SNS and SQS

SQS Dead Letter Queue (DLQ)

• If a consumer fails to process a message within the VisibilityTimeout, the message will be placed back to the queue
• We can set a threshold of how many times a message can be placed back to the queue (MaximumReceives threshold)
• After this threshold is exceeded, the messages are placed back to the DLQ
• DLQ of a FIFO queue must be a FIFO queue. DLQ of a Standard queue must also be a Standard Queue
• Redrive to Source:
  • It is a feature which helps consume messages in the DLQ to understand what is wrong with them
  • When our code is fixed, we can redrive the messages from the DLQ back into source queue (or any other queue) in batches without writing custom code

SNS Dead Letter Queue (DLQ)

• After exhausting the delivery policy (delivery retries) messages that haven’t been delivered are discarded unless we set a DLQ
• Redrive Policy: JSON object that refers to the ARN of the DLQ (SQS or SQS FIFO)
• DLQ is attached to SNS at Subscription-level (rather than the SNS topic)