AWS SQS how does SQS know that processing of a message failed?

I’m reading aws documents on deadletter queue and re-drive policy, and the document mentioned "The redrive policy specifies the source queue, the dead-letter queue, and the conditions under which Amazon SQS moves messages from the former to the latter if the consumer of the source queue fails to process a message a specified number of times".

However, even the document mentioned "message process failed" several times, I do not understand how sqs detects a message processing failure (and thus triggers re-drive or move to the dead letter queue.)

From what I understand, consumer applications call receiveMessage to retrieve the message from SQS, then process the message. The processing function is not passed in to receiveMessage as a lambda. So how does SQS know that message processing has failed?

>Solution :

When a client (e.g. a lambda function) gets a message from the queue, it has limited time to call DeleteMessage. Each msg has also visibility timeout. If the msg is not deleted by the client within the visibility timeout, SQS "assumes" that the processing failed.

Such messages can be then forwarded to SQS depending on how many failed attempts you setup to tollerate.

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