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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>1 Concepts</title><link rel="stylesheet" type="text/css" href="scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="footnote.css" title="default"/><link rel="stylesheet" type="text/css" href="racket.css" title="default"/><link rel="stylesheet" type="text/css" href="manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="manual-racket.css" title="default"/><script type="text/javascript" src="scribble-common.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="scribble-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">▼</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">Marketplace:<span class="mywbr"> </span> Network-<wbr></wbr>Aware Programming</a></td></tr></table></div><div class="tocviewsublisttop" style="display: block;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="Concepts.html" class="tocviewselflink" data-pltdoc="x">Concepts</a></td></tr><tr><td align="right">2 </td><td><a href="high-level-interface.html" class="tocviewlink" data-pltdoc="x">High-<wbr></wbr>level interface</a></td></tr><tr><td align="right">3 </td><td><a href="low-level-interface.html" class="tocviewlink" data-pltdoc="x">Low-<wbr></wbr>level interface</a></td></tr><tr><td align="right">4 </td><td><a href="Drivers.html" class="tocviewlink" data-pltdoc="x">Drivers</a></td></tr><tr><td align="right">5 </td><td><a href="Management_and_Monitoring.html" class="tocviewlink" data-pltdoc="x">Management and Monitoring</a></td></tr><tr><td align="right">6 </td><td><a href="Examples.html" class="tocviewlink" data-pltdoc="x">Examples</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">►</a></td><td>1 </td><td><a href="Concepts.html" class="tocviewselflink" data-pltdoc="x">Concepts</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">1.1 </td><td><a href="Concepts.html#%28part._.What_is_a_process__what_are_event_handlers_%29" class="tocviewlink" data-pltdoc="x">What is a process, what are event handlers?</a></td></tr><tr><td align="right">1.2 </td><td><a href="Concepts.html#%28part._.What_is_a_.V.M_%29" class="tocviewlink" data-pltdoc="x">What is a VM?</a></td></tr><tr><td align="right">1.3 </td><td><a href="Concepts.html#%28part._.Endpoints__.Subscription_and_.Advertisement%29" class="tocviewlink" data-pltdoc="x">Endpoints:<span class="mywbr"> </span> Subscription and Advertisement</a></td></tr><tr><td align="right">1.4 </td><td><a href="Concepts.html#%28part._messages-and-topics%29" class="tocviewlink" data-pltdoc="x">Messages and Topics</a></td></tr><tr><td align="right">1.5 </td><td><a href="Concepts.html#%28part._.Presence%29" class="tocviewlink" data-pltdoc="x">Presence</a></td></tr><tr><td align="right">1.6 </td><td><a href="Concepts.html#%28part._.Nesting__relaying__and_levels_of_discourse%29" class="tocviewlink" data-pltdoc="x">Nesting, relaying, and levels of discourse</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><span class="tocsublinknumber">1.1<tt> </tt></span><a href="#%28part._.What_is_a_process__what_are_event_handlers_%29" class="tocsubseclink" data-pltdoc="x">What is a process, what are event handlers?</a></td></tr><tr><td><span class="tocsublinknumber">1.2<tt> </tt></span><a href="#%28part._.What_is_a_.V.M_%29" class="tocsubseclink" data-pltdoc="x">What is a VM?</a></td></tr><tr><td><span class="tocsublinknumber">1.3<tt> </tt></span><a href="#%28part._.Endpoints__.Subscription_and_.Advertisement%29" class="tocsubseclink" data-pltdoc="x">Endpoints:<span class="mywbr"> </span> Subscription and Advertisement</a></td></tr><tr><td><span class="tocsublinknumber">1.4<tt> </tt></span><a href="#%28part._messages-and-topics%29" class="tocsubseclink" data-pltdoc="x">Messages and Topics</a></td></tr><tr><td><span class="tocsublinknumber">1.5<tt> </tt></span><a href="#%28part._.Presence%29" class="tocsubseclink" data-pltdoc="x">Presence</a></td></tr><tr><td><span class="tocsublinknumber">1.6<tt> </tt></span><a href="#%28part._.Nesting__relaying__and_levels_of_discourse%29" class="tocsubseclink" data-pltdoc="x">Nesting, relaying, and levels of discourse</a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="versionbox"><span class="version">6.0.0.1</span></div><div class="navsettop"><span class="navleft"> </span><span class="navright"> <a href="index.html" title="backward to "Marketplace: Network-Aware Programming"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Marketplace: Network-Aware Programming"" data-pltdoc="x">up</a> <a href="high-level-interface.html" title="forward to "2 High-level interface"" data-pltdoc="x">next →</a></span> </div><h3>1<tt> </tt><a name="(part._.Concepts)"></a>Concepts</h3><p>Marketplace integrates ideas from both distributed systems and
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virtualized operating system designs to obtain an architecture of
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nested <span style="font-style: italic">virtual machines</span> (VMs). Each nested layer is equipped
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with its own publish/subscribe network that also propagates
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<span style="font-style: italic">presence</span> information about the (dis)appearance of services.</p><p>Throughout this manual, diagrams such as the following will illustrate
