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-rw-r--r--system/doc/design_principles/statem.xml184
1 files changed, 57 insertions, 127 deletions
diff --git a/system/doc/design_principles/statem.xml b/system/doc/design_principles/statem.xml
index 8b0fbed7c0..585b1a35f5 100644
--- a/system/doc/design_principles/statem.xml
+++ b/system/doc/design_principles/statem.xml
@@ -84,8 +84,9 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
a server <c>Data</c> besides the state. Because of this, and as
there is no restriction on the number of states
(assuming that there is enough virtual machine memory)
- or on the number of distinct input events, makes
- a state machine implemented with this behavior Turing complete.
+ or on the number of distinct input events,
+ a state machine implemented with this behavior
+ is in fact Turing complete.
But it feels mostly like an Event-Driven Mealy Machine.
</p>
</section>
@@ -101,8 +102,8 @@ State(S) x Event(E) -> Actions(A), State(S')</pre>
<list type="bulleted">
<item>
<p>
- In mode <seealso marker="stdlib:gen_statem#type-callback_mode">
- <c>state_functions</c></seealso>,
+ In mode
+ <seealso marker="stdlib:gen_statem#type-callback_mode"><c>state_functions</c></seealso>,
the state transition rules are written as some Erlang
functions, which conform to the following convention:
</p>
@@ -113,20 +114,19 @@ StateName(EventType, EventContent, Data) ->
</item>
<item>
<p>
- In mode <seealso marker="stdlib:gen_statem#type-callback_mode">
- <c>handle_event_function</c></seealso>,
+ In mode
+ <seealso marker="stdlib:gen_statem#type-callback_mode"><c>handle_event_function</c></seealso>,
only one Erlang function provides all state transition rules:
</p>
<pre>
handle_event(EventType, EventContent, State, Data) ->
.. code for actions here ...
- {next_state, State', Data'}</pre>
+ {next_state, NewState, NewData}</pre>
</item>
</list>
<p>
Both these modes allow other return tuples; see
- <seealso marker="stdlib:gen_statem#Module:StateName/3">
- <c>Module:StateName/3</c></seealso>
+ <seealso marker="stdlib:gen_statem#Module:StateName/3"><c>Module:StateName/3</c></seealso>
in the <c>gen_statem</c> manual page.
These other return tuples can, for example, stop the machine,
execute state transition actions on the machine engine itself,
@@ -172,8 +172,7 @@ handle_event(EventType, EventContent, State, Data) ->
This mode works equally well when you want to focus on
one event at the time or on
one state at the time, but function
- <seealso marker="stdlib:gen_statem#Module:handle_event/4">
- <c>Module:handle_event/4</c></seealso>
+ <seealso marker="stdlib:gen_statem#Module:handle_event/4"><c>Module:handle_event/4</c></seealso>
quickly grows too large to handle without introducing dispatching.
</p>
<p>
@@ -291,9 +290,7 @@ start_link(Code) ->
]]></code>
<p>
<c>start_link</c> calls function
- <seealso marker="stdlib:gen_statem#start_link/4">
- <c>gen_statem:start_link/4</c>
- </seealso>,
+ <seealso marker="stdlib:gen_statem#start_link/4"><c>gen_statem:start_link/4</c></seealso>,
which spawns and links to a new process, a <c>gen_statem</c>.
</p>
<list type="bulleted">
@@ -308,9 +305,7 @@ start_link(Code) ->
Instead its pid must be used. The name can also be specified
as <c>{global,Name}</c>, then the <c>gen_statem</c> is
registered using
- <seealso marker="kernel:global#register_name/2">
- <c>global:register_name/2</c>
- </seealso>
+ <seealso marker="kernel:global#register_name/2"><c>global:register_name/2</c></seealso>
in <c>Kernel</c>.
</p>
</item>
@@ -339,9 +334,7 @@ start_link(Code) ->
<p>
The fourth argument, <c>[]</c>, is a list of options.
