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<title>The Boost Statechart Library - Rationale</title>
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<h3><a href="../../../index.htm"><img alt="C++ Boost" src=
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<h1 align="center">The Boost Statechart Library</h1>
<h2 align="center">Rationale</h2>
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<hr>
<dl class="index">
<dt><a href="#Introduction">Introduction</a></dt>
<dt><a href="#WhyYetAnotherStateMachineFramework">Why yet another state
machine framework</a></dt>
<dt><a href="#StateLocalStorage">State-local storage</a></dt>
<dt><a href="#DynamicConfigurability">Dynamic configurability</a></dt>
<dt><a href="#ErrorHandling">Error handling</a></dt>
<dt><a href="#AsynchronousStateMachines">Asynchronous state
machines</a></dt>
<dt><a href="#MemberFunctionsVsFunctionObjects">User actions: Member
functions vs. function objects</a></dt>
<dt><a href="#Limitations">Limitations</a></dt>
</dl>
<h2><a name="Introduction" id="Introduction">Introduction</a></h2>
<p>Most of the design decisions made during the development of this library
are the result of the following requirements.</p>
<p>Boost.Statechart should ...</p>
<ol>
<li>be fully type-safe. Whenever possible, type mismatches should be
flagged with an error at compile-time</li>
<li>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</li>
<li>allow for easy transformation of a UML statechart (defined in
<a href="http://www.omg.org/cgi-bin/doc?formal/03-03-01">http://www.omg.org/cgi-bin/doc?formal/03-03-01</a>)
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:
<ul>
<li>Hierarchical (composite, nested) states</li>
<li>Orthogonal (concurrent) states</li>
<li>Entry-, exit- and transition-actions</li>
<li>Guards</li>
<li>Shallow/deep history</li>
</ul>
</li>
<li>produce a customizable reaction when a C++ exception is propagated
from user code</li>
<li>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</li>
<li>support the development of arbitrarily large and complex state
machines. Multiple developers should be able to work on the same state
machine simultaneously</li>
<li>allow the user to customize all resource management so that the
library could be used for applications with hard real-time
requirements</li>
<li>enforce as much as possible at compile time. Specifically, invalid
state machines should not compile</li>
<li>offer reasonable performance for a wide range of applications</li>
</ol>
<h2><a name="WhyYetAnotherStateMachineFramework" id=
"WhyYetAnotherStateMachineFramework">Why yet another state machine
framework?</a></h2>
<p>Before I started to develop this library I had a look at the following
frameworks:</p>
<ul>
<li>The framework accompanying the book "Practical Statecharts in C/C++"
by Miro Samek, CMP Books, ISBN: 1-57820-110-1<br>
<a href=
"http://www.quantum-leaps.com">http://www.quantum-leaps.com<br></a> Fails
to satisfy at least the requirements 1, 3, 4, 6, 8.</li>
<li>The framework accompanying "Rhapsody in C++" by ILogix (a code
generator solution)<br>
<a href=
"http://www.ilogix.com/sublevel.aspx?id=53">http://www.ilogix.com/sublevel.aspx?id=53<br>
</a> 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).</li>
<li>The framework accompanying the article "State Machine Design in
C++"<br>
<a href=
"http://www.ddj.com/184401236?pgno=1">http://www.ddj.com/184401236?pgno=1<br>
</a> Fails to satisfy at least the requirements 1, 3, 4, 5 (there is no
direct threading support), 6, 8.</li>
</ul>
<p>I believe Boost.Statechart satisfies all requirements.</p>
<h2><a name="StateLocalStorage" id="StateLocalStorage">State-local
storage</a></h2>
<p>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.&nbsp;</p>
<p>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.</p>
<p>The FAQ item "<a href="faq.html#StateLocalStorage">What's so cool about
state-local storage?</a>" further explains this by comparing the tutorial
StopWatch to a behaviorally equivalent version that does not use
state-local storage.</p>
<h2><a name="DynamicConfigurability" id="DynamicConfigurability">Dynamic
configurability</a></h2>
<h3>Two types of state machine frameworks</h3>
<ul>
<li>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).</li>
<li>On the other side are state machine frameworks which require the
layout to be specified at compile time</li>
</ul>
<p>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 <b>computed
by an algorithm</b>. 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.</p>
<p>However, fully-featured dynamic FSM libraries do exist. So, the question
is:</p>
<h3>Why not use a dynamically configurable FSM library for all state
machines?</h3>
<p>One might argue that a dynamically configurable FSM framework is all one
ever needs because <b>any</b> 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:</p>
<ul>
<li>No compile-time optimizations and validations can be made. For
example, Boost.Statechart determines the <a href=
"definitions.html#InnermostCommonContext">innermost common context</a> 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).</li>
<li>Double dispatch must inevitably be implemented with some kind of a
table. As argued under <a href="performance.html#DoubleDispatch">Double
dispatch</a>, this scales badly.</li>
<li>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.</li>
<li>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.</li>
</ul>
<p>It is for these reasons, that Boost.Statechart was built from ground up
