Amazon SQS short and long polling - Amazon Simple Queue Service
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Amazon SQS short and long polling

Amazon SQS provides short polling and long polling to receive messages from a queue. By default, queues use short polling.

With short polling, the ReceiveMessage request queries only a subset of the servers (based on a weighted random distribution) to find messages that are available to include in the response. Amazon SQS sends the response right away, even if the query found no messages.

With long polling, the ReceiveMessage request queries all of the servers for messages. Amazon SQS sends a response after it collects at least one available message, up to the maximum number of messages specified in the request. Amazon SQS sends an empty response only if the polling wait time expires.

The following sections explain the details of short polling and long polling.

Consuming messages using short polling

When you consume messages from a queue using short polling, Amazon SQS samples a subset of its servers (based on a weighted random distribution) and returns messages from only those servers. Thus, a particular ReceiveMessage request might not return all of your messages. However, if you have fewer than 1,000 messages in your queue, a subsequent request will return your messages. If you keep consuming from your queues, Amazon SQS samples all of its servers, and you receive all of your messages.

The following diagram shows the short-polling behavior of messages returned from a standard queue after one of your system components makes a receive request. Amazon SQS samples several of its servers (in gray) and returns messages A, C, D, and B from these servers. Message E isn't returned for this request, but is returned for a subsequent request.

                    Message Sampling using Short (Standard) Polling

Consuming messages using long polling

When the wait time for the ReceiveMessage API action is greater than 0, long polling is in effect. The maximum long polling wait time is 20 seconds. Long polling helps reduce the cost of using Amazon SQS by eliminating the number of empty responses (when there are no messages available for a ReceiveMessage request) and false empty responses (when messages are available but aren't included in a response). For information about enabling long polling for a new or existing queue using the Amazon SQS console, see the Configuring queue parameters (console). For best practices, see Setting up long polling.

Long polling offers the following benefits:

  • Reduce empty responses by allowing Amazon SQS to wait until a message is available in a queue before sending a response. Unless the connection times out, the response to the ReceiveMessage request contains at least one of the available messages, up to the maximum number of messages specified in the ReceiveMessage action. In rare cases, you might receive empty responses even when a queue still contains messages, especially if you specify a low value for the ReceiveMessageWaitTimeSeconds parameter.

  • Reduce false empty responses by querying all—rather than a subset of—Amazon SQS servers.

  • Return messages as soon as they become available.

For information about how to confirm that a queue is empty, see Confirming that a queue is empty.

Differences between long and short polling

Short polling occurs when the WaitTimeSeconds parameter of a ReceiveMessage request is set to 0 in one of two ways:

  • The ReceiveMessage call sets WaitTimeSeconds to 0.

  • The ReceiveMessage call doesn’t set WaitTimeSeconds, but the queue attribute ReceiveMessageWaitTimeSeconds is set to 0.