ceph/src/boost/libs/statechart/doc/definitions.html

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  11   <title>The Boost Statechart Library - Definitions</title>
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  24         <h1 align="center">The Boost Statechart Library</h1>
  25
  26         <h2 align="center">Definitions</h2>
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  31
  32   <h2>Introduction</h2>
  33
  34   <p>The Boost.Statechart documentation uses a lot of terminology specific to
  35   state machines. Most of it is equal to the one used in the UML
  36   specifications. This document contains only definitions for terminology not
  37   used by the <a href="http://www.omg.org/cgi-bin/doc?formal/03-03-01">UML
  38   standard</a>. A short tour around UML terminology can be found <a href=
  39   "http://www.sts.tu-harburg.de/teaching/ws-99.00/OOA+D/StateDiagrams.pdf">here</a>.</p>
  40
  41   <h2>Definitions</h2>
  42
  43   <dl class="page-index">
  44     <dt><a href="#Context">Context</a></dt>
  45
  46     <dt><a href="#InnermostCommonContext">Innermost common context</a></dt>
  47
  48     <dt><a href="#InnermostState">Innermost state</a></dt>
  49
  50     <dt><a href="#InStateReaction">In-state reaction</a></dt>
  51
  52     <dt><a href="#OutermostState">Outermost state</a></dt>
  53
  54     <dt><a href="#PolymorphicEvents">Polymorphic events</a></dt>
  55
  56     <dt><a href="#Reaction">Reaction</a></dt>
  57
  58     <dt><a href="#UnstableState">Unstable state</a></dt>
  59
  60     <dt><a href="#UnstableStateMachine">Unstable state machine</a></dt>
  61   </dl>
  62
  63   <h3><a name="Context" id="Context">Context</a></h3>
  64
  65   <p>The contexts of a state define its location in the state hierarchy. A
  66   state's <b>direct</b> context is defined by what is passed as the
  67   <code>Context</code> template parameter of the <code><a href=
  68   "reference.html#ClassTemplatesimple_state">simple_state</a></code> and
  69   <code><a href="reference.html#ClassTemplatestate">state</a></code> class
  70   templates. This can either be the state machine (which makes the state an
  71   <a href="#OutermostState">outermost state</a>) or its direct outer state. A
  72   state's <b>indirect</b> contexts follow from the direct context of its
  73   direct context and the direct context of the direct context of its direct
  74   context and so on. Examples:</p>
  75
  76   <p><img alt="OutermostUnstableState" src="OutermostUnstableState.gif"
  77   border="0" width="467" height="572"></p>
  78
  79   <ul>
  80     <li>A's <b>direct</b> context is the state machine (not visible in this
  81     picture). A does not have any indirect contexts</li>
  82
  83     <li>B's <b>direct</b> context is A. B's <b>indirect</b> context is the
  84     state machine (not visible in this picture)</li>
  85
  86     <li>C's <b>direct</b> context is B. C's <b>indirect</b> contexts are B, A
  87     and the state machine (not visible in this picture)</li>
  88
  89     <li>D's <b>direct</b> context is A. D's <b>indirect</b> context is the
  90     state machine (not visible in this picture)</li>
  91   </ul>
  92
  93   <h3><a name="InnermostCommonContext" id="InnermostCommonContext">Innermost
  94   common context</a></h3>
  95
  96   <p>The innermost common context of two states is the first direct or
  97   indirect context that both states have in common. Also known as Least
  98   Common Ancestor (UML).</p>
  99
 100   <h3><a name="InnermostState" id="InnermostState">Innermost state</a></h3>
 101
 102   <p>An innermost state is a state that does not itself have inner states.
 103   Also known as leaf state or simple state (UML). Note that <code><a href=
 104   "reference.html#ClassTemplatesimple_state">boost::statechart::simple_state&lt;&gt;</a></code>
 105   is <b>not</b> a model of the UML simple state.</p>
 106
 107   <h3><a name="InStateReaction" id="InStateReaction">In-state
 108   reaction</a></h3>
 109
 110   <p>An in-state reaction is a <a href="#Reaction">reaction</a> that neither
 111   exits nor enters any states. Also known as inner transition or internal
 112   transition (UML).</p>
 113
 114   <h3><a name="OutermostState" id="OutermostState">Outermost state</a></h3>
 115
 116   <p>An outermost state is a state that does not itself have outer states.
