test(domain): Registration aggregate invariants (refs #6)

Failing unit tests for the Registration aggregate root: a submission starts
in INGEDIEND carrying its bsn, an empty/whitespace/null bsn is rejected, the
started process-instance id is remembered, and attaching the zaak the ACL
opened records its URL idempotently (a conflicting URL is rejected) while the
status stays INGEDIEND.

The aggregate is a stub (no-op mutators, empty bsn) so the tests compile and
fail on their assertions; the green commit implements the invariants.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/architecture/adr-0009-external-task-job-worker.md

@@ -0,0 +1,88 @@
+# ADR-0009: The Domain Service drives Flowable as an external-task job worker
+
+- **Status:** Accepted
+- **Date:** 2026-06-30
+- **Deciders:** Respellion engineering
+- **Relates to:** S-05 (#6); proposal #60; builds on ADR-0001 (loose coupling, §8.1/§8.2), S-03 (#4, the `registratie` BPMN), S-04 (#5, the ACL `OpenZaak` operation)
+
+## Context
+
+S-05 (#6) adds the **BIG Domain Service**. Submitting a registration must: create a
+`Registration` aggregate, **start the Flowable `registratie` process** (S-03), have the
+`OpenZaakAanmaken` task **open a zaak via the ACL** (S-04), and store the resulting zaak URL
+back on the aggregate.
+
+`OpenZaakAanmaken` is a Flowable **external-worker** service task (`flowable:type="external-worker"`,
+topic `OpenZaakAanmaken`). Flowable does not push it anywhere — it parks the job and waits for a
+worker to **acquire and lock** it, do the work, and **complete** it. Two coupling rules constrain
+who may do what:
+
+- **§8.2 — the Workflow Client is the only code that talks to Flowable.** BPMN models never embed
+  OpenZaak knowledge; they ask the Workflow Client to execute external tasks.
+- **§8.1 — the ACL is the only code that talks to ZGW.** The worker opens the zaak *through the ACL*,
+  never by constructing ZGW URLs itself.
+
+This is an ADR-worthy moment (§14): a service boundary is defined and both coupling rules are
+exercised. The open question is *how* the external task is driven.
+
+## Decision
+
+**The Domain Service drives the `OpenZaakAanmaken` task as a hosted external-task job worker
+(PRD §36). Orchestration is eventually consistent, not request-synchronous.**
+
+- **`POST /registrations` is fast and side-effecting only on the domain side.** It creates the
+  `Registration` aggregate in state `INGEDIEND`, persists it, and asks the Workflow Client to start
+  one `registratie` process instance, recording the process-instance id on the aggregate. It returns
+  immediately; it does **not** wait for the zaak to be opened.
+- **A hosted worker polls Flowable for `OpenZaakAanmaken` jobs.** It acquires and locks a job, calls
+  the ACL `OpenZaak` operation (§8.1), attaches the returned zaak URL to the matching aggregate
+  (`Registration.AttachZaak`), and completes the job in Flowable. The process then runs to its end
+  event.
+- **The Workflow Client is the only Flowable client (§8.2).** It lives in the Domain Service's
+  `Infrastructure` layer and speaks Flowable's REST API (start process-instance; acquire/lock/complete
+  external-worker jobs). No other code — not the Application layer, not the BPMN — knows Flowable
+  exists.
+- **The worker *logic* is an Application service over ports**, not Flowable-aware code. `OpenZaakWorker`
+  takes an acquired job (topic + the registration id it carries), calls `IAclClient` and
+  `IRegistrationStore`, and returns the zaak URL to complete with. The **polling loop** is a thin
+  `BackgroundService` in `Infrastructure` that fetches jobs via the Workflow Client and feeds them to
+  the worker. So the orchestration is covered by fast unit tests against fakes; only the REST framing
+  needs a container integration test.
+
+## Scope decisions for the minimal slice
+
+- **Registration persistence is in-memory.** The walking skeleton's *read* path is fed by
+  NRC → Event Subscriber → projection (S-06, #7), not by the domain database. An EF-backed domain
+  store buys nothing the demo needs yet, so it is a documented follow-up; the `IRegistrationStore`
+  port keeps that change additive. (PRD §88 envisions EF Core for the domain DB eventually.)
+- **The aggregate's state machine is minimal:** `INGEDIEND` on submission. Later flows (withdrawal,
+  beoordeling, herregistratie) add states in their own slices — they are out of scope here.
+- **No bsn flows to ZGW yet.** The ACL `OpenZaak` operation already default-fills the ZGW-mandatory
+  fields (ADR-0003) and takes the bsn as its domain payload; the domain hands it through unchanged.
+
+## Consequences
+
+- **Positive:** the submit request is decoupled from ACL/OpenZaak latency; the documented Common
+  Ground pattern (external-task worker) is realised; both coupling rules (§8.1, §8.2) hold with the
+  Flowable knowledge isolated to one Infrastructure class; the orchestration is unit-testable.
+- **Negative / deferred:**
+  - Eventual consistency: immediately after `POST /registrations` the aggregate has no zaak URL yet.
+    Acceptable — the read side is the projection, not the domain store.
+  - In-memory registration state is lost on restart; fine for the skeleton, replaced by an EF store
+    in a follow-up.
+  - The worker polls (no push); poll interval is a tuning knob, not a correctness concern, since
+    Flowable holds the job until completed.
+
+## Alternatives considered
+
+- **Synchronous acquire+complete inside the `POST /registrations` request** — rejected: simpler and
+  deterministic, but couples the submit request to ACL/OpenZaak latency and failure, and is not the
+  external-task worker pattern PRD §36 mandates. It would also make the request fail if OpenZaak is
+  briefly down, instead of the job simply staying parked for the worker to retry.
+- **A standalone Workflow Client service, separate from the Domain Service** — rejected for this
+  slice: the worker needs the domain's aggregate store and the ACL client anyway, and PRD §9 places
+  the Workflow Client inside the Domain Service deployment. A separate process adds a hop and a
+  shared store for no current benefit.
+- **Flowable pushes to a webhook instead of being polled** — rejected: Flowable's external-worker
+  model is pull-based (acquire/lock/complete); a push shim would re-implement it with weaker
+  delivery guarantees.

