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<a class="dropdown-item" tabindex="-1" href="SequenceModels.html#"><b>Chapter 1</b></a>
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<a class="dropdown-item" href="IntroConcSysOverview.html"> 1.1. Introduction to Concurrent Systems</a>
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<a class="dropdown-item" href="SysAndModels.html"> 1.2. Systems and Models</a>
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<a class="dropdown-item" href="Themes.html"> 1.3. Themes and Guiding Principles</a>
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<a class="dropdown-item" href="Architectures.html"> 1.4. System Architectures</a>
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<a class="dropdown-item" href="StateModels.html"> 1.5. State Models in UML</a>
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<a class="dropdown-item" href="SequenceModels.html"> 1.6. Sequence Models in UML</a>
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<a class="dropdown-item" href="StateModelImplementation.html"> 1.7. Extended Example: State Model Implementation</a>
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<a class="dropdown-item disabled"><b>Chapter 2</b></a>
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<a class="dropdown-item" href="ProcessesOverview.html"> 2.1. Processes and OS Basics</a>
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<a class="dropdown-item" href="Multiprogramming.html"> 2.2. Processes and Multiprogramming</a>
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<a class="dropdown-item" href="UnixFile.html"> 2.6. The UNIX File Abstraction</a>
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<a class="dropdown-item" href="EventsSignals.html"> 2.7. Events and Signals</a>
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<a class="dropdown-item" href="Extended2Processes.html"> 2.8. Extended Example: Listing Files with Processes</a>
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<a class="dropdown-item disabled"><b>Chapter 3</b></a>
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<a class="dropdown-item" href="IPCOverview.html"> 3.1. Concurrency with IPC</a>
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<a class="dropdown-item" href="IPCModels.html"> 3.2. IPC Models</a>
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<a class="dropdown-item" href="Pipes.html"> 3.3. Pipes and FIFOs</a>
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<a class="dropdown-item" href="MMap.html"> 3.4. Shared Memory With Memory-mapped Files</a>
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<a class="dropdown-item" href="POSIXvSysV.html"> 3.5. POSIX vs. System V IPC</a>
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<a class="dropdown-item" href="MQueues.html"> 3.6. Message Passing With Message Queues</a>
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<a class="dropdown-item" href="ShMem.html"> 3.7. Shared Memory</a>
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<a class="dropdown-item" href="IPCSems.html"> 3.8. Semaphores</a>
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<a class="dropdown-item" href="Extended3Bash.html"> 3.9. Extended Example: Bash-lite: A Simple Command-line Shell</a>
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<a class="dropdown-item disabled"><b>Chapter 4</b></a>
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<a class="dropdown-item" href="SocketsOverview.html"> 4.1. Networked Concurrency</a>
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<a class="dropdown-item" href="FiveLayer.html"> 4.2. The TCP/IP Internet Model</a>
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<a class="dropdown-item" href="NetApps.html"> 4.3. Network Applications and Protocols</a>
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<a class="dropdown-item" href="Sockets.html"> 4.4. The Socket Interface</a>
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<a class="dropdown-item" href="ThreadsOverview.html"> 6.1. Concurrency with Multithreading</a>
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<a class="dropdown-item" href="RaceConditions.html"> 6.3. Race Conditions and Critical Sections</a>
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<a class="dropdown-item" href="POSIXThreads.html"> 6.4. POSIX Thread Library</a>
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<a class="dropdown-item" href="ThreadArgs.html"> 6.5. Thread Arguments and Return Values</a>
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<a class="dropdown-item" href="ImplicitThreads.html"> 6.6. Implicit Threading and Language-based Threads</a>
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<a class="dropdown-item" href="Extended6Input.html"> 6.7. Extended Example: Keyboard Input Listener</a>
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<a class="dropdown-item" href="Extended6Primes.html"> 6.8. Extended Example: Concurrent Prime Number Search</a>
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<a class="dropdown-item" href="SynchOverview.html"> 7.1. Synchronization Primitives</a>
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<a class="dropdown-item" href="CritSect.html"> 7.2. Critical Sections and Peterson's Solution</a>
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<a class="dropdown-item" href="Locks.html"> 7.3. Locks</a>
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<a class="dropdown-item" href="SynchProblemsOverview.html"> 8.1. Synchronization Patterns and Problems</a>