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various process structures:</p><blockquote class="SCentered"><p><img src="pict.png" alt="image" width="389" height="207"/></p></blockquote><p>Rectangular boxes represent VMs. The processes running within each VM
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are placed atop its box. The narrow rectangular strip at the top of
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each VM’s box represents the network connecting all the VM’s processes
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to each other; it will frequently contain a short description of the
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protocols used for communication across the represented network.</p><p>A central feature of Marketplace is that VMs are nothing more than
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regular processes, making them recursively nestable. Each VM supports
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a collection of processes all its own, and its internal IPC medium
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carries a VM-specific protocol that is often different from the
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protocol spoken by its containing VM.</p><p>The outermost VM is called the <span style="font-style: italic">ground VM</span>. The protocol spoken
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by processes running within the ground VM is a simple protocol
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relating Racket’s <font class="badlink"><span class="techoutside"><span class="techinside">synchronizable events</span></span></font> to Marketplace network
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messages. See <a href="Drivers.html" data-pltdoc="x">Drivers</a> for
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information on using Racket events from Marketplace programs.</p><h4>1.1<tt> </tt><a name="(part._.What_is_a_process__what_are_event_handlers_)"></a>What is a process, what are event handlers?</h4><p>A Marketplace <a name="(tech._proces)"></a><span style="font-style: italic">process</span> is a collection of <a name="(tech._event._handler)"></a><span style="font-style: italic">event
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handlers</span>, plus a piece of private <a name="(tech._process._state)"></a><span style="font-style: italic">process state</span>. Every
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process<span class="NoteBox"><span class="NoteContent">The exception to this rule is the Ground VM, which plays
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a special role.</span></span> runs within a containing VM.</p><p>When an event occurs that is relevant to a process, one of its event
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handlers is called with the process’s current state and a description
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of the event. The handler is expected to return an updated state value
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and a collection of actions for the containing VM to perform. An event
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handler, then, has the following approximate type:</p><blockquote class="SCentered"><p><span style="font-style: italic">State</span> × <span style="font-style: italic">Event</span> → <span style="font-style: italic">State</span> × (Listof <span style="font-style: italic">Action</span>)</p></blockquote><p>Event handlers are registered with the VM by creating <a href="Concepts.html#%28tech._endpoint%29" class="techoutside" data-pltdoc="x"><span class="techinside">endpoints</span></a>
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using the <span class="RktSym">endpoint</span> macro (described in <a href="high-level-interface.html#%28part._endpoint-dsl%29" data-pltdoc="x">Creating endpoints</a>) or
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the low-level <span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._add-endpoint%29%29" class="RktValLink" data-pltdoc="x">add-endpoint</a></span> structure (described in
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<a href="low-level-interface.html#%28part._endpoints-and-messages%29" data-pltdoc="x">Endpoints and Messages</a>).</p><p><a name="(tech._event)"></a><span style="font-style: italic">Events</span>, passed to event handlers, describe the results of
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actions from the outside world, neighbouring processes in the VM, or
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the VM itself. They are implemented as <span class="RktSym"><a href="http://docs.racket-lang.org/reference/define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span>s. See
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<a href="low-level-interface.html#%28part._endpoint-events%29" data-pltdoc="x">Endpoint Events</a> for a description of the available event
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structures.</p><p><a name="(tech._action)"></a><span style="font-style: italic">Actions</span>, passed back to the VM by event handlers, describe
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actions the process wishes to perform. See <a href="low-level-interface.html#%28part._.Actions%29" data-pltdoc="x">Actions</a> for the
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possible actions a process can take.</p><p>Note that the result of an event handler function is actually a