For the available options, see
- <seealso marker="stdlib:gen_statem#start_link/3">
- <c>gen_statem:start_link/3</c>
- </seealso>.
+ <seealso marker="stdlib:gen_statem#start_link/3"><c>gen_statem:start_link/3</c></seealso>.
</p>
</item>
</list>
@@ -350,13 +343,9 @@ start_link(Code) ->
calls callback function <c>code_lock:init(Code)</c>.
This function is expected to return <c>{CallbackMode,State,Data}</c>,
where
- <seealso marker="#callback_modes">
- <c>CallbackMode</c>
- </seealso>
+ <seealso marker="#callback_modes"><c>CallbackMode</c></seealso>
selects callback module state function mode, in this case
- <seealso marker="stdlib:gen_statem#type-callback_mode">
- <c>state_functions</c>
- </seealso>
+ <seealso marker="stdlib:gen_statem#type-callback_mode"><c>state_functions</c></seealso>
through macro <c>?CALLBACK_MODE</c>. That is, each state
has got its own handler function.
<c>State</c> is the initial state of the <c>gen_statem</c>,
@@ -375,23 +364,17 @@ init(Code) ->
{?CALLBACK_MODE,locked,Data}.
]]></code>
<p>Function
- <seealso marker="stdlib:gen_statem#start_link/3">
- <c>gen_statem:start_link</c>
- </seealso>
+ <seealso marker="stdlib:gen_statem#start_link/3"><c>gen_statem:start_link</c></seealso>
is synchronous. It does not return until the <c>gen_statem</c>
is initialized and is ready to receive events.
</p>
<p>
Function
- <seealso marker="stdlib:gen_statem#start_link/3">
- <c>gen_statem:start_link</c>
- </seealso>
+ <seealso marker="stdlib:gen_statem#start_link/3"><c>gen_statem:start_link</c></seealso>
must be used if the <c>gen_statem</c>
is part of a supervision tree, that is, started by a supervisor.
Another function,
- <seealso marker="stdlib:gen_statem#start/3">
- <c>gen_statem:start</c>
- </seealso>
+ <seealso marker="stdlib:gen_statem#start/3"><c>gen_statem:start</c></seealso>
can be used to start a standalone <c>gen_statem</c>, that is,
a <c>gen_statem</c> that is not part of a supervision tree.
</p>
@@ -403,9 +386,7 @@ init(Code) ->
<title>Handling Events</title>
<p>The function notifying the code lock about a button event is
implemented using
- <seealso marker="stdlib:gen_statem#cast/2">
- <c>gen_statem:cast/2</c>:
- </seealso>
+ <seealso marker="stdlib:gen_statem#cast/2"><c>gen_statem:cast/2</c></seealso>:
</p>
<code type="erl"><![CDATA[
button(Digit) ->
@@ -528,9 +509,7 @@ handle_event({call,From}, code_length, #{code := Code} = Data) ->
]]></code>
<p>
This example uses
- <seealso marker="stdlib:gen_statem#call/2">
- <c>gen_statem:call/2</c>
- </seealso>,
+ <seealso marker="stdlib:gen_statem#call/2"><c>gen_statem:call/2</c></seealso>,
which waits for a reply from the server.
The reply is sent with a <c>{reply,From,Reply}</c> tuple
in an action list in the <c>{keep_state,...}</c> tuple
@@ -545,15 +524,13 @@ handle_event({call,From}, code_length, #{code := Code} = Data) ->
<p>
If mode <c>handle_event_function</c> is used,
all events are handled in
- <seealso marker="stdlib:gen_statem#Module:handle_event/4">
- <c>Module:handle_event/4</c>
- </seealso>
+ <seealso marker="stdlib:gen_statem#Module:handle_event/4"><c>Module:handle_event/4</c></seealso>
and we can (but do not have to) use an event-centered approach
where we dispatch on event first and then state:
</p>
<code type="erl"><![CDATA[
...
--define(CALLBACK_MODE, state_functions).
+-define(CALLBACK_MODE, handle_event_function).
...
