1<html><head> 2 <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 3 <title>Concepts</title><link rel="stylesheet" href="boostbook.css" type="text/css"><meta name="generator" content="DocBook XSL-NS Stylesheets V1.75.2"><link rel="home" href="index.html" title="Meta State Machine (MSM)"><link rel="up" href="ch02.html" title="Chapter 2. UML Short Guide"><link rel="prev" href="ch02.html" title="Chapter 2. UML Short Guide"><link rel="next" href="ch02s03.html" title="Added concepts"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Concepts</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch02.html">Prev</a> </td><th width="60%" align="center">Chapter 2. UML Short Guide</th><td width="20%" align="right"> <a accesskey="n" href="ch02s03.html">Next</a></td></tr></table><hr></div><div class="sect1" title="Concepts"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="d0e116"></a>Concepts</h2></div></div></div><p>Thinking in terms of state machines is a bit surprising at first, so let us 4 have a quick glance at the concepts.</p><div class="sect2" title="State machine, state, transition, event"><div class="titlepage"><div><div><h3 class="title"><a name="d0e121"></a>State machine, state, transition, event </h3></div></div></div><p>A state machine is a concrete model describing the behavior of a system. 5 It is composed of a finite number of states and transitions.</p><p> 6 <span class="inlinemediaobject"><img src="../images/sm.gif"></span></p><p>A simple state has no sub states. It can have data, entry and exit 7 behaviors and deferred events. One can provide entry and exit behaviors 8 (also called actions) to states (or state machines), which are executed 9 whenever a state is entered or left, no matter how. A state can also have 10 internal transitions which cause no entry or exit behavior to be called. A 11 state can mark events as deferred. This means the event cannot be processed 12 if this state is active, but it must be retained. Next time a state not 13 deferring this event is active, the event will be processed, as if it had 14 just been fired. </p><p><span class="inlinemediaobject"><img src="../images/state.gif"></span></p><p>A transition is the switching between active states, triggered by an 15 event. Actions and guard conditions can be attached to the transition. The 16 action executes when the transition fires, the guard is a Boolean operation 17 executed first and which can prevent the transition from firing by returning 18 false.</p><p> 19 <span class="inlinemediaobject"><img src="../images/transition.jpg"></span></p><p>An initial state marks the first active state of a state machine. It has 20 no real existence and neither has the transition originating from it.</p><p> 21 <span class="inlinemediaobject"><img src="../images/init_state.gif"></span></p></div><div class="sect2" title="Submachines, orthogonal regions, pseudostates"><div class="titlepage"><div><div><h3 class="title"><a name="d0e151"></a>Submachines, orthogonal regions, pseudostates </h3></div></div></div><p>A composite state is a state containing a region or decomposed in two or 22 more regions. A composite state contains its own set of states and regions. </p><p>A submachine is a state machine inserted as a state in another state 23 machine. The same submachine can be inserted more than once. </p><p>Orthogonal regions are parts of a composite state or submachine, each 24 having its own set of mutually exclusive set of states and transitions. </p><p><span class="inlinemediaobject"><img src="../images/regions.gif" width="60%"></span></p><p>UML also defines a number of pseudo states, which are considered important 25 concepts to model, but not enough to make them first-class citizens. The 26 terminate pseudo state terminates the execution of a state machine (MSM 27 handles this slightly differently. The state machine is not destroyed but no 28 further event processing occurs.). </p><p><span class="inlinemediaobject"><img src="../images/terminate.gif"></span></p><p>An exit point pseudo state exits a composite state or a submachine and 29 forces termination of execution in all contained regions.</p><p><span class="inlinemediaobject"><img src="../images/exit.gif" width="60%"></span></p><p>An entry point pseudo state allows a kind of controlled entry inside a 30 composite. Precisely, it connects a transition outside the composite to a 31 transition inside the composite. An important point is that this mechanism 32 only allows a single region to be entered. In the above diagram, in region1, 33 the initial state would become active. </p><p><span class="inlinemediaobject"><img src="../images/entry_point.gif"></span></p><p>There are also two more ways to enter a submachine (apart the obvious and 34 more common case of a transition terminating on the submachine as shown in 35 the region case). An explicit entry means that an inside state is the target 36 of a transition. Unlike with direct entry, no tentative encapsulation is 37 made, and only one transition is executed. An explicit exit is a transition 38 from an inner state to a state outside the submachine (not supported by 39 MSM). I would not recommend using explicit entry or exit. </p><p><span class="inlinemediaobject"><img src="../images/explicit.gif"></span></p><p>The last entry possibility is using fork. A fork is an explicit entry into 40 one or more regions. Other regions are again activated using their initial 41 state. </p><p><span class="inlinemediaobject"><img src="../images/fork.gif" width="70%"></span></p></div><div class="sect2" title="History"><div class="titlepage"><div><div><h3 class="title"><a name="d0e194"></a> 42 <span class="command"><strong><a name="uml-history"></a></strong></span>History </h3></div></div></div><p>UML defines two kinds of history, shallow history and deep history. 