Merge branch 'main' into fix/91-local-compose-parity

## What & why

Third sub-slice of **S-11 · Withdrawal (Flow 3)** (#12) — the **owner-scoped BFF withdraw endpoint** (backend). S-11a/b made a withdrawal transition the aggregate and cancel the workflow; this adds the citizen-facing entry point through the BFF, gated to the registration's owner.

- **Domain**: `WithdrawRegistrationCommand` carries the caller's `bsn`; the handler returns a `WithdrawOutcome` and refuses a bsn that doesn't own the registration. Unknown and not-owned are **both 404** (indistinguishable — ownership isn't revealed). `POST /registrations/{id}/withdraw` takes `{bsn}` and maps the outcome (204/404).
- **BFF**: `POST /self-service/registrations/{id}/withdraw` (DigiD-authenticated) forwards the token's `bsn` to the domain and relays 204/404. The BFF authenticates; the domain owner-scopes (an aggregate invariant, not the domain doing auth).
- OpenAPI spec + Angular client regenerated for the new endpoint.
- `run-domain-check.sh` withdrawal step now sends the owner `bsn` (verify-stack).

Refs #12 — the self-service "trek aanvraag in" button + e2e (S-11c-2) closes it.

## Definition of Done

- [x] Linked Gitea issue (#12).
- [x] Failing tests committed before the implementation.
- [x] Implementation makes the tests pass.
- [x] Conventional Commits referencing the issue (`refs #12`).
- [ ] CI green — all Gitea Actions jobs.
- [x] `docker compose up` unaffected.
- [x] No ADR needed (owner-scoping is an aggregate invariant; no boundary change).
- [x] Docs — the user-visible demo note lands with S-11c-2.

## Notes for reviewers

- **Full local gate run before pushing this time** (lessons from #89): `dotnet format --verify-no-changes` clean; `make unit` green — Acl 27, EventSubscriber 19, BFF 30, Acceptance 9, Big 95; `api-client` lint+test green.
- Owner mismatch returns 404 (not 403) so the portal can't be used to probe which references exist.

Reviewed-on: #90

docs/architecture/adr-0014-withdrawal-cancels-the-process.md

@@ -29,11 +29,12 @@ task; the Workflow Client correlates a `RegistratieIngetrokken` message to the t
   Client only delivers the message; it never reaches into Flowable to delete an instance. This keeps
   the workflow's control flow in the workflow (§8.2) and leaves an audit trail in Flowable history
   (the process ended via the ingetrokken path, not a raw delete).
-- **Correlated via the Beoordelen task's execution.** The `WithdrawRegistration` handler already
-  knows the registration; it finds the open `Beoordelen` task for it (the same task-query the werkbak
-  uses, §8.2) and asks the Workflow Client to deliver the withdrawal message to that task's execution
-  (`messageEventReceived`). No separate correlation store is needed — the werkbak task set is the
-  authoritative correlation, exactly as the beoordeling decision reuses it (ADR-0013).
+- **Correlated by the registration's own process instance.** The aggregate records its Flowable
+  process instance id at submit, so the `WithdrawRegistration` handler correlates directly by that
+  id — no task lookup. The Workflow Client asks Flowable for the execution **subscribed to** the
+  `RegistratieIngetrokken` message in that instance and delivers `messageEventReceived` to it.
+  Targeting the subscribed execution (not the user task's execution — a message boundary event's
+  subscription lives on its own execution) is what makes the correlation land.
 - **Best-effort, mirroring the beoordeling.** If no open `Beoordelen` task is found (the process has
   not yet parked there — the `OpenZaakAanmaken` window — or has already ended), the withdrawal still
   stands: the aggregate is INGETROKKEN and the werkbak filters it out regardless (S-11b). We complete

infra/docker-compose.local.yml

@@ -17,7 +17,12 @@
 #
 # Port map (host):
 #   8000 OpenZaak · 8001 Open Notificaties · 8080 BFF · 8090 Flowable REST
-#   8100 ACL · 8180 Keycloak   (all admin: admin / admin — dev only)
+#   8100 ACL · 8130 Domain · 8180 Keycloak   (all admin: admin / admin — dev only)
+#   8140 self-service portal · 8141 openbaar register · 8142 behandel portal
+#
+# Portal OIDC on the HOST: browse the portals at their 8140/8141/8142 ports and log in via
+# Keycloak on localhost:8180 (KC_HOSTNAME below pins the issuer there; the BFF still validates
+# in-network via keycloak:8080). Test users are in docs/synthetic-data.md.
 
 services:
 
@@ -205,6 +210,12 @@ services:
       KEYCLOAK_ADMIN_PASSWORD: admin
       KC_HEALTH_ENABLED: "true"
       KC_HTTP_ENABLED: "true"
+      # Pin the frontend/issuer URL to the host-published address so a browser on the host and the
+      # tokens it gets both use localhost:8180. KC_HOSTNAME_BACKCHANNEL_DYNAMIC lets in-network
+      # callers (the BFF via keycloak:8080) still resolve token/jwks endpoints to their request host,
+      # so the BFF validates the localhost:8180 issuer while fetching keys over the compose network.
+      KC_HOSTNAME: http://localhost:8180
+      KC_HOSTNAME_BACKCHANNEL_DYNAMIC: "true"
     ports:
       - "8180:8080"
     volumes:
@@ -295,6 +306,14 @@ services:
       context: ../services/bff
       dockerfile: Dockerfile
     image: register-referentie/bff:dev
+    environment:
+      # Reach Keycloak over the compose network for metadata/keys; the discovered issuer is the
+      # host-pinned localhost:8180 (KC_HOSTNAME above), which is what browser tokens carry — so
+      # validation matches without the BFF ever needing to resolve localhost:8180 itself.
+      Keycloak__Authority: http://keycloak:8080/realms/digid
+      Keycloak__MedewerkerAuthority: http://keycloak:8080/realms/medewerker
+      Downstream__Domain__BaseUrl: http://domain:8080/
+      Downstream__Projection__BaseUrl: http://projection-api:8080/
     ports:
       - "8080:8080"
     healthcheck:
@@ -303,6 +322,39 @@ services:
       timeout: 3s
       retries: 5
       start_period: 10s
+    depends_on:
+      domain:
+        condition: service_healthy
+      projection-api:
+        condition: service_healthy
+      keycloak:
+        condition: service_started
+    networks: [cg]
+
+  # ── BIG Domain Service (S-05) ─────────────────────────────────────────────
+  domain:
+    build:
+      context: ../services/domain
+      dockerfile: Dockerfile
+    image: register-referentie/domain:dev
+    environment:
+      Flowable__BaseUrl: http://flowable-rest:8080/flowable-rest/
+      Flowable__Username: rest-admin
+      Flowable__Password: test
+      Acl__BaseUrl: http://acl:8080/
+    ports:
+      - "8130:8080"
+    healthcheck:
+      test: ["CMD", "curl", "-fsS", "http://localhost:8080/health"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+      start_period: 10s
+    depends_on:
+      acl:
+        condition: service_healthy
+      flowable-init:
+        condition: service_completed_successfully
     networks: [cg]
 
   # ── Read projection (S-06) ────────────────────────────────────────────────
@@ -362,6 +414,73 @@ services:
         condition: service_healthy
     networks: [cg]
 
+  # ── Portals (S-08/S-09/S-12) ──────────────────────────────────────────────
+  # nginx serves each Angular app and reverse-proxies its endpoint group to the BFF (same-origin).
+  # The images bake config.json with the compose authority (keycloak:8080), which a HOST browser
+  # can't resolve — so here we bind-mount a config.json pointing at the host-published localhost:8180
+  # (matching KC_HOSTNAME). openbaar is anonymous and needs no config.
+  self-service:
+    build:
+      context: ..
+      dockerfile: apps/self-service/Dockerfile
+    image: register-referentie/self-service:dev
+    ports:
+      - "8140:80"
+    volumes:
+      - ./local-config/self-service.config.json:/usr/share/nginx/html/config.json:ro,z
+    healthcheck:
+      test: ["CMD-SHELL", "wget -q -O /dev/null http://127.0.0.1/ || exit 1"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+      start_period: 10s
+    depends_on:
+      bff:
+        condition: service_healthy
+      keycloak:
+        condition: service_started
+    networks: [cg]
+
+  openbaar:
+    build:
+      context: ..
+      dockerfile: apps/openbaar/Dockerfile
+    image: register-referentie/openbaar:dev
+    ports:
+      - "8141:80"
+    healthcheck:
+      test: ["CMD-SHELL", "wget -q -O /dev/null http://127.0.0.1/ || exit 1"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+      start_period: 10s
+    depends_on:
+      bff:
+        condition: service_healthy
+    networks: [cg]
+
+  behandel:
+    build:
+      context: ..
+      dockerfile: apps/behandel/Dockerfile
+    image: register-referentie/behandel:dev
+    ports:
+      - "8142:80"
+    volumes:
+      - ./local-config/behandel.config.json:/usr/share/nginx/html/config.json:ro,z
+    healthcheck:
+      test: ["CMD-SHELL", "wget -q -O /dev/null http://127.0.0.1/ || exit 1"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+      start_period: 10s
+    depends_on:
+      bff:
+        condition: service_healthy
+      keycloak:
+        condition: service_started
+    networks: [cg]
+
 volumes:
   oz-db:
   nrc-db:

infra/local-config/behandel.config.json

@@ -0,0 +1,3 @@
+{
+  "authority": "http://localhost:8180/realms/medewerker"
+}

infra/local-config/self-service.config.json

@@ -0,0 +1,3 @@
+{
+  "authority": "http://localhost:8180/realms/digid"
+}

infra/run-domain-check.sh

@@ -142,8 +142,10 @@ done
 [ -n "$task_id2" ] || { echo "FAIL — no Beoordelen task appeared for registration $reg_id2" >&2; docker logs "$dom" 2>&1 | tail -15 >&2; exit 1; }
 echo ">> Beoordelen task $task_id2 is waiting; withdrawing the registration via the domain"
 
+# Owner-scoped: the withdraw carries the same bsn the registration was submitted with (S-11c).
 docker run --rm --network "$net" curlimages/curl:latest \
-  -fsS -X POST "http://$dom_ip:8080/registrations/$reg_id2/withdraw" >/dev/null
+  -fsS -X POST "http://$dom_ip:8080/registrations/$reg_id2/withdraw" \
+  -H 'Content-Type: application/json' -d '{"bsn":"123456782"}' >/dev/null
 
 echo ">> asserting the process was cancelled (no Beoordelen task remains for the registration)"
 gone=""

libs/api-client/src/lib/generated/bff-api.ts

@@ -192,6 +192,40 @@ export class BffApiV1Service {
     );
   }
 
+ postSelfServiceRegistrationsIdWithdraw<TData = void>(id: string, options?: HttpClientBodyOptions): Observable<TData>;
+ postSelfServiceRegistrationsIdWithdraw<TData = void>(id: string, options?: HttpClientEventOptions): Observable<HttpEvent<TData>>;
+ postSelfServiceRegistrationsIdWithdraw<TData = void>(id: string, options?: HttpClientResponseOptions): Observable<AngularHttpResponse<TData>>;
+  postSelfServiceRegistrationsIdWithdraw<TData = void>(
+    id: string, options?: HttpClientObserveOptions): Observable<TData | HttpEvent<TData> | AngularHttpResponse<TData>> {
+    if (options?.observe === 'events') {
+      return this.http.post<TData>(
+      `/self-service/registrations/${id}/withdraw`,
+      undefined,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'events',
+      }
+    );
+    }
+
+    if (options?.observe === 'response') {
+      return this.http.post<TData>(
+      `/self-service/registrations/${id}/withdraw`,
+      undefined,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'response',
+      }
+    );
+    }
+
+    return this.http.post<TData>(
+      `/self-service/registrations/${id}/withdraw`,
+      undefined,{
+        ...(options as Omit<NonNullable<typeof options>, 'observe'>),
+        observe: 'body',
+      }
+    );
+  }
+
  getOpenbaarRegister<TData = OpenbaarEntry[]>(params?: GetOpenbaarRegisterParams, options?: HttpClientBodyOptions): Observable<TData>;
  getOpenbaarRegister<TData = OpenbaarEntry[]>(params?: GetOpenbaarRegisterParams, options?: HttpClientEventOptions): Observable<HttpEvent<TData>>;
  getOpenbaarRegister<TData = OpenbaarEntry[]>(params?: GetOpenbaarRegisterParams, options?: HttpClientResponseOptions): Observable<AngularHttpResponse<TData>>;

services/bff/Bff.Api/DownstreamClients.cs

@@ -22,6 +22,11 @@ public interface IDomainClient
 {
     Task<SubmitAccepted> SubmitRegistrationAsync(string bsn, CancellationToken ct = default);
 
+    /// <summary>Withdraw the caller's own registration ("trek aanvraag in"). Owner-scoped by
+    /// <paramref name="bsn"/>. Returns <c>false</c> when the domain reports the registration is
+    /// unknown or not the caller's (404), so the BFF can relay a 404 rather than a 500.</summary>
+    Task<bool> WithdrawRegistrationAsync(string registrationId, string bsn, CancellationToken ct = default);
+
     /// <summary>The behandelaar's werkbak — registrations awaiting beoordeling.</summary>
     Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default);
 
@@ -47,6 +52,17 @@ public sealed class DomainClient(HttpClient http) : IDomainClient
         return new SubmitAccepted(dto.RegistrationId, dto.Status);
     }
 
+    public async Task<bool> WithdrawRegistrationAsync(string registrationId, string bsn, CancellationToken ct = default)
+    {
+        using var response = await http.PostAsJsonAsync(
+            $"registrations/{registrationId}/withdraw", new { bsn }, ct);
+        // The domain 404s an unknown or not-owned registration; relay that rather than fail hard.
+        if (response.StatusCode == System.Net.HttpStatusCode.NotFound)
+            return false;
+        response.EnsureSuccessStatusCode();
+        return true;
+    }
+
     public async Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)
         => await http.GetFromJsonAsync<List<WerkbakItem>>("behandel/werkbak", ct) ?? [];
 

services/bff/Bff.Api/Program.cs

@@ -86,6 +86,24 @@ app.MapPost("/self-service/registrations", async (ClaimsPrincipal user, IDomainC
     .Produces(StatusCodes.Status400BadRequest)
     .Produces(StatusCodes.Status401Unauthorized);
 
+// Self-service withdrawal (S-11): the signed-in zorgprofessional withdraws their own registration.
+// The bsn comes from the DigiD token and is forwarded to the domain, which owner-scopes the action;
+// a registration that is unknown or not the caller's comes back 404 (ownership is not revealed).
+app.MapPost("/self-service/registrations/{id}/withdraw", async (string id, ClaimsPrincipal user, IDomainClient domain, CancellationToken ct) =>
+{
+    var bsn = user.FindFirstValue("bsn");
+    if (string.IsNullOrWhiteSpace(bsn))
+        return Results.BadRequest("The token carries no bsn claim.");
+
+    var withdrawn = await domain.WithdrawRegistrationAsync(id, bsn, ct);
+    return withdrawn ? Results.NoContent() : Results.NotFound();
+})
+    .RequireAuthorization()
+    .Produces(StatusCodes.Status204NoContent)
+    .Produces(StatusCodes.Status400BadRequest)
+    .Produces(StatusCodes.Status401Unauthorized)
+    .Produces(StatusCodes.Status404NotFound);
+
 // Openbaar register: an anonymous public lookup that exposes only public-safe fields (S-09).
 app.MapGet("/openbaar/register", async (string? q, IProjectionClient projection, CancellationToken ct) =>
 {

services/bff/Bff.Tests/BffFactory.cs

@@ -82,6 +82,18 @@ internal sealed class FakeDomainClient : IDomainClient
         return Task.FromResult(Result);
     }
 
+    public (string RegistrationId, string Bsn)? Withdrawn { get; private set; }
+
+    /// <summary>Whether the fake domain reports the withdrawal as done (true → 204) or not-found/not-owned
+    /// (false → 404). Tests set this to exercise the relay.</summary>
+    public bool WithdrawSucceeds { get; set; } = true;
+
+    public Task<bool> WithdrawRegistrationAsync(string registrationId, string bsn, CancellationToken ct = default)
+    {
+        Withdrawn = (registrationId, bsn);
+        return Task.FromResult(WithdrawSucceeds);
+    }
+
     public (string RegistrationId, string Besluit)? Decided { get; private set; }
 
     public Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)

services/bff/Bff.Tests/SelfServiceEndpointTests.cs

@@ -71,5 +71,46 @@ public class SelfServiceEndpointTests
         Assert.Equal("reg-123", body!.RegistrationId);
     }
 
+    private static HttpRequestMessage Withdraw(string? bearer, string id = "reg-123")
+    {
+        var request = new HttpRequestMessage(HttpMethod.Post, $"/self-service/registrations/{id}/withdraw");
+        if (bearer is not null)
+            request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", bearer);
+        return request;
+    }
+
+    [Fact]
+    public async Task Rejects_a_withdrawal_without_a_token()
+    {
+        using var factory = new BffFactory();
+
+        var response = await factory.CreateClient().SendAsync(Withdraw(bearer: null));
+
+        Assert.Equal(HttpStatusCode.Unauthorized, response.StatusCode);
+        Assert.Null(factory.Domain.Withdrawn);
+    }
+
+    [Fact]
+    public async Task Withdraws_the_callers_registration_forwarding_the_id_and_bsn()
+    {
+        using var factory = new BffFactory();
+
+        var response = await factory.CreateClient().SendAsync(Withdraw(TestTokens.Valid("123456782"), "reg-9"));
+
+        Assert.Equal(HttpStatusCode.NoContent, response.StatusCode);
+        Assert.Equal(("reg-9", "123456782"), factory.Domain.Withdrawn);
+    }
+
+    [Fact]
+    public async Task Relays_not_found_when_the_registration_is_unknown_or_not_the_callers()
+    {
+        using var factory = new BffFactory();
+        factory.Domain.WithdrawSucceeds = false;
+
+        var response = await factory.CreateClient().SendAsync(Withdraw(TestTokens.Valid("123456782")));
+
+        Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);
+    }
+
     private sealed record SubmitAcceptedDto(string RegistrationId, string Status);
 }