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<a class="dropdown-item" href="SynchDesign.html"> 8.2. Basic Synchronization Design Patterns</a>
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<a class="dropdown-item" href="ProdCons.html"> 8.3. Producer-Consumer Problem</a>
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<a class="dropdown-item" href="ReadWrite.html"> 8.4. Readers-Writers Problem</a>
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<a class="dropdown-item" href="DiningPhil.html"> 8.5. Dining Philosophers Problem and Deadlock</a>
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<a class="dropdown-item" href="CigSmokers.html"> 8.6. Cigarette Smokers Problem and the Limits of Semaphores and Locks</a>
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<a class="dropdown-item" href="Extended8ModExp.html"> 8.7. Extended Example: Parallel Modular Exponentiation</a>
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<a class="dropdown-item disabled"><b>Chapter 9</b></a>
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<a class="dropdown-item" href="ParallelDistributedOverview.html"> 9.1. Parallel and Distributed Systems</a>
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<a class="dropdown-item" href="ParVConc.html"> 9.2. Parallelism vs. Concurrency</a>
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<a class="dropdown-item" href="ParallelDesign.html"> 9.3. Parallel Design Patterns</a>
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<a class="dropdown-item" href="Scaling.html"> 9.4. Limits of Parallelism and Scaling</a>
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<a class="dropdown-item" href="DistTiming.html"> 9.5. Timing in Distributed Environments</a>
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<a class="dropdown-item" href="DistDataStorage.html"> 9.6. Reliable Data Storage and Location</a>
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<a class="dropdown-item" href="DistConsensus.html"> 9.7. Consensus in Distributed Systems</a>
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<a class="dropdown-item" href="Extended9Blockchain.html"> 9.8. Extended Example: Blockchain Proof-of-Work</a>
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<a class="dropdown-item disabled"><b>Appendix A</b></a>
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<a class="dropdown-item" href="CLangOverview.html"> A.1. C Language Reintroduction</a>
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<a class="dropdown-item" href="Debugging.html"> A.2. Documentation and Debugging</a>
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<a class="dropdown-item" href="BasicTypes.html"> A.3. Basic Types and Pointers</a>
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<a class="dropdown-item" href="Arrays.html"> A.4. Arrays, Structs, Enums, and Type Definitions</a>
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<script>ODSA.SETTINGS.DISP_MOD_COMP = true;ODSA.SETTINGS.MODULE_NAME = "SequenceModels";ODSA.SETTINGS.MODULE_LONG_NAME = "Sequence Models in UML";ODSA.SETTINGS.MODULE_CHAPTER = "Introduction to Computer Systems"; ODSA.SETTINGS.BUILD_DATE = "2021-06-01 15:31:50"; ODSA.SETTINGS.BUILD_CMAP = false;JSAV_OPTIONS['lang']='en';JSAV_EXERCISE_OPTIONS['code']='java';</script><div class="section" id="sequence-models-in-uml">
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<h1>1.6. Sequence Models in UML<a class="headerlink" href="SequenceModels.html#sequence-models-in-uml" title="Permalink to this headline">¶</a></h1>
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<div class="figure mb-2 align-right" id="id1" style="width: 45%">
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<span id="sequenceuml"></span><a class="reference internal image-reference" href="_images/CSF-Images.1.11.png"><img class="p-3 mb-2 align-center border border-dark rounded-lg" alt="A UML sequence model for a streaming media player execution" src="_images/CSF-Images.1.11.png" style="width: 95%;" /></a>
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<p class="caption align-center px-3"><span class="caption-text"> Figure 1.6.1: A UML sequence model for a streaming media player execution</span></p>
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</div>
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<p>As noted previously, state models omit information about timing or the sequence of events that a
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||
system encounters while it is running. UML <a class="reference internal" href="Glossary.html#term-sequence-model"><span class="xref std std-term">sequence models</span></a> provide a
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||
different view of the system, by looking at the sequence of communication messages that are
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||
exchanged during an execution of the system. <a href="SequenceModels.html#sequenceuml">Figure 1.6.1</a> shows an example of the messages that might
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be exchanged between the streaming media player (whose states are those in <a href="StateModels.html#statemodelsuml">Figure 1.5.1</a>) and the
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server that is providing the data to play. The player starts by connecting to the server; the
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||
server’s response could denote the event that triggers the transition to the <code class="docutils literal notranslate"><span class="pre">Connected</span></code> state.
