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==Message deletion==
SQS does not automatically delete messages once they are sent. When a message is delivered, a receipt handle is generated for that delivery and sent to the recipient. These receipts are not sent with the message but in addition to it. SQS requires the recipient to provide the receipt in order to delete a message. This feature is new as of 2008 where only the message ID was required for message deletion. Because the system is distributed, a message may be sent more than once. In this case, the most recent receipt handle is needed to delete the message. Furthermore, the receipt handle may have other validity constraints; for instance, the receipt handle may only be valid during the visibility timeout (see below).
Once a message is delivered, it has a '''visibility timeout''' to prevent other components from consuming it. The "clock" for the visibility timeout starts once a message is sent, the default time being 30 seconds. If the queue is not told to delete the message during this time, the message becomes visible again and will be present.
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