Implementing ABAP proxies is quite simple. You generate the ABAP classes via SPROXY (which connects to your ESR) and implement the code.
Regarding the Service Interface Design and Implementation you have a choice:
- Outside-In: You define the Data Types and Message Types in the ESR and generate into ABAP
- Inside-Out: You choose what you need in ABAP, import into ESR and use the ABAP perspective. Then you generate into ABAP.
Each approach has pros and cons. Here you see what makes the approaches so different and what fits your requirement best.
- Clear choice if you are using Global Data Types (GDTs) and do not want to deal with the SAP-internal field names in the ESR or towards the Non-SAP applications
- Complex data structures (e.g. BAPIs) & no need to have an abstraction view in ESR
- No usage of GDTs