# OpenZaak / ZGW integration — how the BFF connects (& how to extend)

How the BFF sources cases from a real **OpenZaak** (ZGW APIs) while the frontend stays
unchanged. For the _why_, see [ADR-0005](architecture/0005-openzaak-behind-bff.md); this page
is _how the seam is built and how to add the next slice_. Built in
[WP-49](../project/backlog/WP-49-openzaak-zaken-read-seam.md) (read-only zaken).

## The one rule: OpenZaak sits behind the BFF, never in the browser

The Angular app only ever sees the BFF's decision DTOs (BFF-lite, ADR-0001). All ZGW
awkwardness — URL-as-identity, cross-service joins, JWT auth, pagination — is absorbed by the
.NET BFF. Flipping the data source from local SQLite to OpenZaak is a **backend config change**
with **zero frontend change and no api-client drift**.

## The seam (data source by config)

- `Data/IZaakSource.cs` — the cases READ interface. Returns the existing
  `ApplicationSummaryDto`, so each implementation owns its own mapping.
- `Data/LocalZaakSource.cs` — **default**; reads the local SQLite `ApplicationStore`
  (offline, unchanged behaviour).
- `Zgw/OpenZaakZaakSource.cs` — the OpenZaak client; selected only when `Zgw:Enabled=true`.
- Wiring (`Program.cs`): `if (Zgw:Enabled) AddHttpClient<IZaakSource, OpenZaakZaakSource>()
else AddSingleton<IZaakSource, LocalZaakSource>()`. The `/admin/cases` endpoint resolves
  `IZaakSource` from DI — routes + DTOs untouched.

## The ZGW client (`backend/src/BigRegister.Api/Zgw/`)

- `ZgwOptions.cs` — bound from the `Zgw` appsettings section: `Enabled`, per-service base URLs
  (`ZrcBaseUrl`, `ZtcBaseUrl`), `ClientId`, `Secret`, `UserId`, `UserRepresentation`. The five
  ZGW APIs are separate base URLs; slice 1 needs only Zaken (ZRC) + Catalogi (ZTC).
- `ZgwTokenProvider.cs` — mints an **HS256 JWT per call** (`iss`/`client_id`/`iat`/`user_id`/
  `user_representation`). No refresh flow — OpenZaak expires tokens 1h past `iat`, so per-call
  minting is the recommended pattern. Hand-rolled (no `Microsoft.IdentityModel.*` dependency).
- `ZgwZaakMapper.cs` — the anti-corruption map: ZGW Zaak → `ApplicationSummaryDto`. This is
  where **URL identity** becomes the trailing uuid and the **zaaktype URL** is resolved to a
  human label (the cross-service join).
- `OpenZaakZaakSource.cs` — follows `{count,next,previous,results}` pagination, resolves +
  caches zaaktype labels, attaches `Authorization: Bearer <jwt>`.

## The five ZGW APIs (context for later slices)

| API          | Component | Used by                                             |
| ------------ | --------- | --------------------------------------------------- |
| Zaken        | ZRC       | slice 1 (read), WP-50 (create)                      |
| Catalogi     | ZTC       | slice 1 (zaaktype label; also type URLs for create) |
| Documenten   | DRC       | WP-51 (upload + zaak↔document link)                 |
| Besluiten    | BRC       | later (formal decisions)                            |
| Notificaties | NRC       | WP-52 (live status via webhooks, not polling)       |

## How to add the next slice

1. **Read** — extend `IZaakSource` (or add a sibling interface, e.g. `IDocumentSource`) with
   the new operation; implement it on both `LocalZaakSource` and the OpenZaak source. Keep the
   return type the existing DTO so the FE never changes.
2. **Write** (create-zaak, WP-50) — a create needs a `zaaktype` URL from Catalogi (OpenZaak
   validates it by fetching), then usually a follow-up `status` + `rol`. Route it through the
   existing submit/mutation seam.
3. **Enforce server-side** for anything the FE gates — a config value the FE echoes is never
   the authority (ADR-0001).

