services/domain/Big.Api/Program.cs

@@ -20,6 +20,7 @@ builder.Services.AddSingleton<IRegistrationStore, InMemoryRegistrationStore>();
 builder.Services.AddHttpClient<FlowableWorkflowClient>();
 builder.Services.AddTransient<IWorkflowClient>(sp => sp.GetRequiredService<FlowableWorkflowClient>());
 builder.Services.AddTransient<IExternalWorkerClient>(sp => sp.GetRequiredService<FlowableWorkflowClient>());
+builder.Services.AddTransient<IUserTaskClient>(sp => sp.GetRequiredService<FlowableWorkflowClient>());
 builder.Services.AddHttpClient<IAclClient, AclHttpClient>();
 
 builder.Services.AddScoped<SubmitRegistration>();

services/domain/Big.Application/Ports.cs

@@ -36,6 +36,30 @@ public interface IAclClient
     Task ApproveZaakAsync(Uri zaakUrl, CancellationToken ct = default);
 }
 
+/// <summary>
+/// The port to the behandelaar's user tasks in the workflow engine (S-12). Implemented by the
+/// Workflow Client — the only code that talks to Flowable (§8.2). The application lists the open
+/// <c>Beoordelen</c> tasks (the werkbak), claims one for a behandelaar, and completes it with the
+/// decision; it never names Flowable's REST shapes.
+/// </summary>
+public interface IUserTaskClient
+{
+    /// <summary>The werkbak: the <c>Beoordelen</c> tasks awaiting a behandelaar, each with the
+    /// registration it belongs to.</summary>
+    Task<IReadOnlyList<BeoordelingTask>> GetOpenBeoordelingenAsync(CancellationToken ct = default);
+
+    /// <summary>Claim a beoordeling task for a behandelaar (assigns it to them).</summary>
+    Task ClaimAsync(string taskId, string behandelaar, CancellationToken ct = default);
+
+    /// <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>A <c>Beoordelen</c> user task in the werkbak: the Flowable task id (needed to claim and
+/// 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
 /// (ADR-0009); an EF-backed store is a documented follow-up, and this port keeps that change additive.

services/domain/Big.Infrastructure/FlowableWorkflowClient.cs

@@ -15,12 +15,14 @@ namespace Big.Infrastructure;
 /// The REST contract here is the one verified against a live flowable-rest engine (ADR-0009).
 /// </summary>
 public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions options)
-    : IWorkflowClient, IExternalWorkerClient
+    : IWorkflowClient, IExternalWorkerClient, IUserTaskClient
 {
     private const string Topic = "OpenZaakAanmaken";
     private const string ProcessDefinitionKey = "registratie";
+    private const string BeoordelenTaskKey = "Beoordelen";
     private const string RegistrationIdVariable = "registrationId";
     private const string ZaakUrlVariable = "zaakUrl";
+    private const string BesluitVariable = "besluit";
 
     public async Task<string> StartRegistrationProcessAsync(RegistrationId registrationId, CancellationToken ct = default)
     {
@@ -55,6 +57,34 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
         response.EnsureSuccessStatusCode();
     }
 
+    public async Task<IReadOnlyList<BeoordelingTask>> GetOpenBeoordelingenAsync(CancellationToken ct = default)
+    {
+        var request = new TaskQueryRequest(ProcessDefinitionKey, BeoordelenTaskKey, IncludeProcessVariables: true);
+
+        var page = await PostAsync<TaskQueryRequest, TaskQueryResult>(
+            "service/runtime/tasks/query", request, ct);
+
+        var tasks = page?.Data ?? [];
+        return [.. tasks.Select(t => new BeoordelingTask(t.Id, RegistrationId.Parse(t.RegistrationId())))];
+    }
+
+    public async Task ClaimAsync(string taskId, string behandelaar, CancellationToken ct = default)
+    {
+        using var response = await SendAsync(
+            $"service/runtime/tasks/{taskId}", new ClaimTaskRequest("claim", behandelaar), ct);
+        response.EnsureSuccessStatusCode();
+    }
+
+    public async Task CompleteBeoordelingAsync(string taskId, BeoordelingsBesluit besluit, CancellationToken ct = default)
+    {
+        var request = new CompleteTaskRequest(
+            "complete",
+            [new Variable(BesluitVariable, "string", besluit.ToString().ToLowerInvariant())]);
+
+        using var response = await SendAsync($"service/runtime/tasks/{taskId}", request, ct);
+        response.EnsureSuccessStatusCode();
+    }
+
     private async Task<TResponse?> PostAsync<TRequest, TResponse>(string path, TRequest body, CancellationToken ct)
     {
         using var response = await SendAsync(path, body, ct);
@@ -89,6 +119,32 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
         [property: JsonPropertyName("workerId")] string WorkerId,
         [property: JsonPropertyName("variables")] IReadOnlyList<Variable> Variables);
 
+    private sealed record TaskQueryRequest(
+        [property: JsonPropertyName("processDefinitionKey")] string ProcessDefinitionKey,
+        [property: JsonPropertyName("taskDefinitionKey")] string TaskDefinitionKey,
+        [property: JsonPropertyName("includeProcessVariables")] bool IncludeProcessVariables);
+
+    private sealed record ClaimTaskRequest(
+        [property: JsonPropertyName("action")] string Action,
+        [property: JsonPropertyName("assignee")] string Assignee);
+
+    private sealed record CompleteTaskRequest(
+        [property: JsonPropertyName("action")] string Action,
+        [property: JsonPropertyName("variables")] IReadOnlyList<Variable> Variables);
+
+    private sealed record TaskQueryResult(
+        [property: JsonPropertyName("data")] IReadOnlyList<UserTaskDto>? Data);
+
+    private sealed record UserTaskDto(
+        [property: JsonPropertyName("id")] string Id,
+        [property: JsonPropertyName("processVariables")] IReadOnlyList<Variable>? ProcessVariables)
+    {
+        /// <summary>The registration id this task carries as a process variable.</summary>
+        public string RegistrationId() =>
+            ProcessVariables?.SingleOrDefault(v => v.Name == "registrationId")?.Value
+            ?? throw new InvalidOperationException($"Beoordelen task {Id} carries no registrationId variable.");
+    }
+
     private sealed record Variable(
         [property: JsonPropertyName("name")] string Name,
         [property: JsonPropertyName("type")] string Type,