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<span class="RktSym"><a href="high-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._.Transition%29%29" class="RktValLink" data-pltdoc="x">Transition</a></span> structure; the actual Typed Racket type of event
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handlers is <span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._.Trap.K%29%29" class="RktValLink" data-pltdoc="x">TrapK</a></span>, defined in <a href="low-level-interface.html#%28part._handler-functions%29" data-pltdoc="x">Handler Functions</a>.</p><h4>1.2<tt> </tt><a name="(part._.What_is_a_.V.M_)"></a>What is a VM?</h4><p><a name="(tech._vm)"></a><span style="font-style: italic">Virtual Machines (VMs)</span> are simply a collection
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of processes, plus a shared medium of communication that the contained
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processes use to communicate with each other. VMs offer access to both
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their own <span style="font-style: italic">internal</span> network as well as to the <span style="font-style: italic">external</span>
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network owned by the VM’s own containing VM.<span class="NoteBox"><span class="NoteContent">Again, the only
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exception here is the Ground VM, which interfaces to the underlying
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Racket system and so has no containing VM.</span></span></p><h4>1.3<tt> </tt><a name="(part._.Endpoints__.Subscription_and_.Advertisement)"></a>Endpoints: Subscription and Advertisement</h4><p>The Marketplace operating system’s inter-process communication
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facility is structured around <a name="(tech._pub/sub)"></a><span style="font-style: italic">publish/subscribe (pub/sub)</span> messaging.<span class="NoteBox"><span class="NoteContent">For a survey
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of pub/sub messaging, see
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<a href="http://www.cs.ru.nl/~pieter/oss/manyfaces.pdf">"The Many
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Faces of Publish/Subscribe"</a>, ACM Computing Surveys, Vol. 35, No. 2,
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June 2003, pp. 114–131. There’s also plenty out there on the Internet;
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a good starting point is to google for
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<a href="https://www.google.com/search?q=pub/sub message-oriented middleware">pub/sub message-oriented middleware</a>.</span></span></p><p><a name="(tech._endpoint)"></a><span style="font-style: italic">Endpoints</span> are the representation of a process’s engagement
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in some protocol. They pair a description of the process’s <a href="Concepts.html#%28tech._role%29" class="techoutside" data-pltdoc="x"><span class="techinside">role</span></a>
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in a conversation with an <a href="Concepts.html#%28tech._event._handler%29" class="techoutside" data-pltdoc="x"><span class="techinside">event handler</span></a> that responds to events
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relating to that role.</p><p>A <a name="(tech._role)"></a><span style="font-style: italic">role</span> describes the role some process is playing in a
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conversation. Concretely, roles are represented by <span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._.Role%29%29" class="RktValLink" data-pltdoc="x">Role</a></span>
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structures. A role can be used by the currently-running process to
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describe some role it wishes to play, or can be carried in some
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<span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._.Endpoint.Event%29%29" class="RktValLink" data-pltdoc="x">EndpointEvent</a></span> to describe the role some <span style="font-style: italic">peer</span> process
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is playing in a conversation.</p><p>Roles have three parts:</p><ul><li><p>An <a name="(tech._orientation)"></a><span style="font-style: italic">orientation</span> (type <span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._.Orientation%29%29" class="RktValLink" data-pltdoc="x">Orientation</a></span>) describes
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whether this role is concerned primarily with <span style="font-style: italic">producing</span> or
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<span style="font-style: italic">consuming</span> messages.</p></li><li><p>A <a name="(tech._topic)"></a><span style="font-style: italic">topic</span> is a <a name="(tech._pattern)"></a><span style="font-style: italic">pattern</span> over messages. Topics
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perform double duty: they both scope conversations and filter
