Why do SqS messages sometimes remain in-flight on queue

Question

I\'m using Amazon SQS queues in a very simple way. Usually, messages are written and immediately visible and read. Occasionally, a message is written, and remains In-Flight(Not

Answer 1

It sounds like you are misinterpreting what you are seeing.

Messages "in flight" are not pending delivery, they're messages that have already been delivered but not further acted on by the consumer.

Messages are considered in flight if they have been sent to a client but have not yet been deleted or have not yet reached the end of their visibility window.

^{— http://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/sqs-metricscollected.html}

When a consumer receives a message, it has to -- at some point -- either delete the message, or send a request to increase the timeout for that message; otherwise the message becomes visible again after the timeout expires. If a consumer fails to do one of these things, the message automatically becomes visible again. The visibility timeout is how long the consumer has before one of these things must be done.

Messages should not be "in flight" without something having already received them -- but that "something" can include the console itself, as you'll note on the pop-up you see when you choose "View/Delete Messages" in the console (unless you already checked the "Don't show this again" checkbox):

Messages displayed in the console will not be available to other applications until the console stops polling for messages.

Messages displayed in the console are "in flight" while the console is observing the queue from the "View/Delete Messages" screen.

The part that does not make obvious sense is messages being in flight "for several minutes" if your default visibility timeout is only 5 seconds and nothing in your code is increasing that timeout... however... that could be explained almost perfectly by your consumers not properly disposing of the message, causing it to timeout and immediately be redelivered, giving the impression that a single instance of the message was remaining in-flight, when in fact, the message is briefly transitioning back to visible, only to be claimed almost immediately by another consumer, taking it back to in-flight again.