The Boost Statechart Library - Rationale

###  | # The Boost Statechart Library ## Rationale

Introduction
Why yet another state machine framework
State-local storage
Dynamic configurability
Error handling
Asynchronous state machines
User actions: Member functions vs. function objects
Limitations

## Introduction

Most of the design decisions made during the development of this library are the result of the following requirements.

Boost.Statechart should ...

1. be fully type-safe. Whenever possible, type mismatches should be flagged with an error at compile-time
2. not require the use of a code generator. A lot of the existing FSM solutions force the developer to design the state machine either graphically or in a specialized language. All or part of the code is then generated
3. allow for easy transformation of a UML statechart (defined in http://www.omg.org/cgi-bin/doc?formal/03-03-01) into a working state machine. Vice versa, an existing C++ implementation of a state machine should be fairly trivial to transform into a UML statechart. Specifically, the following state machine features should be supported:
   - Hierarchical (composite, nested) states
   - Orthogonal (concurrent) states
   - Entry-, exit- and transition-actions
   - Guards
   - Shallow/deep history
4. produce a customizable reaction when a C++ exception is propagated from user code
5. support synchronous and asynchronous state machines and leave it to the user which thread an asynchronous state machine will run in. Users should also be able to use the threading library of their choice
6. support the development of arbitrarily large and complex state machines. Multiple developers should be able to work on the same state machine simultaneously
7. allow the user to customize all resource management so that the library could be used for applications with hard real-time requirements
8. enforce as much as possible at compile time. Specifically, invalid state machines should not compile
9. offer reasonable performance for a wide range of applications

## Why yet another state machine framework?

Before I started to develop this library I had a look at the following frameworks:

- The framework accompanying the book "Practical Statecharts in C/C++" by Miro Samek, CMP Books, ISBN: 1-57820-110-1
  http://www.quantum-leaps.com
  Fails to satisfy at least the requirements 1, 3, 4, 6, 8.
- The framework accompanying "Rhapsody in C++" by ILogix (a code generator solution)
  http://www.ilogix.com/sublevel.aspx?id=53
  This might look like comparing apples with oranges. However, there is no inherent reason why a code generator couldn't produce code that can easily be understood and modified by humans. Fails to satisfy at least the requirements 2, 4, 5, 6, 8 (there is quite a bit of error checking before code generation, though).
- The framework accompanying the article "State Machine Design in C++"
  http://www.ddj.com/184401236?pgno=1
  Fails to satisfy at least the requirements 1, 3, 4, 5 (there is no direct threading support), 6, 8.

I believe Boost.Statechart satisfies all requirements.

## State-local storage

This not yet widely known state machine feature is enabled by the fact that every state is represented by a class. Upon state-entry, an object of the class is constructed and the object is later destructed when the state machine exits the state. Any data that is useful only as long as the machine resides in the state can (and should) thus be a member of the state. This feature paired with the ability to spread a state machine over several translation units makes possible virtually unlimited scalability.

In most existing FSM frameworks the whole state machine runs in one environment (context). That is, all resource handles and variables local to the state machine are stored in one place (normally as members of the class that also derives from some state machine base class). For large state machines this often leads to the class having a huge number of data members most of which are needed only briefly in a tiny part of the machine. The state machine class therefore often becomes a change hotspot what leads to frequent recompilations of the whole state machine.

The FAQ item "What's so cool about state-local storage?" further explains this by comparing the tutorial StopWatch to a behaviorally equivalent version that does not use state-local storage.

## Dynamic configurability

### Two types of state machine frameworks

- A state machine framework supports dynamic configurability if the whole layout of a state machine can be defined at runtime ("layout" refers to states and transitions, actions are still specified with normal C++ code). That is, data only available at runtime can be used to build arbitrarily large machines. See "A Multiple Substring Search Algorithm" by Moishe Halibard and Moshe Rubin in June 2002 issue of CUJ for a good example (unfortunately not available online).
- On the other side are state machine frameworks which require the layout to be specified at compile time

State machines that are built at runtime almost always get away with a simple state model (no hierarchical states, no orthogonal states, no entry and exit actions, no history) because the layout is very often computed by an algorithm. On the other hand, machine layouts that are fixed at compile time are almost always designed by humans, who frequently need/want a sophisticated state model in order to keep the complexity at acceptable levels. Dynamically configurable FSM frameworks are therefore often optimized for simple flat machines while incarnations of the static variant tend to offer more features for abstraction.

However, fully-featured dynamic FSM libraries do exist. So, the question is:

### Why not use a dynamically configurable FSM library for all state machines?

One might argue that a dynamically configurable FSM framework is all one ever needs because any state machine can be implemented with it. However, due to its nature such a framework has a number of disadvantages when used to implement static machines:

- No compile-time optimizations and validations can be made. For example, Boost.Statechart determines the innermost common context of the transition-source and destination state at compile time. Moreover, compile time checks ensure that the state machine is valid (e.g. that there are no transitions between orthogonal states).
- Double dispatch must inevitably be implemented with some kind of a table. As argued under Double dispatch, this scales badly.
- To warrant fast table lookup, states and events must be represented with an integer. To keep the table as small as possible, the numbering should be continuous, e.g. if there are ten states, it's best to use the ids 0-9. To ensure continuity of ids, all states are best defined in the same header file. The same applies to events. Again, this does not scale.
- Because events carrying parameters are not represented by a type, some sort of a generic event with a property map must be used and type-safety is enforced at runtime rather than at compile time.

It is for these reasons, that Boost.Statechart was built from ground up to not support dynamic configurability. However, this does not mean that it's impossible to dynamically shape a machine implemented with this library. For example, guards can be used to make different transitions depending on input only available at runtime. However, such layout changes will always be limited to what can be foreseen before compilation. A somewhat related library, the boost::spirit parser framework, allows for roughly the same runtime configurability.

## Error handling

There is not a single word about error handling in the UML state machine semantics specifications. Moreover, most existing FSM solutions also seem to ignore the issue.

### Why an FSM library should support error handling

Consider the following state configuration:

Both states define entry actions (x() and y()). Whenever state A becomes active, a call to x() will immediately be followed by a call to y(). y() could depend on the side-effects of x(). Therefore, executing y() does not make sense if x() fails. This is not an esoteric corner case but happens in every-day state machines all the time. For example, x() could acquire memory the contents of which is later modified by y(). There is a different but in terms of error handling equally critical situation in the Tutorial under Getting state information out of the machine when `Running::~Running()` accesses its outer state `Active`. Had the entry action of `Active` failed and had `Running` been entered anyway then `Running`'s exit action would have invoked undefined behavior. The error handling situation with outer and inner states resembles the one with base and derived classes: If a base class constructor fails (by throwing an exception) the construction is aborted, the derived class constructor is not called and the object never comes to life.
In most traditional FSM frameworks such an error situation is relatively easy to tackle as long as the error can be propagated to the state machine client. In this case a failed action simply propagates a C++ exception into the framework. The framework usually does not catch the exception so that the state machine client can handle it. Note that, after doing so, the client can no longer use the state machine object because it is either in an unknown state or the framework has already reset the state because of the exception (e.g. with a scope guard). That is, by their nature, state machines typically only offer basic exception safety.
However, error handling with traditional FSM frameworks becomes surprisingly cumbersome as soon as a lot of actions can fail and the state machine itself needs to gracefully handle these errors. Usually, a failing action (e.g. x()) then posts an appropriate error event and sets a global error variable to true. Every following action (e.g. y()) first has to check the error variable before doing anything. After all actions have completed (by doing nothing!), the previously posted error event has to be processed what leads to the execution of the remedy action. Please note that it is not sufficient to simply queue the error event as other events could still be pending. Instead, the error event has absolute priority and has to be dealt with immediately. There are slightly less cumbersome approaches to FSM error handling but these usually necessitate a change of the statechart layout and thus obscure the normal behavior. No matter what approach is used, programmers are normally forced to write a lot of code that deals with errors and most of that code is not devoted to error handling but to error propagation.

### Error handling support in Boost.Statechart

C++ exceptions may be propagated from any action to signal a failure. Depending on how the state machine is configured, such an exception is either immediately propagated to the state machine client or caught and converted into a special event that is dispatched immediately. For more information see the Exception handling chapter in the Tutorial.

### Two stage exit

An exit action can be implemented by adding a destructor to a state. Due to the nature of destructors, there are two disadvantages to this approach:

- Since C++ destructors should virtually never throw, one cannot simply propagate an exception from an exit action as one does when any of the other actions fails
- When a `state_machine<>` object is destructed then all currently active states are inevitably also destructed. That is, state machine termination is tied to the destruction of the state machine object

In my experience, neither of the above points is usually problem in practice since ...

- exit actions cannot often fail. If they can, such a failure is usually either
  - not of interest to the outside world, i.e. the failure can simply be ignored
  - so severe, that the application needs to be terminated anyway. In such a situation stack unwind is almost never desirable and the failure is better signaled through other mechanisms (e.g. abort())
- to clean up properly, often exit actions must be executed when a state machine object is destructed, even if it is destructed as a result of a stack unwind

However, several people have put forward theoretical arguments and real-world scenarios, which show that the exit action to destructor mapping can be a problem and that workarounds are overly cumbersome. That's why two stage exit is now supported.

## Asynchronous state machines

### Requirements

For asynchronous state machines different applications have rather varied requirements:

1. In some applications each state machine needs to run in its own thread, other applications are single-threaded and run all machines in the same thread
2. For some applications a FIFO scheduler is perfect, others need priority- or EDF-schedulers
3. For some applications the boost::thread library is just fine, others might want to use another threading library, yet other applications run on OS-less platforms where ISRs are the only mode of (apparently) concurrent execution

### Out of the box behavior

By default, `asynchronous_state_machine<>` subtype objects are serviced by a `fifo_scheduler<>` object. `fifo_scheduler<>` does not lock or wait in single-threaded applications and uses boost::thread primitives to do so in multi-threaded programs. Moreover, a `fifo_scheduler<>` object can service an arbitrary number of `asynchronous_state_machine<>` subtype objects. Under the hood, `fifo_scheduler<>` is just a thin wrapper around an object of its `FifoWorker` template parameter (which manages the queue and ensures thread safety) and a `processor_container<>` (which manages the lifetime of the state machines).

The UML standard mandates that an event not triggering a reaction in a state machine should be silently discarded. Since a `fifo_scheduler<>` object is itself also a state machine, events destined to no longer existing `asynchronous_state_machine<>` subtype objects are also silently discarded. This is enabled by the fact that `asynchronous_state_machine<>` subtype objects cannot be constructed or destructed directly. Instead, this must be done through `fifo_scheduler<>::create_processor<>()` and `fifo_scheduler<>::destroy_processor()` (`processor` refers to the fact that `fifo_scheduler<>` can only host `event_processor<>` subtype objects; `asynchronous_state_machine<>` is just one way to implement such a processor). Moreover, `create_processor<>()` only returns a `processor_handle` object. This must henceforth be used to initiate, queue events for, terminate and destroy the state machine through the scheduler.

### Customization

If a user needs to customize the scheduler behavior she can do so by instantiating `fifo_scheduler<>` with her own class modeling the `FifoWorker` concept. I considered a much more generic design where locking and waiting is implemented in a policy but I have so far failed to come up with a clean and simple interface for it. Especially the waiting is a bit difficult to model as some platforms have condition variables, others have events and yet others don't have any notion of waiting whatsoever (they instead loop until a new event arrives, presumably via an ISR). Given the relatively few lines of code required to implement a custom `FifoWorker` type and the fact that almost all applications will implement at most one such class, it does not seem to be worthwhile anyway. Applications requiring a less or more sophisticated event processor lifetime management can customize the behavior at a more coarse level, by using a custom `Scheduler` type. This is currently also true for applications requiring non-FIFO queuing schemes. However, Boost.Statechart will probably provide a `priority_scheduler` in the future so that custom schedulers need to be implemented only in rare cases.

## User actions: Member functions vs. function objects

All user-supplied functions (`react` member functions, entry-, exit- and transition-actions) must be class members. The reasons for this are as follows:

- The concept of state-local storage mandates that state-entry and state-exit actions are implemented as members
- `react` member functions and transition actions often access state-local data. So, it is most natural to implement these functions as members of the class the data of which the functions will operate on anyway

## Limitations

#### Junction points

UML junction points are not supported because arbitrarily complex guard expressions can easily be implemented with `custom_reaction<>`s.