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incoming messages. More on topics below.</p></li><li><p>An <a name="(tech._interest._type)"></a><span style="font-style: italic">interest-type</span> (type <span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._.Interest.Type%29%29" class="RktValLink" data-pltdoc="x">InterestType</a></span>)
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determines whether the endpoint playing the given role is genuinely
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a participant in matching conversations or is simply observing the
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real participants. See <a href="high-level-interface.html#%28part._endpoint-dsl%29" data-pltdoc="x">Creating endpoints</a> for more on
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interest-types.</p></li></ul><h4>1.4<tt> </tt><a name="(part._messages-and-topics)"></a>Messages and Topics</h4><p><a name="(tech._message)"></a><span style="font-style: italic">Messages</span> are simply Racket data structures. They can be any
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value for which <span class="RktSym"><a href="http://docs.racket-lang.org/reference/booleans.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span> is defined, any <span class="RktPn">#:prefab</span>
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structure, most <span class="RktPn">#:transparent</span> structures, or any non-object
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structure for which <span class="RktSym">prop:struct-map</span> can be defined.</p><p>As mentioned above, topics are simply patterns over messages. They are
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represented as normal data structures <span style="font-style: italic">with embedded wildcards</span>.
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Use <span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-untyped..rkt%29._~3f%29%29" class="RktValLink" data-pltdoc="x">?</a></span> or <span class="RktPn">(</span><span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._wild%29%29" class="RktValLink" data-pltdoc="x">wild</a></span><span class="RktPn">)</span> to construct a wildcard. For
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example, given the following definition,</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">chat-message</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">speaker</span><span class="hspace"> </span><span class="RktSym">text</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:transparent</span><span class="RktPn">)</span></p></blockquote><p>we can not only create instances that might be used with
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<span class="RktSym"><a href="high-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-values..rkt%29._send-message%29%29" class="RktValLink" data-pltdoc="x">send-message</a></span>,</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym">chat-message</span><span class="hspace"> </span><span class="RktVal">"Tony"</span><span class="hspace"> </span><span class="RktVal">"Hello World!"</span><span class="RktPn">)</span></p></blockquote><p>but also create topic patterns using <span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-untyped..rkt%29._~3f%29%29" class="RktValLink" data-pltdoc="x">?</a></span>. For example, this
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pattern matches anything said by <span class="RktVal">"Tony"</span>:</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym">chat-message</span><span class="hspace"> </span><span class="RktVal">"Tony"</span><span class="hspace"> </span><span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-untyped..rkt%29._~3f%29%29" class="RktValLink" data-pltdoc="x">?</a></span><span class="RktPn">)</span></p></blockquote><p>This pattern matches chat-messages sent by anyone saying "Hello":</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym">chat-message</span><span class="hspace"> </span><span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-untyped..rkt%29._~3f%29%29" class="RktValLink" data-pltdoc="x">?</a></span><span class="hspace"> </span><span class="RktVal">"Hello"</span><span class="RktPn">)</span></p></blockquote><p>And finally, this pattern matches any chat-message at all:</p><blockquote class="SCodeFlow"><p><span class="RktPn">(</span><span class="RktSym">chat-message</span><span class="hspace"> </span><span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-untyped..rkt%29._~3f%29%29" class="RktValLink" data-pltdoc="x">?</a></span><span class="hspace"> </span><span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-untyped..rkt%29._~3f%29%29" class="RktValLink" data-pltdoc="x">?</a></span><span class="RktPn">)</span></p></blockquote><p>Patterns can be nested. For instance, given the above definition of
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<span class="RktSym">chat-message</span>, the following pattern matches any chat message
