Amazon SQS Long Polling not returning all messages

Question

I have a requirement to read all messages in my Amazon SQS queue in 1 read and then sort it based on created timestamp and do business logic on it.

To make sure all

Answer 1

As pointed out by Michael - sqlbot, SQS does not guarantee returning all (or the requested number of) messages even in case of Long Polling. Long Polling just ensures that you do not get false empty responses - i.e. your read requests do not return any messages even when there are messages in the queue.

I had done some experiments around this and found that the number of messages returned in the response approaches the number of the messages requested as you increase the number of messages in the queue. Typically, with 1000+ messages in the queue, in my experiments, I could see that it returned 10 messages (which is by the way the max that can be returned for a read request) everytime. In fact this behavior was observed for Short Polling as well. Even with 100+ messages, the number of messages returned was not 10 all the time, although a good percentage of those requests returned 10 messages back. Obviously, this is not guaranteed, but that is what you would typically see.

I had documented the findings from my experiments in one of my blogs - posting a link to the same below in case you would like to see more details of the experiment.

http://pragmaticnotes.com/2017/11/20/amazon-sqs-long-polling-versus-short-polling/

Answer 2

Long polling is supposed to eliminate multiple polls

No, long polling is supposed to eliminate a large number of empty polls and false empty responses when messsages are actually available. A long poll in SQS won't sit and wait for the maximum amount of wait time just looking for more things to return, or keep searching once it's found something. A long poll in SQS only waits long enough to find something:

“Long polling allows the Amazon SQS service to wait until a message is available in the queue before sending a response. So unless the connection times out, the response to the ReceiveMessage request will contain at least one of the available messages (if any) and up to the maximum number requested in the ReceiveMessage call.”

^{— http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-long-polling.html (emphasis added)}

So, the “something” that SQS finds and returns may be all of the messages (up to your max), or a subset of the messages, because, as has been mentioned, SQS is a distributed system. There was likely an architectural decision to be made between "return as quickly as possible once we've found something" and "search the entire system for everything possible up to the maximum number of message the client will accept" ... and, given those alternatives, it seems reasonable that most applications would prefer the faster response of "give me whatever you can, as quickly as you can."

You don't know that you've actually drained a queue until you get back an empty response from a long poll.

Answer 3

Because SQS is, on the back-end, a distributed system, there is no guarantee that any particular request will be able to return the maximum number of messages that are being polled for.

You just have to keep calling, till you are confident enough that you have as many items as you would expect, or that the queue has been emptied.