A. Put the ordering and inventory applications into their own AWS Lambda functions. Have the ordering application write the messages into an Amazon SQS FIFO queue.
B. Put the ordering and inventory applications into their own Amazon ECS containers, and create an Auto Scaling group for each application. Then, deploy the message queuing server in multiple Availability Zones.
C. Put the ordering and inventory applications into their own Amazon EC2 instances, and create an Auto Scaling group for each application. Use Amazon SQS standard queues for the incoming orders, and implement idempotency in the inventory application.
D. Put the ordering and inventory applications into their own Amazon EC2 instances. Write the incoming orders to an Amazon Kinesis data stream. Configure AWS Lambda to poll the stream and update the inventory application.