OData Gateway Part 2: SEGW vs RAP — Two Ways to Build OData

📌 KEY POINTS
✅ SEGW (transaction-based Gateway Service Builder): model entity types and write CRUD logic yourself in DPC_EXT — full control, more code.
✅ RAP (ABAP RESTful Application Programming Model): CDS views + behavior definition + service definition + service binding generate the OData service; the framework handles CRUD, $filter, paging, $expand, draft and ETags.
✅ SAP's strategic model for new development is RAP (and the only one in ABAP Cloud); SEGW remains supported for existing services and older releases.
⚠️ @OData.publish: true on a CDS view was a shortcut in S/4 1511–1909-era systems; it is considered obsolete — do not start new services with it.

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▶ WHERE WE ARE
Part 1 built a small SEGW service. Before going deeper, know the alternative you will meet on every S/4 project.

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▶ 1. SEGW CODE-BASED

• Entity types modelled or imported (DDIC, RFC/BAPI).
• Logic in DPC_EXT methods: GET_ENTITYSET, CREATE_ENTITY, etc.
• You implement filter, paging, sorting, $expand and error handling yourself.
• Works on any NetWeaver 7.x Gateway system.

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▶ 2. SEGW WITH CDS (REFERENCED DATA SOURCE / MAPPING)

• Data Model → Reference → Data Source: a CDS view becomes the entity set; read operations are generated.
• Create/update still coded in DPC_EXT.
• Good middle way on S/4 on-premise when RAP is not possible.

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▶ 3. @OData.PUBLISH (OBSOLETE SHORTCUT)

• Annotation on a CDS view that generates a service automatically (activate in /IWFND/MAINT_SERVICE).
• Read-only unless combined with BOPF; replaced by RAP.

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▶ 4. RAP

• Data model: CDS view entities (root + child with composition).
• Behavior definition (BDEF): create, update, delete, validations, determinations, actions.
• Behavior implementation: ABAP class (managed: framework persists; unmanaged: you call BAPIs / legacy code).
• Service definition: expose entities.
• Service binding: OData V2 or V4, UI or Web API; publish locally.

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▶ 5. DECISION GUIDE

• S/4HANA 2020+ new app or API: RAP.
• Existing SEGW service needing a change: stay in SEGW unless a rewrite is planned.
• ECC or old NetWeaver: SEGW.
• Complex legacy logic (BAPIs): RAP unmanaged or SEGW calling BAPIs.

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▶ STEP BY STEP

1. Open your Part 1 SEGW project and list what you had to code.
2. In ADT, look at a released RAP service (e.g. a service binding in your system) and its service definition.
3. Compare: what does RAP give for free (paging, filter, $expand)?
4. Decide for one planned app which model you would choose and why.

✅ CHECKPOINT: you can explain SEGW vs RAP and choose correctly for a given system and requirement.

On an S/4HANA 2021 project, a team started a new approval app in SEGW out of habit. After three weeks they had written filter, paging, draft-like locking and error mapping by hand, and Fiori elements features like the filter bar needed more custom code.
The architect stopped and rebuilt the app in RAP: CDS view entities, a managed behavior definition and an OData V4 UI service binding. The rebuilt service had fewer than a third of the lines of code, and filter, paging and $expand worked out of the box. Existing SEGW services were left untouched.

• Starting new S/4 services in SEGW without considering RAP.
• Using @OData.publish for new services.
• Rewriting stable SEGW services to RAP without a business reason.
• Assuming RAP is only for greenfield tables — unmanaged RAP can wrap BAPIs.
• Mixing SEGW and RAP logic for the same business object.

• New S/4 apps and APIs: RAP.
• Keep SEGW for existing services and older releases.
• Use SEGW referenced data sources when you need CDS-based reads in SEGW.
• Document the chosen model per service.
• Reuse CDS view entities as the data model for both.

Questions: "SEGW or RAP — which do you choose?", "What is @OData.publish?", "What does RAP generate for you?"
Strong answer: SEGW is code-based with MPC/DPC_EXT where you implement CRUD, filter, paging, $expand yourself; RAP uses CDS, behavior definition, service definition and binding (V2/V4) with framework-handled CRUD and query options, and is SAP's strategic model and mandatory in ABAP Cloud. @OData.publish is an obsolete CDS shortcut. Choose RAP for new S/4 work, SEGW for existing services or old releases.