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greeting anybody at all:</p><blockquote class="SCodeFlow"><table cellspacing="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://docs.racket-lang.org/reference/define-struct.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._struct%29%29" class="RktStxLink" data-pltdoc="x">struct</a></span><span class="hspace"> </span><span class="RktSym">greeting</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">target</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">#:transparent</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym">chat-message</span><span class="hspace"> </span><span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-untyped..rkt%29._~3f%29%29" class="RktValLink" data-pltdoc="x">?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">greeting</span><span class="hspace"> </span><span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-untyped..rkt%29._~3f%29%29" class="RktValLink" data-pltdoc="x">?</a></span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><h4>1.5<tt> </tt><a name="(part._.Presence)"></a>Presence</h4><p><a name="(tech._presence)"></a><span style="font-style: italic">Presence</span> (respectively its opposite, <a name="(tech._absence)"></a><span style="font-style: italic">absence</span>) is
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an indication that a matching conversational partner exists (resp. no
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longer exists) in the network. Presence can be used to synchronize
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conversations, setting up a conversational context before messages are
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sent.</p><p>The term "presence" itself is lifted from Instant Messaging protocols
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like XMPP, where it describes the online/offline status of one’s chat
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buddies. Here, it describes the online/offline status of peer
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processes, in terms of which conversations they are willing to engage
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in.</p><p>The system derives presence information from the set of active pub/sub
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subscription and advertisement endpoints a process has created.
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Creating a new endpoint with a topic pattern that matches some other
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process’s endpoint and an orientation <span style="font-style: italic">opposite</span> to the other
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process’s endpoint causes <span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._presence-event%29%29" class="RktValLink" data-pltdoc="x">presence-event</a></span>s to be sent to both
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endpoints, informing them of the presence of the other. When a process
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crashes, or an endpoint is withdrawn with <span class="RktSym"><a href="high-level-interface.html#%28def._%28%28lib._marketplace%2Fsugar-values..rkt%29._delete-endpoint%29%29" class="RktValLink" data-pltdoc="x">delete-endpoint</a></span>, a
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corresponding <span class="RktSym"><a href="low-level-interface.html#%28def._%28%28lib._marketplace%2Fmain..rkt%29._absence-event%29%29" class="RktValLink" data-pltdoc="x">absence-event</a></span> is sent to the remaining
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endpoint.</p><h4>1.6<tt> </tt><a name="(part._.Nesting__relaying__and_levels_of_discourse)"></a>Nesting, relaying, and levels of discourse</h4><p>Because VMs can be nested, and each VM has an IPC network of its own
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for the use of its processes, information sometimes needs to be
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relayed from a VM’s external network to its internal network and vice
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versa.</p><p>In general, the protocol messages sent across a VM’s internal network
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may be quite different in syntax and meaning from those sent across
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the same VM’s external network: consider the case of the
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<a href="Examples.html#%28part._chat-server-example%29" data-pltdoc="x">TCP chat server</a>, which employs a nested VM to separate
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out TCP-related messages from higher-level, application-specific chat
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messages:</p><blockquote class="SCentered"><p><img src="pict_2.png" alt="image" width="355" height="214"/></p></blockquote><p>Each VM’s network corresponds to a distinct <span style="font-style: italic">level of discourse</span>.
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The nesting of VMs is then roughly analogous to the layering of
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network protocol stacks. For example (and purely hypothetically!) the
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TCP-IP/HTTP/Webapp stack could perhaps be represented as</p><blockquote class="SCentered"><p><img src="pict_3.png" alt="image" width="241" height="260"/></p></blockquote><div class="navsetbottom"><span class="navleft"> </span><span class="navright"> <a href="index.html" title="backward to "Marketplace: Network-Aware Programming"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Marketplace: Network-Aware Programming"" data-pltdoc="x">up</a> <a href="high-level-interface.html" title="forward to "2 High-level interface"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |