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Module plain_fsm

A behaviour/support library for writing plain Erlang FSMs.

This module defines the plain_fsm behaviour.

Required callback functions: code_change/3, data_vsn/0.

Authors: Ulf Wiger, (ulf.wiger@ericsson.com).

Description

This module implements an OTP behaviour for writing plain Erlang FSMs, alleviating a long-standing gripe of mine that the OTP behaviours, for all their power, force programmers into a coding style that is very much different from that taught in the Basic Erlang Course (or the book, or online tutorials, ...) -- the type of programming that made us want to use Erlang in the first place.

Only in my old age have I begun to understand fully what a sacrifice this is. See e.g. my presentation Death by Accidental Complexity (QCon SF 2010) for a more detailed discussion of the issues involved. (Slides also available in the doc/ directory of this repos)

The requirements that drove us away from plain Erlang programming in the first place were:

  • The need to support system messages to control upgrade, state inspection, shutdown, etc. The plain_fsm library solves this in a reasonable way, I think.

  • The need for debugging support. The debugging support in e.g. gen_server is, I believe, rendered obsolete by the new powerful trace support (and dbg) in later versions of OTP.

  • In the case of gen_server, reducing the need to reinvent thewheel, a valid point, but more so for e.g. the client side of gen_server:call(). In a protocol state machine, the only thing that really needs reusing is the handling of system messages.

However, the behaviours provided by OTP for FSM programming, gen_server and gen_fsm (gen_server is perhaps a more common choice than gen_fsm), both have the distinct drawback that you cannot normally start with a classic Erlang design and then migrate to a behaviour without significant code rewrite. In addition, the two behaviours are semantically different from the classic Erlang design

Using plain_fsm

First, write your state machine without worrying about OTP system messages. Once you're happy with it, figure out where you really want to handle system messages. Normally, it will suffice to do it in a fairly stable state. A good rule of thumb is that the top-level state machine should handle system messages, while the transient (sub-) states shouldn't

In the states where you want to handle system messages, you have three choices:

(A) Insert the system messages in the receive clause:

  idle(S) ->
     Parent = plain_fsm:info(parent),
     receive
        {system, From, Req} ->
           plain_fsm:handle_system_msg(
               From, Req, S, fun(S1) -> idle(S1) end);
        {'EXIT', Parent, Reason} ->
           plain_fsm:parent_EXIT(Reason, S);
        ... %% your original code here
     end.

This has the advantage that everyone can understand what's going on. The part that plain_fsm.erl helps you with is the set of functions system_code_change(), system_continue(), system_shutdown(), format_status(), which are required callbacks when you handle system messages directly.

(B) Handle system messages and unknown messages together:

  idle(S) ->
     Parent = plain_fsm:info(parent),
     receive
        ... %% your original code here
        Msg ->
           plain_fsm:handle_msg(Msg, State, fun(S1) -> idle(S1) end)
     end.

This is quite convenient if the receive statement already has a 'catch-all' clause, discarding unknown messages. plain_fsm:handle_msg/3 will handle system messages properly and ignore any other message.

(C) Write a pseudo wrapper function around your receive clause:

  idle(S) ->
     plain_fsm:extended_receive(
        receive
           ... %% your original code
        end).

The function plain_fsm:extended_receive/1 is replaced in a parse_transform into something that looks very much like the previous program (A). The code, as it reads, requires the reader to know that the transformation takes place, otherwise the semantics would be confusing (you cannot solve the problem using a real function that way.) On the plus side, this is a fairly small violation of both the original code and Erlang's semantics.

Note that for this to work, you must include "plain_fsm.hrl"in your module.

Example

In the module fsm_example.erl (included in the plain_fsm package), we choose to handle system messages in the idle state. The example code is runnable, and supports suspend, resume, status inspection, and code change.

Imagine that the code initially looked like this:

  idle(S) ->
      receive
    a ->
        io:format("going to state a~n", []),
        a(S);
    b ->
        io:format("going to state b~n", []),
        b(S)
      after 10000 ->
        io:format("timeout in idle~n", []),
        idle(S)
      end).

The change required to handle system messages is as follows:

  idle(S) ->plain_fsm:extended_receive(
        receive
            a ->
                io:format("going to state a~n", []),
                a(S);
            b ->
                io:format("going to state b~n", []),
                b(S)
        after 10000 ->
                io:format("timeout in idle~n", []),
                idle(S)
        end).

In addition, we change the start function from, in this case:

  spawn_link() ->
      spawn_link(fun() ->
                         process_flag(trap_exit, true),
                         idle(mystate)
                 end).

