Capturing problematic messages in Amazon SQS
To capture all messages that can't be processed, and to collect accurate CloudWatch metrics, configure a dead-letter queue.
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The redrive policy redirects messages to a dead-letter queue after the source queue fails to process a message a specified number of times.
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Using a dead-letter queue decreases the number of messages and reduces the possibility of exposing you to poison pill messages (messages that are received but can't be processed).
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Including a poison pill message in a queue can distort the ApproximateAgeOfOldestMessage CloudWatch metric by giving an incorrect age of the poison pill message. Configuring a dead-letter queue helps avoid false alarms when using this metric.