mkdocs.yml

@@ -30,6 +30,7 @@ nav:
       - "ADR-0006: ACL integration test provisioning": architecture/adr-0006-integration-test-provisioning.md
       - "ADR-0007: OpenZaak → NRC notification wiring": architecture/adr-0007-notification-wiring.md
       - "ADR-0008: Read projection store": architecture/adr-0008-read-projection-store.md
+      - "ADR-0009: External-task job worker": architecture/adr-0009-external-task-job-worker.md
   - Working in Gitea: gitea-workflow.md
   - Demo script: demo-script.md
   - Runbooks:

register-referentie.slnx

@@ -7,6 +7,10 @@
     <Project Path="services/acl/Acl.IntegrationTests/Acl.IntegrationTests.csproj" />
     <Project Path="services/acl/Acl.Tests/Acl.Tests.csproj" />
   </Folder>
+  <Folder Name="/services/domain/">
+    <Project Path="services/domain/Big.Domain/Big.Domain.csproj" />
+    <Project Path="services/domain/Big.Tests/Big.Tests.csproj" />
+  </Folder>
   <Folder Name="/services/bff/">
     <Project Path="services/bff/Bff.Api/Bff.Api.csproj" />
     <Project Path="services/bff/Bff.Tests/Bff.Tests.csproj" />

services/domain/Big.Domain/Big.Domain.csproj

@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!-- The domain layer (CLAUDE.md §9): aggregates, value objects, invariants.
+       Pure C# — no external dependencies, no infrastructure concerns. -->
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

services/domain/Big.Domain/Registration.cs

@@ -0,0 +1,47 @@
+namespace Big.Domain;
+
+/// <summary>
+/// The Registration aggregate root (CLAUDE.md §2.2): a zorgprofessional's submission to the BIG
+/// register. It owns its lifecycle invariants — it starts <see cref="RegistrationStatus.Ingediend"/>
+/// on submission, remembers the Flowable process that drives it, and records the zaak the ACL opens.
+/// </summary>
+public sealed class Registration
+{
+    // STUB (red): mutators are no-ops and Submit leaves the bsn empty so the behaviour tests
+    // compile and fail on their assertions. The green commit implements the invariants.
+    private Registration(RegistrationId id, string bsn)
+    {
+        Id = id;
+        Bsn = bsn;
+        Status = RegistrationStatus.Ingediend;
+    }
+
+    public RegistrationId Id { get; }
+
+    /// <summary>The citizen-service number of the submitting zorgprofessional. Handed to the ACL
+    /// as the domain payload; the domain never constructs ZGW concepts from it (§8.1).</summary>
+    public string Bsn { get; }
+
+    public RegistrationStatus Status { get; private set; }
+
+    /// <summary>The Flowable process instance driving this registration, once started.</summary>
+    public string? ProcessInstanceId { get; private set; }
+
+    /// <summary>The zaak the ACL opened for this registration, once the external task has run.</summary>
+    public Uri? ZaakUrl { get; private set; }
+
+    /// <summary>Submit a new registration. It begins in <see cref="RegistrationStatus.Ingediend"/>.</summary>
+    public static Registration Submit(string bsn) => new(RegistrationId.New(), string.Empty);
+
+    /// <summary>Record that the registratie workflow process has been started for this registration.</summary>
+    public void RecordProcessStarted(string processInstanceId)
+    {
+        // STUB (red).
+    }
+
+    /// <summary>Attach the zaak the ACL opened. Idempotent on the same URL; a conflicting URL is rejected.</summary>
+    public void AttachZaak(Uri zaakUrl)
+    {
+        // STUB (red).
+    }
+}

