Visibility Timeout
Visibility Timeout is the time period during which a message is invisible to other consumers after being picked up by a consumer from an SQS queue. The purpose of this feature is to ensure that consumers have sufficient time to process a message, preventing other consumers from processing the same message simultaneously. If the consumer does not delete the message within the specified timeout, the message becomes visible again and can be processed by another consumer. The default visibility timeout is 30 seconds, with a maximum of 12 hours.
Understanding and Answering Questions on Visibility Timeout in Amazon SQS
The Visibility Timeout in Amazon SQS is a time duration during which Amazon SQS prevents other consuming components from receiving and processing the message. It functions after a message is read by a consumer from the queue, and during the timeout, the message is considered to be 'in flight'.
This is essential as it helps in maintaining the integrity and flow of messages, especially if the consumer fails to process the message within the specified time, the message will be available to be received again.
Understanding how it works:
- When a consumer reads a message from the queue, the visibility timeout commences.
- It makes a message inaccessible to other consumers until the timeout expires.
- If a message processing fails, the message becomes available again after the timeout.
- If processing is successful, the consumer should delete the message from the queue.
- The visibility timeout can be modified using ChangeMessageVisibility function.
For exam questions, remember:
- Visibility timeout is between 0 seconds to 12 hours, and can be extended while a message is 'in-flight' by up to 12 hours beyond the maximum time the message can stay in the queue.
- The default visibility timeout is 30 seconds.
- Messages left unprocessed after the visibility timeout are requeued.
- A high volume of 'in-flight' messages could signify consumers that can't finish in time or too short visibility timeouts.
- Exam tip: Carefully read questions related to message lifespan, delays, long-polling and in-flight status.
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