A company has an application defined with the following architecture.
A fleet of EC2 Instances which are used to accept video uploads from users.
A fleet of EC2 Instances which are used to process the video uploads.
Which of the following would help architect an operationally excellent architecture?

Answer options:

A.Create an SQS queue to store the information for Video uploads. Spin up the processing servers via an Autoscaling Group. Ensure the Group scales based on the Memory utilization of the underlying processing servers.
B.Create an SQS queue to store the information for Video uploads. Spin up the processing servers via an Autoscaling Group. Ensure the Group scales based on the size of the queue.
C.Create an SNS topic to store the information for Video uploads. Spin up the processing servers via an Autoscaling Group. Ensure the Group scales based on the Memory utilization of the underlying processing servers.
D.Create an SNS topic to store the information for Video uploads. Spin up the processing servers via an Autoscaling Group. Ensure the Group scales based on the size of the queue messages.

Answer – B
This architecture is also given in the AWS Documentation.
Option A is incorrect. The ideal approach is to scale the instances based on the size of the queue.
Options C and D are incorrect since you should be using SQS queues. SNS topics are used for notification purposes.
For more information on using SQS queues for Autoscaling, please refer to the below URL
https://docs.aws.amazon.com/autoscaling/ec2/userguide/as-using-sqs-queue.html
As per AWS,
You can use the number of messages stored in an SQS queue as an indicator of the amount of work waiting in line for eventual processing within an Auto Scaling Group comprised of a variable number of EC2 instances. Each SQS queue reports a number of metrics to CloudWatch at five minute intervals, including 
ApproximateNumberOfMessagesVisible
. If your workload is spikey in nature, you may want to build an application that can respond more quickly to changes in the size of the queue.
Memory utilization metrics are custom metrics.For this, you need to install a Cloudwatch agent on the EC2 instances and need to aggregate the dimensions.
However, AWS already has a well-defined architecture based on SQS Queuelength being used for Autoscaling EC2 instances.
For more information, please refer to the link:
https://aws.amazon.com/blogs/aws/auto-scaling-with-sqs/