A FIFO behaves similarly to a standard queue, but supports odering and exactly-once processing.
Either receiving a response from the client confirming receipt of a message, or performing a status check at regular intervals.
The observer pattern is a software design pattern in which an object, named the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. wiki
Unit testing, service testing, and end-to-end testing form a system-under-test.
FIFO Queue - an amazon SQS service that provides for FIFO functionality and exactly-once processing.
Pub/Sub - an asynchronous messaging service that decouples services that produce events from services that process events.