Is changed into:

  spawn_link() ->plain_fsm:spawn_link(?MODULE, fun() ->
                                            process_flag(trap_exit,true),
                                            idle(mystate)
                                    end).

See also spawn/2 and spawn_opt/3 for information on other possible start functions.

To be fully compliant, you also need to supply a code_change/3 function. See behaviour_info/1 for details.

Function Index

behaviour_info/1Defines which functions this behaviour expects to be exported from the user's callback module.
current_function/0Virtual function for extracting the current function.
extended_receive/1Virtual function used to wrap receive clauses.
handle_msg/3Called in a "catch-all" clause within a receive statement.
handle_system_msg/4Called when the process receives a system message.
hibernate/3Virtual function used to wrap a call to the BIF erlang:hibernate/3.
info/1retrieves meta-data for the plain_fsm process.
parent_EXIT/2Handles parent termination properly.
spawn/2Equivalent to proc_lib:spawn(StartF).
spawn_link/2Equivalent to proc_lib:spawn_link(StartF).
spawn_opt/3Equivalent to proc_lib:spawn_opt(StartF, Opts).
spawn_opt/4Equivalent to proc_lib:spawn_opt(Node, StartF, Opts).
start_opt/4Similar to proc_lib:start(M,F,A, Timeout, Opts).
store_name/1stores an internal name for the FSM (for sys:get_status()).
tail_apply/5Helper function to dispatch blocking calls as tail calls.
wake_up/5

Function Details

behaviour_info/1

behaviour_info(Other::atom()) -> term()



Defines which functions this behaviour expects to be exported from the user's callback module. plain_fsm requires only code_change/3 to be present. The semantics of Mod:code_change/3 are as follows:

    code_change(OldVsn, State, Extra) -> {ok, NewState}.

The above code is just like it would look like in a gen_server callback module.

    code_change(OldVsn, State, Extra) -> {ok, NewState, Options}.

where Options may be any of

  • {mod, module()}, allowing you to switch callback modules during a code change.

  • {name, name()}, allowing you to rename the process (note that you have to handle name registration yourself.)

  • {cont, atom() | function(1)}, allowing you to provide another continuation (point of entry into your own code after the code change.)

current_function/0

current_function() -> {Module, Function, Arity}



Virtual function for extracting the current function.

This function call is expanded by the plain_fsm parse transform into the name and arity ({Module, Function, Arity}) of the current function. It cannot be used from code that hasn't been transformed.

extended_receive/1

extended_receive(Expr) -> VOID



Virtual function used to wrap receive clauses.

This function cannot be called directly, but is intended as a syntactic wrapper around a receive clause. It will be transformed at compile time to a set of receive patterns handling system messages and parent termination according to the OTP rules. The transform requires that the surrounding function has exactly one argument (the "State" or "Loop Data".)

To trigger the parse_transform, include the file plain_fsm.hrl (found in plain_fsm/inc/) in your module, and the Erlang compiler must be able to find the module plain_fsm_xform.beam. If erlc is used, this is accomplished by adding -pa .../plain_fsm/ebin to the erlc command.

handle_msg/3

handle_msg(Other::Msg, State, Cont::cont()) -> NEVER_RETURNS



Called in a "catch-all" clause within a receive statement.

This function never returns. It will handle system messages properly and ignore anything else. Example:

  idle(S) ->
    receive
       ...
       Msg ->
           plain_fsm:handle_msg(Msg, S, fun(S1) ->
                                                 idle(S1)
                                        end)
    end.

Note that this function should only be used if it is known to be safe to discard unknown messages. In most state machines there should be at least one state where unknown messages are discarded; in these states, the handle_msg/3 function can be a convenient way to handle both unknown messages and system messages.

The Cont argument should be either a fun with one argument (the new state), which jumps back into the user code in the proper place, or it can be the name of a function (in this case, 'idle'). In the latter case, the function in question must be exported; in the former case, this is not necessary.

handle_system_msg/4

handle_system_msg(Req, From, State, Cont::cont()) -> NEVER_RETURNS



Called when the process receives a system message.

This function never returns. If the program handles system messages explicitly, this function can be called to handle them in the plain_fsm way. Example:

  idle(S) ->
    receive
       {system, From, Req} ->
           plain_fsm:handle_system_msg(From, Req, S, fun(S1) ->
                                                            idle(S1)
                                                     end);
       ...
    end.

The Cont argument should be either a fun with one argument (the new state), which jumps back into the user code in the proper place, or it can be the name of a function (in this case, 'idle'). In the latter case, the function in question must be exported; in the former case, this is not necessary.

hibernate/3

hibernate(M::atom(), F::atom(), A::[IntState]) -> NEVER_RETURNS



Virtual function used to wrap a call to the BIF erlang:hibernate/3.

