Software Architect's Handbook

Port binding

Cloud-native applications are completely self-contained. Services should make themselves available to other services by specified ports. Similar to any backing services your application may be using, the API you expose may be the backing service for another application. This allows one app to be the backing service of another app.

Just as accessing a backing service should be achievable through a simple binding, such as a URL, other applications should be able to interface with your applications through a simple binding.