SAP Gateway Part 3: SEGW Model, Registration, Metadata and Transports

📌 KEY POINTS
✅ SEGW flow: model → generate runtime artifacts (MPC, DPC, *_EXT) → implement DPC_EXT → register → test.
✅ Only change the *_EXT classes; generated base classes are overwritten on regeneration.
✅ Register the service in /IWFND/MAINT_SERVICE (on the hub in hub scenarios) with the right system alias.
⚠️ Transport SEGW project, classes AND the service registration — missing one breaks QAS/PRD.

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▶ WHERE WE ARE
Parts 1–2 explained the runtime. This part shows how a classic SEGW service is built and moved through the landscape. (New services: use RAP; this knowledge is for maintaining existing ones.)

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▶ 1. MODEL IN SEGW

• Create project ZPURCHREQ (package, description).
• Data Model: entity types (PurchaseRequest, PurchaseRequestItem) with properties, keys, and flags (creatable, updatable, filterable, sortable); entity sets; associations and navigation properties; function imports.
• Model from a DDIC structure, from RFC/BAPI, or manually.

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▶ 2. GENERATE RUNTIME ARTIFACTS
"Generate Runtime Objects" creates:

• ZCL_ZPURCHREQ_MPC and ZCL_ZPURCHREQ_MPC_EXT — model provider (metadata).
• ZCL_ZPURCHREQ_DPC and ZCL_ZPURCHREQ_DPC_EXT — data provider (CRUD methods).
• Registration objects: technical model and technical service name (ZPURCHREQ_SRV).

📌 REMEMBER: implement and enhance only in the _EXT classes.\n\n━━━━━━━━━━━━━━━━━━━━\n▶ 3. IMPLEMENT DPC_EXT\nRedefine the methods you need: GET_ENTITYSET, GET_ENTITY, CREATE_ENTITY, UPDATE_ENTITY, DELETE_ENTITY, CREATE_DEEP_ENTITY, EXECUTE_ACTION. Keep business logic in separate classes (reusable, testable); DPC_EXT maps OData ↔ business logic.
MPC_EXT: redefine DEFINE for extra annotations or metadata tweaks.

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▶ 4. REGISTER AND ACTIVATE

• /IWFND/MAINT_SERVICE → Add Service → system alias (LOCAL for embedded) → find ZPURCHREQ_SRV → add with a package and transport.
• This creates the ICF node and the service entry (with version).
• Test with /IWFND/GW_CLIENT: $metadata, then entity sets.

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▶ 5. METADATA AND CHANGES

• Model changes → regenerate → implement → clean metadata cache on the backend and hub.
• Keep property names stable once consumers exist (same contract rules as in RAP Service Definitions Part 3).

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▶ 6. TRANSPORTS

• Workbench transport: SEGW project, MPC/DPC(_EXT) classes, business logic classes.
• The service registration on the gateway system (hub or embedded) is a separate transportable entry created during registration — check it is in a transport.
• After import: verify service active, system alias correct, cache cleaned.

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▶ STEP BY STEP

1. Create a small SEGW project with one entity set.
2. Generate, redefine GET_ENTITYSET in DPC_EXT.
3. Register in /IWFND/MAINT_SERVICE; test in GW client.
4. Add a property; regenerate; clean cache; verify $metadata.

✅ CHECKPOINT: you can build, register and transport a SEGW service safely.

A SEGW service worked in DEV but returned 404 in QAS. The classes and project had been transported, but the service registration created in /IWFND/MAINT_SERVICE was saved as a local object in DEV, so QAS had no service entry.
The team registered the service with a transport, imported it, cleaned the metadata caches and verified the ICF node. They added "registration in a transport?" to the SEGW release checklist.

• Changing generated MPC/DPC base classes instead of _EXT.
• Business logic directly in DPC_EXT.
• Registration saved as local object, not transported.
• Wrong system alias when registering on the hub.
• Forgetting cache cleanup after model changes.
• Renaming properties used by consumers.

• Use SEGW only to maintain or extend existing services; RAP for new ones.
• Implement only in _EXT classes; keep logic in reusable classes.
• Register services with a transport and correct alias.
• Clean metadata caches after changes.
• Keep a release checklist for SEGW transports.

Questions: "Explain the SEGW development process", "What are MPC and DPC classes?", "Why do services fail after transport?"
Strong answer: model entity types/sets/associations in SEGW, generate MPC (metadata) and DPC (data) classes plus _EXT subclasses, implement CRUD by redefining DPC_EXT methods (logic in separate classes), register in /IWFND/MAINT_SERVICE with system alias and transport, test in GW client. Typical transport issues: missing registration, wrong alias, stale caches, inactive ICF node.