services/bff/openapi.json

@@ -30,6 +30,37 @@
         }
       }
     },
+    "/self-service/registrations/{id}/withdraw": {
+      "post": {
+        "tags": [
+          "Bff.Api"
+        ],
+        "parameters": [
+          {
+            "name": "id",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "204": {
+            "description": "No Content"
+          },
+          "400": {
+            "description": "Bad Request"
+          },
+          "401": {
+            "description": "Unauthorized"
+          },
+          "404": {
+            "description": "Not Found"
+          }
+        }
+      }
+    },
     "/openbaar/register": {
       "get": {
         "tags": [

services/domain/Big.Api/Program.cs

@@ -76,16 +76,19 @@ app.MapPost("/registrations/{id}/decide", async (string id, DecideRequest body,
 });
 
 // Withdraw a registration (S-11): the zorgprofessional pulls their own still-open submission back,
-// advancing it to INGETROKKEN. Idempotent. The BFF reaches this behind a digid token, owner-scoped
-// to the caller's bsn (S-11c); the domain trusts its callers (§8.3). Cancelling the running Flowable
-// process is a later sub-slice (S-11b).
-app.MapPost("/registrations/{id}/withdraw", async (string id, WithdrawRegistration withdraw, CancellationToken ct) =>
+// advancing it to INGETROKKEN and cancelling its workflow. Owner-scoped by the caller's bsn (the BFF
+// forwards it from the DigiD token, S-11c); a registration that is unknown or not the caller's is
+// 404 (indistinguishable, so ownership isn't leaked). Idempotent.
+app.MapPost("/registrations/{id}/withdraw", async (string id, WithdrawRequest body, WithdrawRegistration withdraw, CancellationToken ct) =>
 {
     if (!Guid.TryParse(id, out var guid))
         return Results.NotFound();
 
-    await withdraw.HandleAsync(new WithdrawRegistrationCommand(new RegistrationId(guid)), ct);
-    return Results.NoContent();
+    if (string.IsNullOrWhiteSpace(body?.Bsn))
+        return Results.BadRequest(new { error = "A bsn is required to withdraw a registration." });
+
+    var outcome = await withdraw.HandleAsync(new WithdrawRegistrationCommand(new RegistrationId(guid), body.Bsn), ct);
+    return outcome == WithdrawOutcome.Withdrawn ? Results.NoContent() : Results.NotFound();
 });
 
 // The behandelaar's werkbak (S-12): the registrations awaiting beoordeling, read from the open
@@ -113,6 +116,8 @@ public sealed record SubmitRegistrationRequest(string Bsn);
 
 public sealed record DecideRequest(string Besluit);
 
+public sealed record WithdrawRequest(string Bsn);
+
 public sealed record RegistrationResponse(string RegistrationId, string Status, string? ZaakUrl);
 
 public partial class Program;

services/domain/Big.Application/Ports.cs

@@ -15,6 +15,14 @@ public interface IWorkflowClient
     /// aggregate. Returns the process instance id.
     /// </summary>
     Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Cancel a running <c>registratie</c> process on withdrawal (S-11): correlate the
+    /// <c>RegistratieIngetrokken</c> message to the instance, tripping the interrupting message event
+    /// that ends it (ADR-0014). Best-effort — if the instance is not waiting on that message (already
+    /// ended, or not yet parked) it is a no-op; the aggregate is INGETROKKEN regardless.
+    /// </summary>
+    Task WithdrawProcessAsync(string processInstanceId, CancellationToken ct = default);
 }
 
 /// <summary>
@@ -54,18 +62,11 @@ public interface IUserTaskClient
     /// <summary>Complete a beoordeling task, carrying the decision into the process as the
     /// <c>besluit</c> variable so the workflow can continue on the chosen branch.</summary>
     Task CompleteBeoordelingAsync(string taskId, BeoordelingsBesluit besluit, CancellationToken ct = default);
-
-    /// <summary>Deliver the withdrawal message to a <c>Beoordelen</c> task's execution, tripping the
-    /// process's interrupting message boundary event so the registratie process cancels (S-11,
-    /// ADR-0014). The registration itself is already INGETROKKEN in the domain; this ends its
-    /// workflow so the case leaves the werkbak.</summary>
-    Task WithdrawBeoordelingAsync(string executionId, CancellationToken ct = default);
 }
 
 /// <summary>A <c>Beoordelen</c> user task in the werkbak: the Flowable task id (needed to claim and
-/// complete it), the execution it runs in (needed to deliver the withdrawal message to its boundary
-/// event), and the registration it carries as a process variable.</summary>
-public sealed record BeoordelingTask(string TaskId, string ExecutionId, RegistrationId RegistrationId);
+/// complete it) and the registration it carries as a process variable.</summary>
+public sealed record BeoordelingTask(string TaskId, RegistrationId RegistrationId);
 
 /// <summary>
 /// Persistence port for the <see cref="Registration"/> aggregate. In-memory for the minimal slice

services/domain/Big.Application/WithdrawRegistration.cs

@@ -2,43 +2,54 @@ using Big.Domain;
 
 namespace Big.Application;
 
-/// <summary>A zorgprofessional's request to withdraw their own registration ("trek aanvraag in").</summary>
-public sealed record WithdrawRegistrationCommand(RegistrationId RegistrationId);
+/// <summary>A zorgprofessional's request to withdraw their own registration ("trek aanvraag in").
+/// <paramref name="Bsn"/> is the authenticated caller (from the DigiD token, forwarded by the BFF):
+/// only the registration's own bsn may withdraw it.</summary>
+public sealed record WithdrawRegistrationCommand(RegistrationId RegistrationId, string Bsn);
+
+/// <summary>The outcome of a withdrawal request.</summary>
+public enum WithdrawOutcome
+{
+    /// <summary>The registration is now (or already was) INGETROKKEN.</summary>
+    Withdrawn,
+
+    /// <summary>No registration with that id belongs to the caller — unknown, or owned by someone
+    /// else (the two are deliberately indistinguishable, so the endpoint reveals neither).</summary>
+    NotFound,
+}
 