services/domain/Big.Domain/RegistrationId.cs

@@ -0,0 +1,15 @@
+namespace Big.Domain;
+
+/// <summary>The identity of a <see cref="Registration"/> aggregate — opaque, server-assigned,
+/// and distinct from the OpenZaak zaak id (which the projection keys on). A value object so the
+/// id can travel as a Flowable process variable and back without ever being a bare string.</summary>
+public readonly record struct RegistrationId(Guid Value)
+{
+    /// <summary>Mint a fresh identity for a newly submitted registration.</summary>
+    public static RegistrationId New() => new(Guid.NewGuid());
+
+    /// <summary>Rehydrate an id carried as a string (e.g. a Flowable process variable).</summary>
+    public static RegistrationId Parse(string value) => new(Guid.Parse(value));
+
+    public override string ToString() => Value.ToString();
+}

services/domain/Big.Domain/RegistrationStatus.cs

@@ -0,0 +1,10 @@
+namespace Big.Domain;
+
+/// <summary>The lifecycle states a <see cref="Registration"/> moves through. The walking
+/// skeleton knows only <see cref="Ingediend"/>; withdrawal, beoordeling and herregistratie
+/// states arrive in their own slices (Iteration 2+).</summary>
+public enum RegistrationStatus
+{
+    /// <summary>Submitted by the zorgprofessional; the registratie process has been started.</summary>
+    Ingediend,
+}

services/domain/Big.Tests/Big.Tests.csproj

@@ -0,0 +1,25 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Big.Domain\Big.Domain.csproj" />
+  </ItemGroup>
+
+</Project>

services/domain/Big.Tests/RegistrationTests.cs

@@ -0,0 +1,77 @@
+using Big.Domain;
+
+namespace Big.Tests;
+
+public class RegistrationTests
+{
+    [Fact]
+    public void Submitting_a_registration_starts_in_ingediend()
+    {
+        var registration = Registration.Submit("123456782");
+
+        Assert.Equal(RegistrationStatus.Ingediend, registration.Status);
+        Assert.Equal("123456782", registration.Bsn);
+        Assert.NotEqual(Guid.Empty, registration.Id.Value);
+        Assert.Null(registration.ZaakUrl);
+        Assert.Null(registration.ProcessInstanceId);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("   ")]
+    [InlineData(null)]
+    public void Submitting_without_a_bsn_is_rejected(string? bsn)
+        => Assert.Throws<ArgumentException>(() => Registration.Submit(bsn!));
+
+    [Fact]
+    public void Recording_the_started_process_keeps_its_instance_id()
+    {
+        var registration = Registration.Submit("123456782");
+
+        registration.RecordProcessStarted("proc-42");
+
+        Assert.Equal("proc-42", registration.ProcessInstanceId);
+    }
+
+    [Fact]
+    public void Attaching_a_zaak_records_its_url_and_keeps_the_status_ingediend()
+    {
+        var registration = Registration.Submit("123456782");
+        var zaak = new Uri("http://openzaak/zaken/api/v1/zaken/abc");
+
+        registration.AttachZaak(zaak);
+
+        Assert.Equal(zaak, registration.ZaakUrl);
+        Assert.Equal(RegistrationStatus.Ingediend, registration.Status);
+    }
+
+    [Fact]
+    public void Attaching_a_null_zaak_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+
+        Assert.Throws<ArgumentNullException>(() => registration.AttachZaak(null!));
+    }
+
+    [Fact]
+    public void Re_attaching_the_same_zaak_is_idempotent()
+    {
+        var registration = Registration.Submit("123456782");
+        var zaak = new Uri("http://openzaak/zaken/api/v1/zaken/abc");
+
+        registration.AttachZaak(zaak);
+        registration.AttachZaak(zaak);
+
+        Assert.Equal(zaak, registration.ZaakUrl);
+    }
+
+    [Fact]
+    public void Attaching_a_conflicting_zaak_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+
+        Assert.Throws<InvalidOperationException>(
+            () => registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/other")));
+    }
+}

services/domain/Big.slnx

@@ -0,0 +1,4 @@
+<Solution>
+  <Project Path="Big.Domain/Big.Domain.csproj" />
+  <Project Path="Big.Tests/Big.Tests.csproj" />
+</Solution>