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54 | <H3><A NAME="SECTION009541000000000000000"></A><A NAME="semaphore-examples"></A> | |
55 | <BR> | |
56 | 7.5.4.1 <tt class="class">Semaphore</tt> Example | |
57 | </H3> | |
58 | ||
59 | <P> | |
60 | Semaphores are often used to guard resources with limited capacity, for | |
61 | example, a database server. In any situation where the size of the resource | |
62 | size is fixed, you should use a bounded semaphore. Before spawning any | |
63 | worker threads, your main thread would initialize the semaphore: | |
64 | ||
65 | <P> | |
66 | <div class="verbatim"><pre> | |
67 | maxconnections = 5 | |
68 | ... | |
69 | pool_sema = BoundedSemaphore(value=maxconnections) | |
70 | </pre></div> | |
71 | ||
72 | <P> | |
73 | Once spawned, worker threads call the semaphore's acquire and release | |
74 | methods when they need to connect to the server: | |
75 | ||
76 | <P> | |
77 | <div class="verbatim"><pre> | |
78 | pool_sema.acquire() | |
79 | conn = connectdb() | |
80 | ... use connection ... | |
81 | conn.close() | |
82 | pool_sema.release() | |
83 | </pre></div> | |
84 | ||
85 | <P> | |
86 | The use of a bounded semaphore reduces the chance that a programming error | |
87 | which causes the semaphore to be released more than it's acquired will go | |
88 | undetected. | |
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