Databases and external systems that are accessed in the application are called resources. It should make no difference to the system where an external service or database is part of the application. The resources should be attached to the application in a loosely coupled way. External systems and databases should be able to be replaced by new instances without affecting the application.

Applications abstract the accessed external system, first of all in the communication technology being used. Communication via HTTP or JDBC, for example, abstracts the implementations and enables systems to be replaced by others. By doing so, applications are only coupled to their contract: the communication protocol and defined schemas. JPA, JAX-RS, and JSON-B are examples that support this approach.

Container orchestration frameworks take this approach even further and abstract services into logic names. As shown previously applications can use service names as hostnames, resolved by DNS.

In general, application developers should loosely couple systems together, ideally only depending on protocols and schemas. At a code level backing services are abstracted into own components, such as individual controls with clean interfaces. This minimizes changes if attached resources change.