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The player then starts requesting data in chunks of 16 KB, buffering the data until it is ready to
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||
be played.</p>
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<p>UML sequence models use <em>lifelines</em> to denote one entity’s perspective over time;
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the start time would be at the top and later time is farther down. UML sequence models use a solid
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||
arrow head (such as the <code class="docutils literal notranslate"><span class="pre">Connect</span></code> message) to denote <a class="reference internal" href="Glossary.html#term-synchronous"><span class="xref std std-term">synchronous</span></a> messages, with the open
|
||
arrow head (such as the request for bytes 16384-32767) to denote <a class="reference internal" href="Glossary.html#term-asynchronous"><span class="xref std std-term">asynchronous</span></a> messages. The
|
||
distinction indicates whether or not the sender must wait while the message is sent or if it can do
|
||
something else during that time. For instance, it would make sense for the <code class="docutils literal notranslate"><span class="pre">Connect</span></code> and the first
|
||
request to be synchronous messages, because that connection and initial data are necessary to get
|
||
started. Then, once the system has started playing the buffered data, additional requests for more
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||
data could be sent as asynchronous messages. Throughout this book, we will mostly be modeling
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||
communication as asynchronous messages, which is common for concurrent systems.</p>
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||
<p><a href="SequenceModels.html#sequenceuml">Figure 1.6.1</a> illustrates three additional key features of UML sequence models.
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||
First, the dotted message lines indicate that a message is a <em>reply</em>. This notation is
|
||
especially useful for synchronous messages to illustrate the linkage between the request and the
|
||
response. The <code class="docutils literal notranslate"><span class="pre">Play</span> <span class="pre">Movie</span></code> message is a <em>self-message</em>, which indicates a meaningful event
|
||
on that lifeline. That is, from that message, the movie is being played, as long as there is data
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||
available. While the movie is playing, additional asynchronous messages are sent and more data is
|
||
received for buffering. Lastly, the message that ends at a circle between the lifelines denotes a
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||
<em>lost message</em>. This notation indicates that the media player sent the message, but the server
|
||
did not receive it. This type of lost message is common in networked systems, as we will explore in
|
||
Chapters 4 and 5.</p>
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||
<p>Throughout this book, we will use both UML state models and sequence models to illustrate key ideas
|
||
in relation to concurrent systems. Understanding how to read and interpret these models is an
|
||
important skill in this area. Unlike state models, however, there is no particular translation for
|
||
sequence models. Specifically, the type of message that would need to be sent depends on the
|
||
underlying form of communication, such as <a class="reference internal" href="Glossary.html#term-pipe"><span class="xref std std-term">pipes</span></a> or <a class="reference internal" href="Glossary.html#term-socket"><span class="xref std std-term">sockets</span></a>, which we
|
||
will discuss in later chapters.</p>
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||
<div class="section" id="summary-questions">
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||
<h2>1.6.1. Summary Questions<a class="headerlink" href="SequenceModels.html#summary-questions" title="Permalink to this headline">¶</a></h2>
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