to <b>not</b> 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.</p>
<h2><a name="ErrorHandling" id="ErrorHandling">Error handling</a></h2>
<p>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.&nbsp;</p>
<h3>Why an FSM library should support error handling</h3>
<p>Consider the following state configuration:</p>
<p><img alt="A" src="A.gif" border="0" width="230" height="170"></p>
<p>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 <a href=
"tutorial.html#GettingStateInformationOutOfTheMachine">Getting state
information out of the machine</a> when <code>Running::~Running()</code>
accesses its outer state <code>Active</code>. Had the entry action of
<code>Active</code> failed and had <code>Running</code> been entered anyway
then <code>Running</code>'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.<br>
In most traditional FSM frameworks such an error situation is relatively
easy to tackle <b>as long as the error can be propagated to the state
machine client</b>. 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.<br>
However, error handling with traditional FSM frameworks becomes
surprisingly cumbersome as soon as a lot of actions can fail and the state
machine <b>itself</b> 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 <b>not</b> devoted to error
handling but to error propagation.</p>
<h3>Error handling support in Boost.Statechart</h3>
<p>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 <a href="tutorial.html#ExceptionHandling">Exception
handling</a> chapter in the Tutorial.</p>
<h3>Two stage exit</h3>
<p>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:</p>
<ul>
<li>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</li>
<li>When a <code>state_machine&lt;&gt;</code> 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</li>
</ul>
<p>In my experience, neither of the above points is usually problem in
practice since ...</p>
<ul>
<li>exit actions cannot often fail. If they can, such a failure is
usually either
<ul>
<li>not of interest to the outside world, i.e. the failure can simply
be ignored</li>
<li>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())</li>
</ul>
</li>
<li>to clean up properly, often exit actions <b>must</b> be executed when
a state machine object is destructed, even if it is destructed as a
result of a stack unwind</li>
</ul>
<p>However, several people have put forward theoretical arguments and
real-world scenarios, which show that the exit action to destructor mapping
<b>can</b> be a problem and that workarounds are overly cumbersome. That's
why <a href="tutorial.html#TwoStageExit">two stage exit</a> is now
supported.</p>
<h2><a name="AsynchronousStateMachines" id=
"AsynchronousStateMachines">Asynchronous state machines</a></h2>
<h3>Requirements</h3>
<p>For asynchronous state machines different applications have rather
varied requirements:</p>
<ol>
<li>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</li>
<li>For some applications a FIFO scheduler is perfect, others need
priority- or EDF-schedulers</li>
<li>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</li>
</ol>
<h3>Out of the box behavior</h3>
<p>By default, <code>asynchronous_state_machine&lt;&gt;</code> subtype
objects are serviced by a <code>fifo_scheduler&lt;&gt;</code> object.
<code>fifo_scheduler&lt;&gt;</code> does not lock or wait in
single-threaded applications and uses boost::thread primitives to do so in
multi-threaded programs. Moreover, a <code>fifo_scheduler&lt;&gt;</code>
object can service an arbitrary number of
<code>asynchronous_state_machine&lt;&gt;</code> subtype objects. Under the
hood, <code>fifo_scheduler&lt;&gt;</code> is just a thin wrapper around an
object of its <code>FifoWorker</code> template parameter (which manages the
queue and ensures thread safety) and a
<code>processor_container&lt;&gt;</code> (which manages the lifetime of the
state machines).</p>
<p>The UML standard mandates that an event not triggering a reaction in a
state machine should be silently discarded. Since a
<code>fifo_scheduler&lt;&gt;</code> object is itself also a state machine,
events destined to no longer existing
<code>asynchronous_state_machine&lt;&gt;</code> subtype objects are also
silently discarded. This is enabled by the fact that
<code>asynchronous_state_machine&lt;&gt;</code> subtype objects cannot be
constructed or destructed directly. Instead, this must be done through
<code>fifo_scheduler&lt;&gt;::create_processor&lt;&gt;()</code> and
<code>fifo_scheduler&lt;&gt;::destroy_processor()</code>
(<code>processor</code> refers to the fact that
<code>fifo_scheduler&lt;&gt;</code> can only host
<code>event_processor&lt;&gt;</code> subtype objects;
<code>asynchronous_state_machine&lt;&gt;</code> is just one way to
implement such a processor). Moreover,
<code>create_processor&lt;&gt;()</code> only returns a
<code>processor_handle</code> object. This must henceforth be used to
initiate, queue events for, terminate and destroy the state machine through
the scheduler.</p>
<h3>Customization</h3>
<p>If a user needs to customize the scheduler behavior she can do so by
instantiating <code>fifo_scheduler&lt;&gt;</code> with her own class
modeling the <code>FifoWorker</code> 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 <code>FifoWorker</code> 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 <code>Scheduler</code> type. This is
currently also true for applications requiring non-FIFO queuing schemes.
However, Boost.Statechart will probably provide a
<code>priority_scheduler</code> in the future so that custom schedulers
need to be implemented only in rare cases.</p>
<h2><a name="MemberFunctionsVsFunctionObjects" id=
"MemberFunctionsVsFunctionObjects">User actions: Member functions vs.
function objects</a></h2>
<p>All user-supplied functions (<code>react</code> member functions,
entry-, exit- and transition-actions) must be class members. The reasons
for this are as follows:</p>
<ul>
<li>The concept of state-local storage mandates that state-entry and
state-exit actions are implemented as members</li>
<li><code>react</code> 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</li>
</ul>
<h2><a name="Limitations" id="Limitations">Limitations</a></h2>
<h4>Junction points</h4>
<p>UML junction points are not supported because arbitrarily complex guard
expressions can easily be implemented with
<code>custom_reaction&lt;&gt;</code>s.</p>
<h4>Dynamic choice points</h4>
<p>Currently there is no direct support for this UML element because its
behavior can often be implemented with
<code>custom_reaction&lt;&gt;</code>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:</p>
<pre>
struct make_choice : sc::event&lt; make_choice &gt; {};
// universal choice point base class template
template&lt; class MostDerived, class Context &gt;
struct choice_point : sc::state&lt; MostDerived, Context &gt;
{
typedef sc::state&lt; MostDerived, Context &gt; base_type;
typedef typename base_type::my_context my_context;
typedef choice_point my_base;
choice_point( my_context ctx ) : base_type( ctx )
{
this-&gt;post_event( boost::intrusive_ptr&lt; make_choice &gt;(
new make_choice() ) );
}
};
// ...
struct MyChoicePoint;
struct Machine : sc::state_machine&lt; Machine, MyChoicePoint &gt; {};
struct Dest1 : sc::simple_state&lt; Dest1, Machine &gt; {};
struct Dest2 : sc::simple_state&lt; Dest2, Machine &gt; {};
struct Dest3 : sc::simple_state&lt; Dest3, Machine &gt; {};
struct MyChoicePoint : choice_point&lt; MyChoicePoint, Machine &gt;
{
MyChoicePoint( my_context ctx ) : my_base( ctx ) {}
sc::result react( const make_choice &amp; )
{
if ( /* ... */ )
{
return transit&lt; Dest1 &gt;();
}
else if ( /* ... */ )
{
return transit&lt; Dest2 &gt;();
}
else
{
return transit&lt; Dest3 &gt;();
}
}
};
</pre>
<p><code>choice_point&lt;&gt;</code> 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
<code>custom_reaction&lt;&gt;</code>. If the demand is high enough I will
add it to the library.</p>
<h4>Deep history of orthogonal regions</h4>
<p>Deep history of states with orthogonal regions is currently not
supported:</p>
<p><img alt="DeepHistoryLimitation1" src="DeepHistoryLimitation1.gif"
border="0" width="331" height="346"></p>
<p>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:</p>
<p><img alt="DeepHistoryLimitation2" src="DeepHistoryLimitation2.gif"
border="0" width="332" height="347"></p>
<p>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.</p>
<h4>Synchronization (join and fork) bars</h4>
<p><img alt="JoinAndFork" src="JoinAndFork.gif" border="0" width="541"
height="301"></p>
<p>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 <b>much</b>
more complex and they are only needed rarely.</p>
<h4>Event dispatch to orthogonal regions</h4>
<p>The Boost.Statechart event dispatch algorithm is different to the one
specified in <a href=
"http://www.wisdom.weizmann.ac.il/~dharel/SCANNED.PAPERS/Statecharts.pdf">David
Harel's original paper</a> and in the <a href=
"http://www.omg.org/cgi-bin/doc?formal/03-03-01">UML standard</a>. Both
mandate that each event is dispatched to all orthogonal regions of a state
machine. Example:</p>
<p><img alt="EventDispatch" src="EventDispatch.gif" border="0" width="436"
height="211"></p>
<p>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 <a href=
"http://www.wisdom.weizmann.ac.il/~dharel/SCANNED.PAPERS/Statecharts.pdf">his
paper</a>, 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 <a href=
"definitions.html#Reaction">reactions</a> 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:</p>
<p><img alt="SimpleEventDispatch" src="SimpleEventDispatch.gif" border="0"
width="466" height="226"></p>
<h4>Transitions across orthogonal regions</h4>
<p><img alt="TransAcrossOrthRegions" src="TransAcrossOrthRegions.gif"
border="0" width="226" height="271"></p>
<p>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!</p>
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<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->03 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38512" --></p>
<p><i>Copyright &copy; 2003-<!--webbot bot="Timestamp" s-type="EDITED" s-format="%Y" startspan -->2006<!--webbot bot="Timestamp" endspan i-checksum="770" -->
<a href="contact.html">Andreas Huber D&ouml;nni</a></i></p>
<p><i>Distributed under the Boost Software License, Version 1.0. (See
accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
copy at <a href=
"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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