When Oracle acquired Collaxa and its BPEL product, there was no Oracle ESB yet. All adapters -such as file, FTP, JMS and database adapters- and complementary routing and transformation functionality were directly used from BPEL processes. Later on, Oracle introduced its ESB product and advocated to place adapters in the ESB instead of BPEL PM. That made sense since it promotes a cleaner separation of concerns. After all, adapters are more a technical, infrastructural concern than an orchestration concern. Mostly due to backwards compatibility, adapter functionality remained a part of the BPEL PM product. This caused much confusion since it was not always clear from where to use these adapters.
When you follow the above design guideline of placing adapter functionality in the ESB-layer, you could end up with masses of ESB projects in a real-life SOA implementation. Think of it, there can be tens -or maybe hundreds of BPEL processes and composite services- of which several will invoke partnerlinks that are in fact ESB projects wrapping adapter functionality. Examples are retrieving data from a database, publishing an event, FTP-ing an order file to a supplier, etc. This leads to “over-servicing” since some of these ESB projects are not reusable (low-level) services, but local to a single composite service or process only.
The amount of ‘none-reusable’ services depends on the approach taken. Losing overview is one of the risks of implementing SOA in bottom-up fashion only. This can result in too many granular, and possibly none-reusable services. It can be prevented by applying a more top-down or meet-in-the-middle approach. This will lead to less none-reusable services. However, you’ll always end-up with some services that are ‘private’ or ‘local’, meaning that they are only used by one other process or composite service. In such cases you would like the ESB service to be local -or encapsulated- to the composite service or process without moving the adapter functionality to BPEL.
The great thing about the Service Component Architecture (SCA) standard -on which Oracle SOA Suite 11g is based- is that you do not need to expose low-level mediator services and adapter functionality as a separate external service. It can be a local component to a composite and therefore not visible to other service composites. This promotes much better integration and encapsulation of infrastructure and low-level services. You only need to expose those artifacts that are -or will be- reusable. This resembles some of the OO-principles such as encapsulation. However, if a project ends up with lots and lots of none-reusable or ‘private’ services, it might be a good idea to analyze the services and process in a more top-down fashion to be able to create better reusable services.