#### Dynamic choice points

Currently there is no direct support for this UML element because its behavior can often be implemented with `custom_reaction<>`s. In rare cases this is not possible, namely when a choice point happens to be the initial state. Then, the behavior can easily be implemented as follows:

    struct make_choice : sc::event< make_choice > {};

    // universal choice point base class template
    template< class MostDerived, class Context >
    struct choice_point : sc::state< MostDerived, Context >
    {
      typedef sc::state< MostDerived, Context > base_type;
      typedef typename base_type::my_context my_context;
      typedef choice_point my_base;

      choice_point( my_context ctx ) : base_type( ctx )
      {
        this->post_event( boost::intrusive_ptr< make_choice >(
          new make_choice() ) );
      }
    };

    // ...

    struct MyChoicePoint;
    struct Machine : sc::state_machine< Machine, MyChoicePoint > {};

    struct Dest1 : sc::simple_state< Dest1, Machine > {};
    struct Dest2 : sc::simple_state< Dest2, Machine > {};
    struct Dest3 : sc::simple_state< Dest3, Machine > {};

    struct MyChoicePoint : choice_point< MyChoicePoint, Machine >
    {
      MyChoicePoint( my_context ctx ) : my_base( ctx ) {}

      sc::result react( const make_choice & )
      {
        if ( /* ... */ )
        {
          return transit< Dest1 >();
        }
        else if ( /* ... */ )
        {
          return transit< Dest2 >();
        }
        else
        {
          return transit< Dest3 >();
        }
      }
    };

`choice_point<>` is not currently part of Boost.Statechart, mainly because I fear that beginners could use it in places where they would be better off with `custom_reaction<>`. If the demand is high enough I will add it to the library.

#### Deep history of orthogonal regions

Deep history of states with orthogonal regions is currently not supported:

Attempts to implement this statechart will lead to a compile-time error because B has orthogonal regions and its direct or indirect outer state contains a deep history pseudo state. In other words, a state containing a deep history pseudo state must not have any direct or indirect inner states which themselves have orthogonal regions. This limitation stems from the fact that full deep history support would be more complicated to implement and would consume more resources than the currently implemented limited deep history support. Moreover, full deep history behavior can easily be implemented with shallow history:

Of course, this only works if C, D, E or any of their direct or indirect inner states do not have orthogonal regions. If not so then this pattern has to be applied recursively.

#### Synchronization (join and fork) bars

Synchronization bars are not supported, that is, a transition always originates at exactly one state and always ends at exactly one state. Join bars are sometimes useful but their behavior can easily be emulated with guards. The support of fork bars would make the implementation much more complex and they are only needed rarely.

#### Event dispatch to orthogonal regions

The Boost.Statechart event dispatch algorithm is different to the one specified in David Harel's original paper and in the UML standard. Both mandate that each event is dispatched to all orthogonal regions of a state machine. Example:

Here the Harel/UML dispatch algorithm specifies that the machine must transition from (B,D) to (C,E) when an EvX event is processed. Because of the subtleties that Harel describes in chapter 7 of his paper, an implementation of this algorithm is not only quite complex but also much slower than the simplified version employed by Boost.Statechart, which stops searching for reactions as soon as it has found one suitable for the current event. That is, had the example been implemented with this library, the machine would have transitioned non-deterministically from (B,D) to either (C,D) or (B,E). This version was chosen because, in my experience, in real-world machines different orthogonal regions often do not specify transitions for the same events. For the rare cases when they do, the UML behavior can easily be emulated as follows:

#### Transitions across orthogonal regions

Transitions across orthogonal regions are currently flagged with an error at compile time (the UML specifications explicitly allow them while Harel does not mention them at all). I decided to not support them because I have erroneously tried to implement such a transition several times but have never come across a situation where it would make any sense. If you need to make such transitions, please do let me know!

Revised 03 December, 2006

Copyright © 2003-2006 Andreas Huber Dönni

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)