 /// <summary>
 /// The withdrawal use case (S-11): a zorgprofessional pulls a still-open registration back. It
 /// advances the aggregate to INGETROKKEN, persists it, then cancels the running registratie process
-/// by delivering the withdrawal message to its open <c>Beoordelen</c> task (ADR-0014), so the case
-/// leaves the behandelaar's werkbak. Idempotent — a repeated or redelivered withdrawal of an
-/// already-withdrawn registration is a no-op (not persisted or cancelled again). Cancelling is
-/// best-effort: if no <c>Beoordelen</c> task is open (the process has not parked there yet, or has
-/// already ended) the withdrawal still stands — mirroring <see cref="BeoordeelRegistratie"/>.
+/// by correlating the withdrawal message to its instance (ADR-0014), so the case leaves the
+/// behandelaar's werkbak. Idempotent — a repeated or redelivered withdrawal of an already-withdrawn
+/// registration is a no-op (not persisted or cancelled again). Cancelling is best-effort: if the
+/// registration never started a process the withdrawal still stands (the process cancel is skipped),
+/// mirroring how <see cref="BeoordeelRegistratie"/> completes its task best-effort.
 /// </summary>
-public sealed class WithdrawRegistration(IRegistrationStore store, IUserTaskClient tasks)
+public sealed class WithdrawRegistration(IRegistrationStore store, IWorkflowClient workflow)
 {
-    public async Task HandleAsync(WithdrawRegistrationCommand command, CancellationToken ct = default)
+    public async Task<WithdrawOutcome> HandleAsync(WithdrawRegistrationCommand command, CancellationToken ct = default)
     {
         ArgumentNullException.ThrowIfNull(command);
 
-        var registration = await store.GetAsync(command.RegistrationId, ct)
-            ?? throw new InvalidOperationException($"No registration {command.RegistrationId} to withdraw.");
+        var registration = await store.GetAsync(command.RegistrationId, ct);
+
+        // Unknown, or not the caller's registration: report NotFound either way (don't reveal which).
+        if (registration is null || registration.Bsn != command.Bsn)
+            return WithdrawOutcome.NotFound;
 
         // A repeated withdrawal is a no-op: don't persist or cancel the already-withdrawn one again.
         if (registration.Status == RegistrationStatus.Ingetrokken)
-            return;
+            return WithdrawOutcome.Withdrawn;
 
         registration.Withdraw();
         await store.SaveAsync(registration, ct);
-        await CancelWorkflowTaskAsync(command.RegistrationId, ct);
-    }
 
-    // Cancel the workflow: deliver the withdrawal message to the open Beoordelen task's execution so
-    // the process's boundary event ends it. If none is open the withdrawal still stands.
-    private async Task CancelWorkflowTaskAsync(RegistrationId registrationId, CancellationToken ct)
-    {
-        var open = await tasks.GetOpenBeoordelingenAsync(ct);
-        var task = open.FirstOrDefault(t => t.RegistrationId == registrationId);
-        if (task is not null)
-            await tasks.WithdrawBeoordelingAsync(task.ExecutionId, ct);
+        // Cancel the running process (if one was started) so its Beoordelen task leaves the werkbak.
+        if (registration.ProcessInstanceId is not null)
+            await workflow.WithdrawProcessAsync(registration.ProcessInstanceId, ct);
+
+        return WithdrawOutcome.Withdrawn;
     }
 }

services/domain/Big.Infrastructure/FlowableWorkflowClient.cs

@@ -66,7 +66,7 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
             "service/query/tasks", request, ct);
 
         var tasks = page?.Data ?? [];
-        return [.. tasks.Select(t => new BeoordelingTask(t.Id, t.ExecutionId, RegistrationId.Parse(t.RegistrationId())))];
+        return [.. tasks.Select(t => new BeoordelingTask(t.Id, RegistrationId.Parse(t.RegistrationId())))];
     }
 
     public async Task ClaimAsync(string taskId, string behandelaar, CancellationToken ct = default)
@@ -86,18 +86,35 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
         response.EnsureSuccessStatusCode();
     }
 
-    public async Task WithdrawBeoordelingAsync(string executionId, CancellationToken ct = default)
+    public async Task WithdrawProcessAsync(string processInstanceId, CancellationToken ct = default)
     {
-        // Deliver the withdrawal message to the Beoordelen task's execution, tripping the process's
-        // interrupting message boundary event so the registratie instance ends (ADR-0014). Flowable
-        // takes messageEventReceived as a PUT on the subscribed execution.
-        var request = new MessageEventRequest("messageEventReceived", IngetrokkenMessage);
+        // Correlate the withdrawal message to the instance: find the execution subscribed to it (the
+        // interrupting message event's own execution — NOT the user task's), then deliver
+        // messageEventReceived to that execution so the process ends (ADR-0014). If nothing is
+        // subscribed (the process is not parked at Beoordelen) this is a best-effort no-op.
+        var subscribed = await GetAsync<ExecutionQueryResult>(
+            $"service/runtime/executions?messageEventSubscriptionName={IngetrokkenMessage}&processInstanceId={processInstanceId}",
+            ct);
 
+        var execution = subscribed?.Data?.FirstOrDefault();
+        if (execution is null)
+            return;
+
+        var request = new MessageEventRequest("messageEventReceived", IngetrokkenMessage);
         using var response = await SendAsync(
-            $"service/runtime/executions/{executionId}", request, ct, HttpMethod.Put);
+            $"service/runtime/executions/{execution.Id}", request, ct, HttpMethod.Put);
         response.EnsureSuccessStatusCode();
     }
 
