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| 55 | <H2><A NAME="SECTION00131610000000000000000"> | |
| 56 | 11.16.1 Server Creation Notes</A> | |
| 57 | </H2> | |
| 58 | ||
| 59 | <P> | |
| 60 | There are five classes in an inheritance diagram, four of which represent | |
| 61 | synchronous servers of four types: | |
| 62 | ||
| 63 | <P> | |
| 64 | <div class="verbatim"><pre> | |
| 65 | +------------+ | |
| 66 | | BaseServer | | |
| 67 | +------------+ | |
| 68 | | | |
| 69 | v | |
| 70 | +-----------+ +------------------+ | |
| 71 | | TCPServer |------->| UnixStreamServer | | |
| 72 | +-----------+ +------------------+ | |
| 73 | | | |
| 74 | v | |
| 75 | +-----------+ +--------------------+ | |
| 76 | | UDPServer |------->| UnixDatagramServer | | |
| 77 | +-----------+ +--------------------+ | |
| 78 | </pre></div> | |
| 79 | ||
| 80 | <P> | |
| 81 | Note that <tt class="class">UnixDatagramServer</tt> derives from <tt class="class">UDPServer</tt>, not | |
| 82 | from <tt class="class">UnixStreamServer</tt> - the only difference between an IP and a | |
| 83 | Unix stream server is the address family, which is simply repeated in both | |
| 84 | unix server classes. | |
| 85 | ||
| 86 | <P> | |
| 87 | Forking and threading versions of each type of server can be created using | |
| 88 | the <tt class="class">ForkingMixIn</tt> and <tt class="class">ThreadingMixIn</tt> mix-in classes. For | |
| 89 | instance, a threading UDP server class is created as follows: | |
| 90 | ||
| 91 | <P> | |
| 92 | <div class="verbatim"><pre> | |
| 93 | class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass | |
| 94 | </pre></div> | |
| 95 | ||
| 96 | <P> | |
| 97 | The mix-in class must come first, since it overrides a method defined in | |
| 98 | <tt class="class">UDPServer</tt>. Setting the various member variables also changes the | |
| 99 | behavior of the underlying server mechanism. | |
| 100 | ||
| 101 | <P> | |
| 102 | To implement a service, you must derive a class from | |
| 103 | <tt class="class">BaseRequestHandler</tt> and redefine its <tt class="method">handle()</tt> method. You | |
| 104 | can then run various versions of the service by combining one of the server | |
| 105 | classes with your request handler class. The request handler class must be | |
| 106 | different for datagram or stream services. This can be hidden by using the | |
| 107 | handler subclasses <tt class="class">StreamRequestHandler</tt> or <tt class="class">DatagramRequestHandler</tt>. | |
| 108 | ||
| 109 | <P> | |
| 110 | Of course, you still have to use your head! For instance, it makes no sense | |
| 111 | to use a forking server if the service contains state in memory that can be | |
| 112 | modified by different requests, since the modifications in the child process | |
| 113 | would never reach the initial state kept in the parent process and passed to | |
| 114 | each child. In this case, you can use a threading server, but you will | |
| 115 | probably have to use locks to protect the integrity of the shared data. | |
| 116 | ||
| 117 | <P> | |
| 118 | On the other hand, if you are building an HTTP server where all data is | |
| 119 | stored externally (for instance, in the file system), a synchronous class | |
| 120 | will essentially render the service "deaf" while one request is being | |
| 121 | handled - which may be for a very long time if a client is slow to receive | |
| 122 | all the data it has requested. Here a threading or forking server is | |
| 123 | appropriate. | |
| 124 | ||
| 125 | <P> | |
| 126 | In some cases, it may be appropriate to process part of a request | |
| 127 | synchronously, but to finish processing in a forked child depending on the | |
| 128 | request data. This can be implemented by using a synchronous server and | |
| 129 | doing an explicit fork in the request handler class <tt class="method">handle()</tt> | |
| 130 | method. | |
| 131 | ||
| 132 | <P> | |
| 133 | Another approach to handling multiple simultaneous requests in an | |
| 134 | environment that supports neither threads nor <tt class="function">fork()</tt> (or where | |
| 135 | these are too expensive or inappropriate for the service) is to maintain an | |
| 136 | explicit table of partially finished requests and to use <tt class="function">select()</tt> | |
| 137 | to decide which request to work on next (or whether to handle a new incoming | |
| 138 | request). This is particularly important for stream services where each | |
| 139 | client can potentially be connected for a long time (if threads or | |
| 140 | subprocesses cannot be used). | |
| 141 | ||
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