This function cannot be called directly, but translates to the call erlang:hibernate(plain_fsm,wake_up,[data_vsn(),Module,M,F,A]) where Module:data_vsn() and Module:code_change/3 are expected to exist (the parse_transform will add and export the function data_vsn() -< 0, if it doesn't already exist.)

The function plain_fsm:wake_up/5 will begin by calling Module:data_vsn(), and if it is the same as before, simply call apply(M,F,A). Otherwise, Module:code_change(OldVsn, IntState, hibernate) will be called first. This allows a plain_fsm behaviour module to be "bootstrapped" to a new version during hibernation.

info/1

info(What::atom()) -> term()
  • What = debug | name | mod | parent

retrieves meta-data for the plain_fsm process.

Description of available meta-data:

      debug : See the manual for sys.erl
      name  : Internal name, normally the same as the registered name.
              initially undefined, can be set via plain_fsm:store_name/1.
      mod   : Name of the callback module.
      parent: The pid() of the parent process.
###parent_EXIT/2##
parent_EXIT(Reason, State) -> EXIT



Handles parent termination properly.

This function is called when the parent of a plain_fsm instance dies. The OTP rules state that the child should die with the same reason as the parent (especially in the case of Reason='shutdown'.)

spawn/2

spawn(Mod::atom(), StartF::function()) -> pid()



Equivalent to proc_lib:spawn(StartF). This function also initializes the plain_fsm meta-data.

spawn_link/2

spawn_link(Mod::atom(), StartF::function()) -> pid()



Equivalent to proc_lib:spawn_link(StartF). This function also initializes the plain_fsm meta-data.

spawn_opt/3

spawn_opt(Mod::atom(), StartF::function(), Opts::list()) -> pid()



Equivalent to proc_lib:spawn_opt(StartF, Opts). This function also initializes the plain_fsm meta-data.

spawn_opt/4

spawn_opt(Node::atom(), Mod::atom(), StartF::function(), Opts::list()) -> pid()



Equivalent to proc_lib:spawn_opt(Node, StartF, Opts). This function also initializes the sysFsm meta-data.

start_opt/4

start_opt(Mod::atom(), InitF::function(), Timeout::integer(), Opts::list()) -> {ok, pid()} | {error, Reason}



Similar to proc_lib:start(M,F,A, Timeout, Opts).

This function works in a similar fashion to proc_lib:start/5, but takes a fun instead of a {M,F,A} argument.

InitF() may return one of the following:

  • {reply, Reply, Cont}, where Reply will be sent back to the parent, and Cont is a continuation function with no arguments.
  • {noreply, Cont}, which sends no ack message back to the parent (presumably, this is done elsewhere in the code then).

store_name/1

store_name(Name::term()) -> ok



stores an internal name for the FSM (for sys:get_status()). This can be used if the FSM were started as an anonymous process (the only kind currently supported). Note that this function does not register the name. The name stored is the one that shows up in sys:get_status/1. No restriction is made here regarding the data type.

tail_apply/5

tail_apply(F::Fun, OldVsn, Module, ContF, S) -> NEVER_RETURNS



Helper function to dispatch blocking calls as tail calls. During code change, it can be a problem that processes lie in blocking calls - say, e.g., to gen_tcp:connect(...). If the module is reloaded, the calling function will still be on the call stack, and may eventually get the process killed (as the VM only holds two versions of the module).

This function is most easily called using the macro ?tail_apply(F, ContF, S), which expands to

  plain_fsm:tail_apply(F, ?MODULE:data_vsn(), ?MODULE, ContF, S)

In this case, ?MODULE:data_vsn() will have been automatically generated by plain_fsm, or is manually updated whenever the internal representation of the state S is changed.

ContF represents an exported function in the calling module, ContF(Status, Result, S) Status :: ok | error Result :: fun() | any()

If the call to Fun() fails, the exception (throw, error or exit) will be caught, and Result will be a fun (arity 0), which can be called to "re-throw" the exception. This way, the continuation function can catch exceptions in its own try/catch pattern.

'Status' will be error if Fun() fails, otherwise ok.

Thus, the simplest implementation of ContF would be:

  ContF(ok, Result, S) ->
      handle_result(Result, S);
  ContF(error, E, _S) ->
      E().

Note that this solution does not throw away the call stack, as e.g. a call to hibernate/3 does. Thus, it is basically only tail-recursive as regards the calling function, placing plain_fsm:tail_apply/5 on the call stack rather than a function in the user module.

wake_up/5

wake_up(OldVsn, Module, M, F, A) -> any()

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