From 6cf6b9305b2d1fe96d1cf93a520d7dfc724bf300 Mon Sep 17 00:00:00 2001 From: Flavio Percoco Date: Thu, 30 Oct 2014 11:23:41 +0100 Subject: [PATCH] Why do we increment the counter after the insert? This patchs adds a comment explaining why the counter in the mongodb driver is incremented *after* the insert happens. Change-Id: I143c8e214736eec4261e09561824088775eb6ac1 --- zaqar/queues/storage/mongodb/messages.py | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/zaqar/queues/storage/mongodb/messages.py b/zaqar/queues/storage/mongodb/messages.py index cdd47ac2b..87a689217 100644 --- a/zaqar/queues/storage/mongodb/messages.py +++ b/zaqar/queues/storage/mongodb/messages.py @@ -514,6 +514,13 @@ class MessageController(storage.Message): collection = self._collection(queue_name, project) # Set the next basis marker for the first attempt. + # + # Note that we don't increment the counter right away because + # if 2 concurrent posts happen and the one with the higher counter + # ends before the one with the lower counter, there's a window + # where a client paging through the queue may get the messages + # with the higher counter and skip the previous ones. This would + # make our FIFO guarantee unsound. next_marker = self._queue_ctrl._get_counter(queue_name, project) # Unique transaction ID to facilitate atomic batch inserts