43 Shallow history is a pseudo state representing the most recent substate of a 44 submachine. A submachine can have at most one shallow history. A transition 45 with a history pseudo state as target is equivalent to a transition with the 46 most recent substate as target. And very importantly, only one transition 47 may originate from the history. Deep history is a shallow history 48 recursively reactivating the substates of the most recent substate. It is 49 represented like the shallow history with a star (H* inside a 50 circle).</p><p> 51 <span class="inlinemediaobject"><img src="../images/history.gif" width="60%"></span></p><p>History is not a completely satisfying concept. First of all, there can be 52 just one history pseudo state and only one transition may originate from it. 53 So they do not mix well with orthogonal regions as only one region can be 54 “remembered”. Deep history is even worse and looks like a last-minute 55 addition. History has to be activated by a transition and only one 56 transition originates from it, so how to model the transition originating 57 from the deep history pseudo state and pointing to the most recent substate 58 of the substate? As a bonus, it is also inflexible and does not accept new 59 types of histories. Let's face it, history sounds great and is useful in 60 theory, but the UML version is not quite making the cut. And therefore, MSM 61 provides a different version of this useful concept. </p></div><div class="sect2" title="Completion transitions / anonymous transitions"><div class="titlepage"><div><div><h3 class="title"><a name="d0e208"></a><span class="command"><strong><a name="uml-anonymous"></a></strong></span>Completion transitions / anonymous 62 transitions</h3></div></div></div><p>Completion events (or transitions), also called anonymous transitions, are 63 defined as transitions having no defined event triggering them. This means 64 that such transitions will immediately fire when a state being the source of 65 an anonymous transition becomes active, provided that a guard allows it. 66 They are useful in modeling algorithms as an activity diagram would normally 67 do. In the real-time world, they have the advantage of making it easier to 68 estimate how long a periodically executed action will last. For example, 69 consider the following diagram. </p><p><span class="inlinemediaobject"><img src="../images/completion.gif"></span></p><p>The designer now knows at any time that he will need a maximum of 4 70 transitions. Being able to estimate how long a transition takes, he can 71 estimate how much of a time frame he will need to require (real-time tasks 72 are often executed at regular intervals). If he can also estimate the 73 duration of actions, he can even use graph algorithms to better estimate his 74 timing requirements. </p></div><div class="sect2" title="Internal transitions"><div class="titlepage"><div><div><h3 class="title"><a name="d0e220"></a><span class="command"><strong><a name="UML-internal-transition"></a></strong></span> Internal transitions </h3></div></div></div><p>Internal transitions are transitions executing in the scope of the active 75 state, being a simple state or a submachine. One can see them as a 76 self-transition of this state, without an entry or exit action 77 called.</p></div><div class="sect2" title="Conflicting transitions"><div class="titlepage"><div><div><h3 class="title"><a name="d0e226"></a> 78 <span class="command"><strong><a name="transition-conflict"></a></strong></span>Conflicting transitions </h3></div></div></div><p>If, for a given event, several transitions are enabled, they are said to 79 be in conflict. There are two kinds of conflicts: </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>For a given source state, several transitions are defined, 80 triggered by the same event. Normally, the guard condition in 81 each transition defines which one is fired.</p></li><li class="listitem"><p>The source state is a submachine or simple state and the 82 conflict is between a transition internal to this state and a 83 transition triggered by the same event and having as target 84 another state.</p></li></ul></div><p>The first one is simple; one only needs to define two or more 85 rows in the transition table, with the same source and trigger, with a 86 different guard condition. Beware, however, that the UML standard wants 87 these conditions to be not overlapping. If they do, the standard says 88 nothing except that this is incorrect, so the implementer is free to 89 implement it the way he sees fit. In the case of MSM, the transition 90 appearing last in the transition table gets selected first, if it returns 91 false (meaning disabled), the library tries with the previous one, and so 92 on.</p><p> 93 <span class="inlinemediaobject"><img src="../images/conflict1.gif"></span></p><p>In the second case, UML defines that the most inner transition gets 94 selected first, which makes sense, otherwise no exit point pseudo state 95 would be possible (the inner transition brings us to the exit point, from 96 where the containing state machine can take over). </p><p><span class="inlinemediaobject"><img src="../images/conflict2.gif" width="60%"></span></p><p>MSM handles both cases itself, so the designer needs only concentrate on 97 its state machine and the UML subtleties (not overlapping conditions), not 98 on implementing this behavior himself. </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch02.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch02.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch02s03.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 2. 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