You can use asynchronous integration as a decoupling technique to allow the Sterling Order Management Software to be unavailable while external systems are still available.

The most common decoupling technique is asynchronous message communication between business entities. Take, for example, the need to send orders created at different external systems to Sterling Order Management Software. These systems could send the order creation requests to Sterling Order Management Software:

Synchronously using protocols such as HTTP, Web services, EJB/RMI and so forth; or

Asynchronously using messages.

Both approaches have their strengths and weaknesses. From a high-availability standpoint, the loosely-coupled asynchronous approach allows Sterling Order Management Software to be unavailable as a result of a scheduled or unscheduled outage, without affecting the external systems. The external systems can queue up the order creation requests into an integration message queue.

In contrast, if the communication is synchronous, Sterling Order Management Software must be available for the external system to create the request. In this architecture, the availability requirements of Sterling Order Management Software have to be the greatest of all the availability requirements of all tightly connected systems combined.

This scenario is, of course, simplistic since it may not take into account other synchronous interfaces (like inventory lookups or pricing) that the order creation process is dependent upon.

https://docs.aws.amazon.com/prescriptive-guidance/latest/modernization-integrating-microservices/decouple-messaging.html