From 2c3865b210c9c9cd5a2fc0a21e315b9d6da03084 Mon Sep 17 00:00:00 2001 From: Raymond Hettinger Date: Tue, 18 Jan 2011 22:58:33 +0000 Subject: [PATCH] Expand barrier example to show time-outs. --- Doc/whatsnew/3.2.rst | 25 ++++++++++++++++++++----- 1 file changed, 20 insertions(+), 5 deletions(-) diff --git a/Doc/whatsnew/3.2.rst b/Doc/whatsnew/3.2.rst index 5a7b72ccb34..b53f1a3e611 100644 --- a/Doc/whatsnew/3.2.rst +++ b/Doc/whatsnew/3.2.rst @@ -847,11 +847,6 @@ are suitable for use in loops. The separate *filling* and *draining* phases assure that all threads get released (drained) before any one of them can loop back and re-enter the barrier. The barrier fully resets after each cycle. -If any of the predecessor tasks can hang or be delayed, a barrier can be created -with an optional *timeout* parameter. Then if the timeout period elapses before -all the predecessor tasks reach the barrier point, all waiting threads are -released and a :exc:`~threading.BrokenBarrierError` exception is raised. - Example of using barriers:: def get_votes(site): @@ -870,6 +865,26 @@ is similar to one with :meth:`threading.Thread.join`, but the threads stay alive and continue to do work (summarizing ballots) after the barrier point is crossed. +If any of the predecessor tasks can hang or be delayed, a barrier can be created +with an optional *timeout* parameter. Then if the timeout period elapses before +all the predecessor tasks reach the barrier point, all waiting threads are +released and a :exc:`~threading.BrokenBarrierError` exception is raised:: + + def get_votes(site): + ballots = conduct_election(site) + try: + all_polls_closed.wait(timeout = midnight - time.now()) + except BrokenBarrerError: + lockbox = seal_ballots(ballots) + queue.put(lockbox) + else: + totals = summarize(ballots) + publish(site, totals) + +In this example, the barrier enforces a more robust rule. If some election +sites do not finish before midnight, the barrier times-out and the ballots are +sealed and deposited in a queue for later handling. + See `Barrier Synchronization Patterns `_ for more examples of how barriers can be used in parallel computing. Also, there is