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| 54 | <H3><A NAME="SECTION009541000000000000000"></A><A NAME="semaphore-examples"></A> |
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| 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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