-export([handle_event/4]).
@@ -604,11 +581,7 @@ handle_event(timeout, _, open, Data) ->
strategy must be a time-out value and the <c>gen_statem</c> must
in function <c>init/1</c> set itself to trap exit signals
by calling
- <seealso marker="erts:erlang#process_flag/2">
- <c>process_flag(trap_exit, true)</c>
- </seealso>.
- When ordered to shut down, the <c>gen_statem</c> then calls
- callback function <c>terminate(shutdown, State, Data)</c>:
+ <seealso marker="erts:erlang#process_flag/2"><c>process_flag(trap_exit, true)</c></seealso>:
</p>
<code type="erl"><![CDATA[
init(Args) ->
@@ -617,6 +590,10 @@ init(Args) ->
...
]]></code>
<p>
+ When ordered to shut down, the <c>gen_statem</c> then calls
+ callback function <c>terminate(shutdown, State, Data)</c>.
+ </p>
+ <p>
In the following example, function <c>terminate/3</c>
locks the door if it is open, so we do not accidentally leave the door
open when the supervision tree terminates:
@@ -633,9 +610,7 @@ terminate(_Reason, State, _Data) ->
<p>
If the <c>gen_statem</c> is not part of a supervision tree,
it can be stopped using
- <seealso marker="stdlib:gen_statem#stop/1">
- <c>gen_statem:stop</c>
- </seealso>,
+ <seealso marker="stdlib:gen_statem#stop/1"><c>gen_statem:stop</c></seealso>,
preferably through an API function:
</p>
<code type="erl"><![CDATA[
@@ -660,7 +635,7 @@ stop() ->
<title>Actions</title>
<p>
In the first sections actions were mentioned as a part of
- the general state machine model. These actions
+ the general state machine model. These general actions
are implemented with the code that callback module
<c>gen_statem</c> executes in an event-handling
callback function before returning
@@ -671,17 +646,11 @@ stop() ->
that a callback function can order the <c>gen_statem</c>
engine to do after the callback function return.
These are ordered by returning a list of
- <seealso marker="stdlib:gen_statem#type-action">
- actions
- </seealso>
+ <seealso marker="stdlib:gen_statem#type-action">actions</seealso>
in the
- <seealso marker="stdlib:gen_statem#type-state_function_result">
- return tuple
- </seealso>
+ <seealso marker="stdlib:gen_statem#type-state_function_result">return tuple</seealso>
from the
- <seealso marker="stdlib:gen_statem#Module:StateName/3">
- callback function
- </seealso>.
+ <seealso marker="stdlib:gen_statem#Module:StateName/3">callback function</seealso>.
These state transition actions affect the <c>gen_statem</c>
engine itself and can do the following:
</p>
@@ -697,9 +666,7 @@ stop() ->
and replying to a caller.
An example of event postponing is included later in this chapter.
For details, see the
- <seealso marker="stdlib:gen_statem#type-action">
- <c>gen_statem(3)</c>
- </seealso>
+ <seealso marker="stdlib:gen_statem#type-action"><c>gen_statem(3)</c></seealso>
manual page.
You can, for example, reply to many callers
and generate multiple next events to handle.
@@ -712,9 +679,7 @@ stop() ->
<title>Event Types</title>
<p>
The previous sections mentioned a few
- <seealso marker="stdlib:gen_statem#type-event_type">
- event types
- </seealso>.
+ <seealso marker="stdlib:gen_statem#type-event_type">event types</seealso>.
Events of all types are handled in the same callback function,
for a given state, and the function gets
<c>EventType</c> and <c>EventContent</c> as arguments.
@@ -727,22 +692,16 @@ stop() ->
<tag><c>cast</c></tag>
<item>
Generated by
- <seealso marker="stdlib:gen_statem#cast/2">
- <c>gen_statem:cast</c>.
- </seealso>
+ <seealso marker="stdlib:gen_statem#cast/2"><c>gen_statem:cast</c></seealso>.