+    private async Task<TResponse?> GetAsync<TResponse>(string path, CancellationToken ct)
+    {
+        var message = new HttpRequestMessage(HttpMethod.Get, new Uri(options.BaseUrl, path));
+        message.Headers.Authorization = new AuthenticationHeaderValue("Basic", BasicCredentials());
+        using var response = await http.SendAsync(message, ct);
+        response.EnsureSuccessStatusCode();
+        return await response.Content.ReadFromJsonAsync<TResponse>(ct);
+    }
+
     private async Task<TResponse?> PostAsync<TRequest, TResponse>(string path, TRequest body, CancellationToken ct)
     {
         using var response = await SendAsync(path, body, ct);
@@ -154,7 +171,6 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
 
     private sealed record UserTaskDto(
         [property: JsonPropertyName("id")] string Id,
-        [property: JsonPropertyName("executionId")] string ExecutionId,
         // Flowable's task-query returns the (included) process variables under "variables", not
         // "processVariables"; the request opts in via includeProcessVariables.
         [property: JsonPropertyName("variables")] IReadOnlyList<Variable>? Variables)
@@ -172,6 +188,11 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
 
     private sealed record ProcessInstance([property: JsonPropertyName("id")] string Id);
 
+    private sealed record ExecutionQueryResult(
+        [property: JsonPropertyName("data")] IReadOnlyList<ExecutionDto>? Data);
+
+    private sealed record ExecutionDto([property: JsonPropertyName("id")] string Id);
+
     private sealed record AcquiredJob(
         [property: JsonPropertyName("id")] string Id,
         [property: JsonPropertyName("variables")] IReadOnlyList<Variable> Variables)

services/domain/Big.Tests/BeoordeelRegistratieTests.cs

@@ -19,7 +19,7 @@ public class BeoordeelRegistratieTests
     }
 
     private static FakeUserTaskClient TaskFor(Registration registration) =>
-        new([new BeoordelingTask("task-1", "exec-1", registration.Id)]);
+        new([new BeoordelingTask("task-1", registration.Id)]);
 
     [Fact]
     public async Task Goedkeuren_sets_the_zaak_status_via_the_acl_and_marks_the_registration_ingeschreven()

services/domain/Big.Tests/Fakes.cs

@@ -32,6 +32,7 @@ internal sealed class FakeWorkflowClient(string processInstanceId = "proc-1", Ac
     : IWorkflowClient
 {
     public RegistrationId? StartedFor { get; private set; }
+    public string? WithdrawnProcessInstanceId { get; private set; }
 
     public Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
     {
@@ -39,6 +40,12 @@ internal sealed class FakeWorkflowClient(string processInstanceId = "proc-1", Ac
         StartedFor = registrationId;
         return Task.FromResult(processInstanceId);
     }
+
+    public Task WithdrawProcessAsync(string processInstanceId, CancellationToken ct = default)
+    {
+        WithdrawnProcessInstanceId = processInstanceId;
+        return Task.CompletedTask;
+    }
 }
 
 /// <summary>A fake user-task client for the werkbak/decision use cases: returns a scripted set of
@@ -47,7 +54,6 @@ internal sealed class FakeUserTaskClient(IReadOnlyList<BeoordelingTask> open) :
 {
     public (string TaskId, string Behandelaar)? Claimed { get; private set; }
     public (string TaskId, BeoordelingsBesluit Besluit)? Completed { get; private set; }
-    public string? WithdrawnExecutionId { get; private set; }
 
     public Task<IReadOnlyList<BeoordelingTask>> GetOpenBeoordelingenAsync(CancellationToken ct = default)
         => Task.FromResult(open);
@@ -63,12 +69,6 @@ internal sealed class FakeUserTaskClient(IReadOnlyList<BeoordelingTask> open) :
         Completed = (taskId, besluit);
         return Task.CompletedTask;
     }
-
-    public Task WithdrawBeoordelingAsync(string executionId, CancellationToken ct = default)
-    {
-        WithdrawnExecutionId = executionId;
-        return Task.CompletedTask;
-    }
 }
 
 /// <summary>A fake ACL client that records the bsn it was asked to open a zaak for and returns a

services/domain/Big.Tests/FlowableWorkflowClientTests.cs

@@ -158,14 +158,13 @@ public class FlowableWorkflowClientTests
         var capture = new RequestCapture();
         var client = Client(capture.Responds(HttpStatusCode.OK,
             $$"""
-            {"data":[{"id":"task-1","executionId":"exec-1","variables":[{"name":"registrationId","type":"string","value":"{{rid}}"}]}],"total":1}
+            {"data":[{"id":"task-1","variables":[{"name":"registrationId","type":"string","value":"{{rid}}"}]}],"total":1}
             """));
 
         var tasks = await client.GetOpenBeoordelingenAsync();
 
         var task = Assert.Single(tasks);
         Assert.Equal("task-1", task.TaskId);
-        Assert.Equal("exec-1", task.ExecutionId);
         Assert.Equal(rid, task.RegistrationId);
         Assert.Equal(HttpMethod.Post, capture.Seen!.Method);
         Assert.Equal("http://flowable/flowable-rest/service/query/tasks",
@@ -244,29 +243,58 @@ public class FlowableWorkflowClientTests
     }
 
     [Fact]
-    public async Task Withdraw_beoordeling_delivers_the_message_to_the_task_execution()
+    public async Task Withdraw_process_correlates_the_message_to_the_subscribed_execution()
     {
-        var capture = new RequestCapture();
-        var client = Client(capture.Responds(HttpStatusCode.OK));
+        HttpRequestMessage? getReq = null;
+        HttpRequestMessage? putReq = null;
+        string? putBody = null;
+        var client = Client(new StubHandler(async req =>
+        {
+            if (req.Method == HttpMethod.Get)
+            {
+                getReq = req;
+                return new HttpResponseMessage(HttpStatusCode.OK)
+                {
+                    Content = new StringContent("""{"data":[{"id":"exec-9"}]}""", Encoding.UTF8, "application/json"),
+                };
+            }
+            putReq = req;
+            putBody = await req.Content!.ReadAsStringAsync();
+            return new HttpResponseMessage(HttpStatusCode.OK);
+        }));
 