## Coupling

Low and one-directional. Consumer coupling is near zero — `IZaakSource` is injected at one
endpoint, and the FE is fully decoupled by the DTO. The producer side is contained in `Zgw/`:
add a slice by adding a source method + a mapper case, not by touching the FE or the contract.
Watch the **sync-over-async** `ponytail:` note in `OpenZaakZaakSource` — make the cases read
path async if OpenZaak becomes the default.

## Config

```jsonc
// appsettings.json — off by default (POC runs offline on the local store)
"Zgw": {
  "Enabled": true,
  "ZrcBaseUrl": "https://open-zaak.example/zaken/api/v1",
  "ZtcBaseUrl": "https://open-zaak.example/catalogi/api/v1",
  "ClientId": "big-register", "Secret": "<from a secret store>",
  "UserId": "<session user>", "UserRepresentation": "<session name>"
}
```

## Anti-corruption layer — two nested boundaries (what to learn)

This setup is an anti-corruption layer (ACL) **twice over**, and seeing them as a pair is the
lesson worth taking away:

1. **The BFF guards everything against upstream systems.** OpenZaak's foreign model —
   URL-as-identity, a `zaaktype` that is a URL _into another service_, `{count,next,previous,
results}` pagination, HS256 JWT auth — never leaves the BFF. `ZgwZaakMapper` translates it
   into the BFF's own `ApplicationSummaryDto`; `IZaakSource` makes the boundary swappable
   (`LocalZaakSource` vs `OpenZaakZaakSource` return the _same_ DTO).
2. **The Angular app guards itself against the BFF.** `infrastructure/` is the only layer that
   touches the network (lint-enforced); every response crosses a `parse*` (`Result`) trust
   boundary + a `toDomain` mapper before any domain/UI code sees it (ADR-0001, ARCHITECTURE §6).

The DTO at `/api/v1` is the membrane between them — which is why wiring OpenZaak touched **zero
frontend code and produced zero api-client drift**. That was the proof the ACL held.

Principles this demonstrates:

- **An ACL is a _mapping_, not a passthrough.** A DTO that is the upstream shape renamed is
  corruption with extra steps; the valuable ACLs here (`ZgwZaakMapper`, the `parse*`/`toDomain`
  pairs) actively translate a foreign model into a local one.
- **Put the ACL where trust changes, and make it the _only_ place.** One choke point per
  boundary — the `Zgw/` folder + `IZaakSource` server-side, `infrastructure/` client-side.
- **Decision DTOs and the ACL are complementary.** BFF-lite (server decides, FE renders) is an
  ACL against _business-rule_ drift, layered on the ACL against _data-shape_ drift.
- **A real seam is swap-testable offline.** Because the ACL returns a stable DTO, the ZGW
  client is unit-testable with fixtures + a stub handler — no live OpenZaak.
- **Mark the honest edges.** The `ponytail:` sync-over-async note and the "coarse status map"
  comment in `ZgwZaakMapper` show where the ACL is deliberately thin — an ACL need not be
  complete on day one, but its shortcuts should be visible.

Caveat: today only the cases **read** path has a source interface (`IZaakSource`). Other BFF
endpoints still read `SeedData`/static stores directly — ACL-ready (the DTO seam exists) but not
yet swappable. That is the WP-50/51/52 roadmap.

## See also

- [ADR-0005 — OpenZaak behind the BFF](architecture/0005-openzaak-behind-bff.md) — the decision.
- [ADR-0001 — BFF-lite + decision DTOs](architecture/0001-bff-lite-decision-dtos.md) — why the FE doesn't change.
- [WP-49](../project/backlog/WP-49-openzaak-zaken-read-seam.md) (this), WP-50/51/52 (later slices).
- `backend/src/BigRegister.Api/Zgw/` — the client; `Data/IZaakSource.cs` — the seam.
- [ZGW standard (VNG)](https://vng-realisatie.github.io/gemma-zaken/) · [OpenZaak auth docs](https://open-zaak.readthedocs.io/en/stable/client-development/authentication.html).