</item>
<tag><c>{call,From}</c></tag>
<item>
Generated by
- <seealso marker="stdlib:gen_statem#call/2">
- <c>gen_statem:call</c>
- </seealso>,
+ <seealso marker="stdlib:gen_statem#call/2"><c>gen_statem:call</c></seealso>,
where <c>From</c> is the reply address to use
when replying either through the state transition action
<c>{reply,From,Msg}</c> or by calling
- <seealso marker="stdlib:gen_statem#reply/1">
- <c>gen_statem:reply</c>
- </seealso>.
+ <seealso marker="stdlib:gen_statem#reply/1"><c>gen_statem:reply</c></seealso>.
</item>
<tag><c>info</c></tag>
<item>
@@ -759,7 +718,7 @@ stop() ->
<item>
Generated by state transition action
<c>{next_event,internal,EventContent}</c>.
- All event types above can be generated using
+ All event types above can also be generated using
<c>{next_event,EventType,EventContent}</c>.
</item>
</taglist>
@@ -780,9 +739,7 @@ stop() ->
or you want to start a timer in one state and respond
to the time-out in another. This can be accomplished
with a regular Erlang timer:
- <seealso marker="erts:erlang#start_timer/4">
- <c>erlang:start_timer</c>.
- </seealso>
+ <seealso marker="erts:erlang#start_timer/4"><c>erlang:start_timer</c></seealso>.
</p>
<p>
For the example so far in this chapter: using the
@@ -818,9 +775,7 @@ open(cast, {button,_}, Data) ->
]]></code>
<p>
If you need to cancel a timer because of some other event, you can use
- <seealso marker="erts:erlang#cancel_timer/2">
- <c>erlang:cancel_timer(Tref)</c>
- </seealso>.
+ <seealso marker="erts:erlang#cancel_timer/2"><c>erlang:cancel_timer(Tref)</c></seealso>.
Notice that a time-out message cannot arrive after this,
unless you have postponed it (see the next section) before,
so ensure that you do not accidentally postpone such messages.
@@ -844,9 +799,7 @@ open(cast, {button,_}, Data) ->
</p>
<p>
Postponing is ordered by the state transition
- <seealso marker="stdlib:gen_statem#type-action">
- action
- </seealso>
+ <seealso marker="stdlib:gen_statem#type-action">action</seealso>
<c>postpone</c>.
</p>
<p>
@@ -861,7 +814,7 @@ open(cast, {button,_}, Data) ->
...
]]></code>
<p>
- A postponed event is only retried after a state change
+ The fact that a postponed event is only retried after a state change
translates into a requirement on the event and state space.
If you have a choice between storing a state data item
in the <c>State</c> or in the <c>Data</c>:
@@ -953,9 +906,7 @@ do_unlock() ->
</p>
<p>
The state transition
- <seealso marker="stdlib:gen_statem#type-action">
- action
- </seealso>
+ <seealso marker="stdlib:gen_statem#type-action">action</seealso>
<c>postpone</c> is designed to model
selective receives. A selective receive implicitly postpones
any not received events, but the <c>postpone</c>
@@ -977,16 +928,12 @@ do_unlock() ->
It can sometimes be beneficial to be able to generate events
to your own state machine.
This can be done with the state transition
- <seealso marker="stdlib:gen_statem#type-action">
- action
- </seealso>
+ <seealso marker="stdlib:gen_statem#type-action">action</seealso>
<c>{next_event,EventType,EventContent}</c>.
</p>
<p>
You can generate events of any existing
- <seealso marker="stdlib:gen_statem#type-action">
- type
- </seealso>,
+ <seealso marker="stdlib:gen_statem#type-action">type</seealso>,
but the <c>internal</c> type can only be generated through action
<c>next_event</c>. Hence, it cannot come from an external source,
so you can be certain that an <c>internal</c> event is an event
@@ -1150,9 +1097,9 @@ code_change(_Vsn, State, Data, _Extra) ->
<p>
This section describes what to change in the example
to use one <c>handle_event/4</c> function.
- The following clean first-dispatch-on-event approach
- does not work that well because of the generated
- entry actions:
+ The previously used clean first-dispatch-on-event approach
+ does not work that well here because of the generated
+ entry actions so this example dispatches on state first:
</p>
<code type="erl"><![CDATA[
...
@@ -1227,13 +1174,9 @@ handle_event({call,From}, code_length, _State, #{code := Code}) ->
<p>
To avoid this, you can format the internal state
that gets in the error log and gets returned from
- <seealso marker="stdlib:sys#get_status/1">
- <c>sys:get_status/1,2</c>
- </seealso>
+ <seealso marker="stdlib:sys#get_status/1"><c>sys:get_status/1,2</c></seealso>
by implementing function
- <seealso marker="stdlib:gen_statem#Module:format_status/2">
- <c>Module:format_status/2</c>
- </seealso>,
+ <seealso marker="stdlib:gen_statem#Module:format_status/2"><c>Module:format_status/2</c></seealso>,
for example like this:
</p>
<code type="erl"><![CDATA[
@@ -1259,9 +1202,7 @@ format_status(Opt, [_PDict,State,Data]) ->
]]></code>
<p>
It is not mandatory to implement a
- <seealso marker="stdlib:gen_statem#Module:format_status/2">
- <c>Module:format_status/2</c>
- </seealso>
+ <seealso marker="stdlib:gen_statem#Module:format_status/2"><c>Module:format_status/2</c></seealso>
function. If you do not, a default implementation is used that
does the same as this example function without filtering
the <c>Data</c> term, that is, <c>StateData = {State,Data}</c>.
@@ -1274,13 +1215,9 @@ format_status(Opt, [_PDict,State,Data]) ->
<title>Complex State</title>
<p>
The callback mode
- <seealso marker="stdlib:gen_statem#type-callback_mode">
- <c>handle_event_function</c>
- </seealso>
+ <seealso marker="stdlib:gen_statem#type-callback_mode"><c>handle_event_function</c></seealso>
enables using a non-atom state as described in section
- <seealso marker="#callback_modes">
- Callback Modes
- </seealso>,
+ <seealso marker="#callback_modes">Callback Modes</seealso>,
for example, a complex state term like a tuple.
</p>
<p>
@@ -1308,8 +1245,7 @@ format_status(Opt, [_PDict,State,Data]) ->
<p>
So we make the <c>button/1</c> function synchronous
by using
- <seealso marker="stdlib:gen_statem#call/2">
- <c>gen_statem:call</c></seealso>
+ <seealso marker="stdlib:gen_statem#call/2"><c>gen_statem:call</c></seealso>
and still postpone its events in the <c>open</c> state.
Then a call to <c>button/1</c> during the <c>open</c>
state does not return until the state transits to <c>locked</c>,
@@ -1462,16 +1398,12 @@ format_status(Opt, [_PDict,State,Data]) ->
and the amount of heap memory all these servers need
is a problem, then the memory footprint of a server
can be mimimized by hibernating it through
- <seealso marker="stdlib:proc_lib#hibernate/3">
- <c>proc_lib:hibernate/3</c>.
- </seealso>
+ <seealso marker="stdlib:proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>.
</p>
<note>
<p>
It is rather costly to hibernate a process; see
- <seealso marker="erts:erlang#hibernate/3">
- <c>erlang:hibernate/3</c>
- </seealso>.
+ <seealso marker="erts:erlang#hibernate/3"><c>erlang:hibernate/3</c></seealso>.
It is not something you want to do after every event.
</p>
</note>
@@ -1495,9 +1427,7 @@ handle_event(
]]></code>
<p>
The
- <seealso marker="stdlib:gen_statem#type-hibernate">
- <c>[hibernate]</c>
- </seealso>
+ <seealso marker="stdlib:gen_statem#type-hibernate"><c>[hibernate]</c></seealso>
action list on the last line
when entering the <c>{open,_}</c> state is the only change.
If any event arrives in the <c>{open,_},</c> state, we