-        await client.WithdrawBeoordelingAsync("exec-1");
+        await client.WithdrawProcessAsync("pi-1");
 
-        // A PUT messageEventReceived on the execution trips the Beoordelen boundary event (ADR-0014).
-        Assert.Equal(HttpMethod.Put, capture.Seen!.Method);
-        Assert.Equal("http://flowable/flowable-rest/service/runtime/executions/exec-1",
-            capture.Seen.RequestUri!.ToString());
-        Assert.Equal("rest-admin:test", DecodeBasic(capture.Seen));
-        Assert.Contains("\"action\":\"messageEventReceived\"", capture.Body);
-        Assert.Contains("\"messageName\":\"RegistratieIngetrokken\"", capture.Body);
+        // 1. Find the execution subscribed to the withdrawal message for this instance.
+        Assert.Equal(HttpMethod.Get, getReq!.Method);
+        Assert.Contains("service/runtime/executions", getReq.RequestUri!.ToString());
+        Assert.Contains("messageEventSubscriptionName=RegistratieIngetrokken", getReq.RequestUri!.Query);
+        Assert.Contains("processInstanceId=pi-1", getReq.RequestUri!.Query);
+        // 2. Deliver messageEventReceived to that execution (PUT), tripping the interrupting event.
+        Assert.Equal(HttpMethod.Put, putReq!.Method);
+        Assert.Equal("http://flowable/flowable-rest/service/runtime/executions/exec-9",
+            putReq.RequestUri!.ToString());
+        Assert.Contains("\"action\":\"messageEventReceived\"", putBody);
+        Assert.Contains("\"messageName\":\"RegistratieIngetrokken\"", putBody);
     }
 
     [Fact]
-    public async Task Withdraw_beoordeling_throws_when_flowable_rejects_the_request()
+    public async Task Withdraw_process_is_a_no_op_when_no_execution_is_subscribed()
     {
-        var capture = new RequestCapture();
-        var client = Client(capture.Responds(HttpStatusCode.InternalServerError));
+        var methods = new List<HttpMethod>();
+        var client = Client(new StubHandler(req =>
+        {
+            methods.Add(req.Method);
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = new StringContent("""{"data":[]}""", Encoding.UTF8, "application/json"),
+            });
+        }));
 
-        await Assert.ThrowsAsync<HttpRequestException>(() => client.WithdrawBeoordelingAsync("exec-1"));
+        await client.WithdrawProcessAsync("pi-1");
+
+        // No subscribed execution → no message delivered (best-effort), no throw.
+        Assert.DoesNotContain(HttpMethod.Put, methods);
     }
 
     [Fact]

services/domain/Big.Tests/WerkbakTests.cs

@@ -18,7 +18,7 @@ public class WerkbakTests
         registration.AttachZaak(FakeAclClient.DefaultZaakUrl);
         registration.TakeIntoBehandeling();
         store.Seed(registration);
-        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", "exec-1",registration.Id)]);
+        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", registration.Id)]);
         var werkbak = new Werkbak(tasks, store);
 
         var items = await werkbak.GetAsync();
@@ -41,7 +41,7 @@ public class WerkbakTests
     public async Task Skips_a_task_whose_registration_is_unknown()
     {
         // Defensive: the werkbak never invents an item for a task the domain has no aggregate for.
-        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", "exec-1",RegistrationId.New())]);
+        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", RegistrationId.New())]);
         var werkbak = new Werkbak(tasks, new FakeRegistrationStore());
 
         Assert.Empty(await werkbak.GetAsync());
@@ -56,7 +56,7 @@ public class WerkbakTests
         var registration = Registration.Submit("123456782");
         registration.Withdraw();
         store.Seed(registration);
-        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", "exec-1",registration.Id)]);
+        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", registration.Id)]);
         var werkbak = new Werkbak(tasks, store);
 
         Assert.Empty(await werkbak.GetAsync());

services/domain/Big.Tests/WithdrawRegistrationTests.cs

@@ -5,75 +5,101 @@ namespace Big.Tests;
 
 public class WithdrawRegistrationTests
 {
-    private static FakeUserTaskClient NoTasks() => new([]);
+    private const string Bsn = "123456782";
+
+    private static Registration Submitted(string processInstanceId = "proc-1")
+    {
+        var registration = Registration.Submit(Bsn);
+        registration.RecordProcessStarted(processInstanceId);
+        return registration;
+    }
+
+    private static WithdrawRegistrationCommand Command(RegistrationId id, string bsn = Bsn) => new(id, bsn);
 
     [Fact]
     public async Task Withdrawing_marks_the_registration_ingetrokken_and_persists_it()
     {
         var store = new FakeRegistrationStore();
-        var registration = Registration.Submit("123456782");
+        var registration = Submitted();
         store.Seed(registration);
-        var handler = new WithdrawRegistration(store, NoTasks());
+        var handler = new WithdrawRegistration(store, new FakeWorkflowClient());
 
-        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+        var outcome = await handler.HandleAsync(Command(registration.Id));
 
+        Assert.Equal(WithdrawOutcome.Withdrawn, outcome);
         var saved = await store.GetAsync(registration.Id);
         Assert.Equal(RegistrationStatus.Ingetrokken, saved!.Status);
         Assert.Equal(1, store.SaveCount);
     }
 
     [Fact]
-    public async Task Withdrawing_cancels_the_open_beoordelen_task_via_its_execution()
+    public async Task Withdrawing_cancels_the_running_process()
     {
         var store = new FakeRegistrationStore();
-        var registration = Registration.Submit("123456782");
+        var registration = Submitted("proc-42");
         store.Seed(registration);
-        var tasks = new FakeUserTaskClient([new BeoordelingTask("task-1", "exec-1", registration.Id)]);
-        var handler = new WithdrawRegistration(store, tasks);
+        var workflow = new FakeWorkflowClient();
+        var handler = new WithdrawRegistration(store, workflow);
 
-        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+        await handler.HandleAsync(Command(registration.Id));
 
