collection()} | :no_request | :timeout
Wait for any request response in a collection.
Waits for a response in req_id_collection
. All request identifiers
of req_id_collection
must correspond to requests that have been made
using send_request/2
or send_request/4
, and all requests
must have been made by the process calling this function.
The label
in the response is the label
associated with
the request identifier that the response corresponds to.
The label
of a request identifier is associated
when adding the request id to a collection,
or when sending the request using send_request/4
.
Compared to wait_response/2
, the returned result or exception
associated with a specific request identifier will be wrapped
in a 3-tuple {response, label, new_req_id_collection}
.
response
is the value that would have been produced
by wait_response/2
, label
is the value associated with
the specific request identifier
and new_req_id_collection
is a possibly modified
request identifier collection.
If req_id_collection
is empty, no_request
is returned.
If no response is received before wait_time
has expired,
timeout
is returned. It is valid to continue waiting
for a response as many times as needed up until a response
has been received and completed by check_response()
,
receive_response()
, or wait_response()
.
The difference between receive_response/3
and wait_response/3
is that receive_response/3
abandons requests at time-out
so that potential future responses are ignored, while
wait_response/3
does not.
If delete
is true
, the association with label
has been deleted
from req_id_collection
in the resulting new_req_id_collection
. If
delete
is false
, new_req_id_collection
will equal req_id_collection
.
Note that deleting an association is not for free and that a collection
containing already handled requests can still be used by subsequent calls to
wait_response/3
, check_response/3
, and receive_response/3
.
However, without deleting handled associations, the above calls will not be
able to detect when there are no more outstanding requests to handle, so you
will have to keep track of this some other way than relying on a :no_request
return. Note that if you pass a collection only containing associations of
already handled or abandoned requests to this function, it will always block
until wait_time
expires and then return :timeout
.