You have a web application that processes customer orders. The frontend application forwards the order messages to an SQS queue. The backend contains an Elastic Load Balancer and an Auto Scaling group. You want the ASG to auto-scale depending on the queue size. Which of the following CloudWatch metrics would you choose to discover the SQS queue length?

Answer options:

A.ApproximateNumberOfMessagesVisible
B.NumberOfMessagesReceived
C.NumberOfMessagesDeleted
D.ApproximateNumberOfMessagesNotVisible

Correct Answer – A
The backend nodes can scale based on the queue length. Check the reference in https://aws.amazon.com/blogs/compute/building-loosely-coupled-scalable-c-applications-with-amazon-sqs-and-amazon-sns/.
Option A is CORRECT: ApproximateNumberOfMessagesVisible describes the number of messages available for retrieval. It can be used to decide the queue length.
Option B is incorrect: Because NumberOfMessagesReceived is the number of messages returned by calls to the ReceiveMessage action. It does not measure the queue length.
Option C is incorrect: Because NumberOfMessagesDeleted is the number of messages deleted from the queue. It is not suitable to be used in this scenario.
Option D is incorrect: Because ApproximateNumberOfMessagesNotVisible measures the number of messages in flight. It should not be used here.