-        // The withdrawal message is delivered to the Beoordelen task's execution, tripping the
-        // boundary event that ends the process (ADR-0014).
-        Assert.Equal("exec-1", tasks.WithdrawnExecutionId);
+        // The process the registration recorded at submit is cancelled (ADR-0014).
+        Assert.Equal("proc-42", workflow.WithdrawnProcessInstanceId);
     }
 
     [Fact]
-    public async Task Withdrawing_with_no_open_task_still_marks_ingetrokken()
+    public async Task Withdrawing_before_a_process_was_started_still_marks_ingetrokken()
     {
-        // If the process has not parked at Beoordelen yet (or already ended), the withdrawal stands:
-        // the aggregate is INGETROKKEN and nothing is cancelled.
         var store = new FakeRegistrationStore();
-        var registration = Registration.Submit("123456782");
+        var registration = Registration.Submit(Bsn); // no RecordProcessStarted
         store.Seed(registration);
-        var tasks = NoTasks();
-        var handler = new WithdrawRegistration(store, tasks);
+        var workflow = new FakeWorkflowClient();
+        var handler = new WithdrawRegistration(store, workflow);
 
-        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+        await handler.HandleAsync(Command(registration.Id));
 
         Assert.Equal(RegistrationStatus.Ingetrokken, (await store.GetAsync(registration.Id))!.Status);
-        Assert.Null(tasks.WithdrawnExecutionId);
+        Assert.Null(workflow.WithdrawnProcessInstanceId);
+    }
+
+    [Fact]
+    public async Task A_different_bsn_cannot_withdraw_the_registration()
+    {
+        // Owner-scoping: only the zorgprofessional who submitted may withdraw. Another bsn is told
+        // NotFound (we don't reveal the registration exists) and nothing is changed.
+        var store = new FakeRegistrationStore();
+        var registration = Submitted();
+        store.Seed(registration);
+        var workflow = new FakeWorkflowClient();
+        var handler = new WithdrawRegistration(store, workflow);
+
+        var outcome = await handler.HandleAsync(Command(registration.Id, bsn: "999999990"));
+
+        Assert.Equal(WithdrawOutcome.NotFound, outcome);
+        Assert.Equal(RegistrationStatus.Ingediend, (await store.GetAsync(registration.Id))!.Status);
+        Assert.Equal(0, store.SaveCount);
+        Assert.Null(workflow.WithdrawnProcessInstanceId);
     }
 
     [Fact]
     public async Task Rejects_a_null_command_without_touching_the_store()
     {
         var store = new FakeRegistrationStore();
-        var handler = new WithdrawRegistration(store, NoTasks());
+        var handler = new WithdrawRegistration(store, new FakeWorkflowClient());
 
         await Assert.ThrowsAsync<ArgumentNullException>(() => handler.HandleAsync(null!));
         Assert.Equal(0, store.SaveCount);
     }
 
     [Fact]
-    public async Task Withdrawing_an_unknown_registration_throws()
+    public async Task Withdrawing_an_unknown_registration_is_not_found()
     {
         var store = new FakeRegistrationStore();
-        var handler = new WithdrawRegistration(store, NoTasks());
+        var handler = new WithdrawRegistration(store, new FakeWorkflowClient());
 
-        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
-            () => handler.HandleAsync(new WithdrawRegistrationCommand(RegistrationId.New())));
-        Assert.Contains("No registration", ex.Message);
+        var outcome = await handler.HandleAsync(Command(RegistrationId.New()));
+
+        Assert.Equal(WithdrawOutcome.NotFound, outcome);
         Assert.Equal(0, store.SaveCount);
     }
 
@@ -81,14 +107,16 @@ public class WithdrawRegistrationTests
     public async Task Re_withdrawing_an_already_ingetrokken_registration_is_idempotent()
     {
         var store = new FakeRegistrationStore();
-        var registration = Registration.Submit("123456782");
+        var registration = Submitted();
         store.Seed(registration);
-        var handler = new WithdrawRegistration(store, NoTasks());
+        var workflow = new FakeWorkflowClient();
+        var handler = new WithdrawRegistration(store, workflow);
 
-        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
-        await handler.HandleAsync(new WithdrawRegistrationCommand(registration.Id));
+        await handler.HandleAsync(Command(registration.Id));
+        var second = await handler.HandleAsync(Command(registration.Id));
 
-        // The second withdrawal is a no-op: the aggregate is not persisted again.
+        // The second withdrawal is a no-op: still Withdrawn, but the aggregate is not persisted again.
+        Assert.Equal(WithdrawOutcome.Withdrawn, second);
         Assert.Equal(1, store.SaveCount);
         Assert.Equal(RegistrationStatus.Ingetrokken, (await store.GetAsync(registration.Id))!.Status);
     }

tests/acceptance/Support/BffAcceptanceHost.cs

@@ -69,6 +69,9 @@ public sealed class CapturingDomainClient : IDomainClient
         return Task.FromResult(new SubmitAccepted("reg-acc-1", "Ingediend"));
     }
 
+    public Task<bool> WithdrawRegistrationAsync(string registrationId, string bsn, CancellationToken ct = default)
+        => Task.FromResult(true);
+
     public Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)
         => Task.FromResult<IReadOnlyList<WerkbakItem>>([]);
 

tests/acceptance/Support/InMemoryDomainPorts.cs

@@ -11,12 +11,19 @@ public sealed class InMemoryWorkflowClient : IWorkflowClient
     public const string StartedProcessInstanceId = "proc-acc-1";
 
     public RegistrationId? StartedFor { get; private set; }
+    public string? WithdrawnProcessInstanceId { get; private set; }
 
     public Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
     {
         StartedFor = registrationId;
         return Task.FromResult(StartedProcessInstanceId);
     }
+
+    public Task WithdrawProcessAsync(string processInstanceId, CancellationToken ct = default)
+    {
+        WithdrawnProcessInstanceId = processInstanceId;
+        return Task.CompletedTask;
+    }
 }
 
 /// <summary>An in-memory ACL stand-in: records the bsn it opened a zaak for and returns a fixed URL,