SQS long polling isn’t just a way to wait longer for messages; it’s a fundamental shift in how your application interacts with the queue, drastically reducing costs and latency.
Let’s see it in action. Imagine you have a queue my-processing-queue.
Here’s how you’d send a message with the AWS CLI:
aws sqs send-message --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/my-processing-queue --message-body "Process this data"
Now, here’s the crucial difference.
Short Polling (the default, often invisible):
When you short poll, your application asks SQS, "Got anything for me?" SQS immediately responds, either with messages or an empty response. If it’s empty, your application has to wait a bit (or immediately ask again) before asking again. This creates a lot of "wasted" API calls.
# Example of a short poll (wait time is 0 seconds by default)
aws sqs receive-message --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/my-processing-queue
If no messages are available, the response is quick and empty. Your application then has to decide when to poll again. This constant back-and-forth, especially if messages are infrequent, can rack up API call costs and introduce latency as your application waits for the next poll cycle.
Long Polling (the smart choice for many):
With long polling, you tell SQS, "I’m looking for messages, and I’m willing to wait up to X seconds for one." SQS then holds your request open. If a message arrives within that X-second window, SQS immediately returns it. If X seconds pass and still no message arrives, SQS responds with an empty result. The key is that SQS only responds once after X seconds if nothing is found, rather than responding immediately with "nothing."
You enable this by setting the WaitTimeSeconds parameter.
# Example of a long poll, waiting up to 20 seconds
aws sqs receive-message --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/my-processing-queue --wait-time-seconds 20
The real magic happens when messages are sparse. Instead of your application constantly pinging SQS every second (or few seconds) and getting empty responses, SQS does the waiting for you. Your application only makes a ReceiveMessage API call when it knows there’s a good chance a message will be there, or when the WaitTimeSeconds has expired. This dramatically reduces the number of API calls, lowering costs and the time it takes for your application to actually get a message when one becomes available.
The problem long polling solves is the inefficiency and cost of frequent, empty ReceiveMessage calls when message arrival is unpredictable or infrequent. Short polling is like repeatedly knocking on a door every few seconds to see if anyone’s home. Long polling is like knocking once and saying, "I’ll wait here for 20 seconds, let me know if you come to the door."
The internal mechanism is that SQS maintains the connection open on its side. When a message arrives, it’s immediately dispatched to waiting consumers. If the WaitTimeSeconds elapses without a message, SQS simply closes the connection and returns an empty response. Your client then initiates a new ReceiveMessage request, potentially with a new WaitTimeSeconds value. The WaitTimeSeconds can be set between 0 and 20 seconds. A value of 0 effectively turns it back into short polling.
The most surprising aspect of long polling is that it can actually decrease the overall latency experienced by your application for message retrieval. While it introduces a potential wait of up to WaitTimeSeconds, it eliminates the sum of many small, immediate waits and subsequent polls that occur with short polling when messages are infrequent. The system is designed to be efficient; SQS handles the waiting, not your application’s CPU cycles and network.
The next concept you’ll want to grasp is how to manage visibility timeouts effectively alongside long polling to prevent message processing race conditions.