6.3.3 Refreshing semaphores

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  • Redis in Action – Home
  • Foreword
  • Preface
  • Acknowledgments
  • About this Book
  • About the Cover Illustration
  • Part 1: Getting Started
  • Part 2: Core concepts
  • Part 3: Next steps
  • Appendix A
  • Appendix B
  • Buy the paperback

    6.3.3 Refreshing semaphores

    As the API for the marketplace was being completed, it was decided that there should be a method for users to stream all item listings as they happen, along with a stream for all purchases that actually occur. The semaphore method that we created only supports a timeout of 10 seconds, primarily to deal with timeouts and possible bugs on our side of things. But users of the streaming portion of the API will want to keep connected for much longer than 10 seconds, so we need a method for refreshing the semaphore so that it doesn’t time out.

    Because we already separated the timeout ZSET from the owner ZSET, we can actually refresh timeouts quickly by updating our time in the timeout ZSET, shown in the following listing.

    Listing 6.16 The refresh_fair_semaphore() function
    def refresh_fair_semaphore(conn, semname, identifier):
    
     
       if conn.zadd(semname, identifier, time.time()):
    

    Update our semaphore.

          release_fair_semaphore(conn, semname, identifier)
          return False
    

    We lost our semaphore; report back.

       return True
    

    We still have our semaphore.

     

    As long as we haven’t already timed out, we’ll be able to refresh the semaphore. If we were timed out in the past, we’ll go ahead and let the semaphore be lost and report to the caller that it was lost. When using a semaphore that may be refreshed, we need to be careful to refresh often enough to not lose the semaphore.

    Now that we have the ability to acquire, release, and refresh a fair semaphore, it’s time to deal with our final race condition.