 117   Note that an outermost state is different from the UML top state. A state
 118   machine can have an arbitrary number of the former but only exactly one of
 119   the latter. Boost.Statechart only supports outermost states.</p>
 120
 121   <h3><a name="PolymorphicEvents" id="PolymorphicEvents">Polymorphic
 122   events</a></h3>
 123
 124   <p>An FSM library supports polymorphic events if events can inherit from
 125   each other without restrictions <b>and</b> if it allows the definition of
 126   reactions for leafs and nodes of the resulting event inheritance
 127   tree.</p>
 128
 129   <p>Example (using a hypothetical FSM library, as Boost.Statechart does not
 130   support polymorphic events):</p>
 131   <pre>
 132 struct EvButtonPressed : Event // node
 133 {
 134   /* common button pressed properties */
 135 };
 136
 137 struct EvPlayButtonPressed : EvButtonPressed {}; // leaf
 138 struct EvStopButtonPressed : EvButtonPressed {}; // leaf
 139 struct EvForwardButtonPressed : EvButtonPressed {}; // leaf
 140 </pre>
 141
 142   <p>If a state machine needs to react whenever <b>any</b> button (including
 143   the ones that may be added in the future) is pressed, a reaction for
 144   <code>EvButtonPressed</code> can be defined.</p>
 145
 146   <h3><a name="Reaction" id="Reaction">Reaction</a></h3>
 147
 148   <p>A reaction consists of all the side effects caused by the processing of
 149   one event. Reactions can be categorized as follows:</p>
 150
 151   <ol>
 152     <li>In-state reaction</li>
 153
 154     <li>Event deferral</li>
 155
 156     <li>Transition</li>
 157
 158     <li>Termination, also known as transition to the final state (UML)</li>
 159   </ol>
 160
 161   <p>Note that it is possible to mix a reaction of type 1 with one of the
 162   other types (the in-state reaction is always executed first) but it is not
 163   possible to mix a reaction of type 2-4 with anything else but type 1.</p>
 164
 165   <p>A reaction is always associated with exactly one state type and exactly
 166   one event type.</p>
 167
 168   <h3><a name="UnstableState" id="UnstableState">Unstable state</a></h3>
 169
 170   <p>A state is unstable from the moment when it has been entered until after
 171   its last <b>direct</b> inner state has been entered. A state is also
 172   unstable from the moment just before its first <b>direct</b> inner state is
 173   exited until right before the state itself is exited.</p>
 174
 175   <h3><a name="UnstableStateMachine" id="UnstableStateMachine">Unstable state
 176   machine</a></h3>
 177
 178   <p>A state machine is unstable if at least one of its currently active
 179   states is unstable. This is the case during the following three
 180   operations:</p>
 181
 182   <ul>
 183     <li>Initiation: From the moment after the first state has been entered
 184     until after the last state of the initial state configuration has been
 185     entered</li>
 186
 187     <li>Transition: From the moment just before the first state of the
 188     current state configuration is exited until after the last state of the
 189     destination state configuration has been entered</li>
 190
 191     <li>Termination: From the moment just before the first state is exited
 192     until right before the last terminated state is exited. A successfully
 193     executed termination (no exception was thrown) never leaves any states
 194     unstable. For example, consider the active state A with two orthogonal
 195     regions in which the inner states B and C are each active. Terminating
 196     either B or C does not make A unstable. Neither does terminating both, as
 197     that inevitably also terminates A</li>
 198   </ul>
 199
 200   <p>Under normal circumstances a state machine has Run-To-Completion
 201   semantics, that is, it is always stable before the machine returns to the
 202   client or before the next event is dequeued. So, a state machine is usually
 203   only unstable when it is busy processing an event and becomes stable again
 204   right before it has finished processing the event. However, this can not be
 205   guaranteed when entry, exit or transition actions fail. Such a failure is
 206   reported by an event, which must be processed while the state machine is
 207   unstable. However, exception event processing rules ensure that a state
 208   machine is never unstable when it returns to the client (see <code><a href=
 209   "reference.html#process_event">state_machine&lt;&gt;::process_event()</a></code>
 210   for details).</p>
 211   <hr>
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 217   <p>Revised
 218   <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->29 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38526" --></p>
 219
 220   <p><i>Copyright &copy; 2003-<!--webbot bot="Timestamp" s-type="EDITED" s-format="%Y" startspan -->2006<!--webbot bot="Timestamp" endspan i-checksum="770" -->
 221   <a href="contact.html">Andreas Huber D&ouml;nni</a></i></p>
 222
 223   <p><i>Distributed under the Boost Software License, Version 1.0. (See
 224   accompanying file <a href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
 225   copy at <a href=
 226   "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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