Added Multi stream projection exercise to introduction to Event Sourcing workshop

EventSourcing.NetCore.slnx

@@ -251,9 +251,15 @@
     <Project Path="Workshops/IntroductionToEventSourcing/12-Projections.SingleStream/12-Projections.SingleStream.csproj" />
     <Project Path="Workshops/IntroductionToEventSourcing/13-Projections.SingleStream.Idempotency/13-Projections.SingleStream.Idempotency.csproj" />
     <Project Path="Workshops/IntroductionToEventSourcing/14-Projections.SingleStream.EventualConsistency/14-Projections.SingleStream.EventualConsistency.csproj" />
-    <Project Path="Workshops/IntroductionToEventSourcing/15-EventsDefinition/15-EventsDefinition.csproj" />
-    <Project Path="Workshops/IntroductionToEventSourcing/16-EntitiesDefinition/16-EntitiesDefinition.csproj" />
-    <Project Path="Workshops/IntroductionToEventSourcing/17-BusinessProcesses/17-BusinessProcesses.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/15-Projections.MultiStream.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/16-Projections.MultiStream.OutOfOrder/16-Projections.MultiStream.OutOfOrder.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/17-Projections.MultiStream.Marten/17-Projections.MultiStream.Marten.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/18-Projections.MultiStream.OutOfOrder.Marten/18-Projections.MultiStream.OutOfOrder.Marten.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/19-EventsDefinition/19-EventsDefinition.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/20-EntitiesDefinition/20-EntitiesDefinition.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/21-BusinessProcesses/21-BusinessProcesses.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/22-Idempotency/22-Idempotency.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/23-Consistency/23-Consistency.csproj" />
   </Folder>
   <Folder Name="/Workshops/IntroductionToEventSourcing/docker/">
     <File Path="Workshops/IntroductionToEventSourcing/docker-compose.yml" />
@@ -278,9 +284,15 @@
     <Project Path="Workshops/IntroductionToEventSourcing/Solved/12-Projections.SingleStream/12-Projections.SingleStream.csproj" />
     <Project Path="Workshops/IntroductionToEventSourcing/Solved/13-Projections.SingleStream.Idempotency/13-Projections.SingleStream.Idempotency.csproj" />
     <Project Path="Workshops/IntroductionToEventSourcing/Solved/14-Projections.SingleStream.EventualConsistency/14-Projections.SingleStream.EventualConsistency.csproj" />
-    <Project Path="Workshops/IntroductionToEventSourcing/Solved/15-EventsDefinition/15-EventsDefinition.csproj" />
-    <Project Path="Workshops/IntroductionToEventSourcing/Solved/16-EntitiesDefinition/16-EntitiesDefinition.csproj" />
-    <Project Path="Workshops/IntroductionToEventSourcing/Solved/17-BusinessProcesses/17-BusinessProcesses.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/Solved/15-Projections.MultiStream/15-Projections.MultiStream.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/Solved/16-Projections.MultiStream.OutOfOrder/16-Projections.MultiStream.OutOfOrder.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/Solved/17-Projections.MultiStream.Marten/17-Projections.MultiStream.Marten.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/Solved/18-Projections.MultiStream.OutOfOrder.Marten/18-Projections.MultiStream.OutOfOrder.Marten.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/Solved/19-EventsDefinition/19-EventsDefinition.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/Solved/20-EntitiesDefinition/20-EntitiesDefinition.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/Solved/21-BusinessProcesses/21-BusinessProcesses.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/Solved/22-Idempotency/22-Idempotency.csproj" />
+    <Project Path="Workshops/IntroductionToEventSourcing/Solved/23-Consistency/23-Consistency.csproj" />
   </Folder>
   <Properties Name="RiderSharedRunConfigurations" Scope="PostLoad" />
 </Solution>

Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/15-Projections.MultiStream.csproj

@@ -0,0 +1,26 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <RootNamespace>IntroductionToEventSourcing.Projections.MultiStream</RootNamespace>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="FluentAssertions" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" />
+        <PackageReference Include="GitHubActionsTestLogger">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
+        <PackageReference Include="xunit" />
+        <PackageReference Include="xunit.runner.visualstudio">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+        </PackageReference>
+        <PackageReference Include="coverlet.collector">
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+            <PrivateAssets>all</PrivateAssets>
+        </PackageReference>
+    </ItemGroup>
+
+</Project>

Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/ProjectionsTests.cs

@@ -0,0 +1,131 @@
+using FluentAssertions;
+using IntroductionToEventSourcing.Projections.MultiStream.Tools;
+using Xunit;
+
+namespace IntroductionToEventSourcing.Projections.MultiStream;
+
+// EVENTS
+public record PaymentRecorded(
+    Guid PaymentId,
+    Guid OrderId,
+    decimal Amount
+);
+
+public record MerchantLimitsChecked(
+    Guid PaymentId,
+    Guid MerchantId,
+    bool IsWithinLimits
+);
+
+public record FraudScoreCalculated(
+    Guid PaymentId,
+    decimal Score,
+    bool IsAcceptable
+);
+
+public record PaymentVerificationCompleted(
+    Guid PaymentId,
+    bool IsApproved
+);
+
+// ENUMS
+public enum VerificationStatus
+{
+    Pending,
+    Passed,
+    Failed
+}
+
+public enum PaymentStatus
+{
+    Pending,
+    Approved,
+    Rejected
+}
+
+// READ MODEL
+public class PaymentVerification
+{
+    public Guid Id { get; set; }
+    public PaymentStatus Status { get; set; }
+}
+
+public class ProjectionsTests
+{
+    [Fact]
+    [Trait("Category", "SkipCI")]
+    public void MultiStreamProjection_ForPaymentVerification_ShouldSucceed()
+    {
+        var payment1Id = Guid.CreateVersion7();
+        var payment2Id = Guid.CreateVersion7();
+        var payment3Id = Guid.CreateVersion7();
+        var payment4Id = Guid.CreateVersion7();
+
+        var order1Id = Guid.CreateVersion7();
+        var order2Id = Guid.CreateVersion7();
+        var order3Id = Guid.CreateVersion7();
+        var order4Id = Guid.CreateVersion7();
+
+        var merchant1Id = Guid.CreateVersion7();
+        var merchant2Id = Guid.CreateVersion7();
+
+        var fraudCheck1Id = Guid.CreateVersion7();
+        var fraudCheck2Id = Guid.CreateVersion7();
+        var fraudCheck3Id = Guid.CreateVersion7();
+
+        var eventStore = new EventStore();
+        var database = new Database();
+
+        // TODO:
+        // 1. Create a PaymentVerificationProjection class that handles each event type.
+        //    Events arrive on different streams (payment, merchant, fraud check),
+        //    but they share PaymentId — use PaymentId as the read model key.
+        // 2. Register your event handlers using `eventStore.Register`.
+
+        // Payment 1: Approved — all checks pass
+        eventStore.Append(payment1Id, new PaymentRecorded(payment1Id, order1Id, 100m));
+        eventStore.Append(merchant1Id, new MerchantLimitsChecked(payment1Id, merchant1Id, true));
+        eventStore.Append(fraudCheck1Id, new FraudScoreCalculated(payment1Id, 0.1m, true));
+        eventStore.Append(payment1Id, new PaymentVerificationCompleted(payment1Id, true));
+
+        // Payment 2: Merchant rejected — exceeds merchant limits
+        eventStore.Append(payment2Id, new PaymentRecorded(payment2Id, order2Id, 5000m));
+        eventStore.Append(merchant2Id, new MerchantLimitsChecked(payment2Id, merchant2Id, false));
+        eventStore.Append(fraudCheck2Id, new FraudScoreCalculated(payment2Id, 0.2m, true));
+        eventStore.Append(payment2Id, new PaymentVerificationCompleted(payment2Id, false));
+
+        // Payment 3: Fraud rejected — high fraud score
+        eventStore.Append(payment3Id, new PaymentRecorded(payment3Id, order3Id, 200m));
+        eventStore.Append(merchant1Id, new MerchantLimitsChecked(payment3Id, merchant1Id, true));
+        eventStore.Append(fraudCheck3Id, new FraudScoreCalculated(payment3Id, 0.95m, false));
+        eventStore.Append(payment3Id, new PaymentVerificationCompleted(payment3Id, false));
+
+        // Payment 4: Pending — still awaiting fraud check and final decision
+        eventStore.Append(payment4Id, new PaymentRecorded(payment4Id, order4Id, 50m));
+        eventStore.Append(merchant1Id, new MerchantLimitsChecked(payment4Id, merchant1Id, true));
+
+        // Assert Payment 1: Approved
+        var payment1 = database.Get<PaymentVerification>(payment1Id)!;
+        payment1.Should().NotBeNull();
+        payment1.Id.Should().Be(payment1Id);
+        payment1.Status.Should().Be(PaymentStatus.Approved);
+
+        // Assert Payment 2: Merchant rejected
+        var payment2 = database.Get<PaymentVerification>(payment2Id)!;
+        payment2.Should().NotBeNull();
+        payment2.Id.Should().Be(payment2Id);
+        payment2.Status.Should().Be(PaymentStatus.Rejected);
+
+        // Assert Payment 3: Fraud rejected
+        var payment3 = database.Get<PaymentVerification>(payment3Id)!;
+        payment3.Should().NotBeNull();
+        payment3.Id.Should().Be(payment3Id);
+        payment3.Status.Should().Be(PaymentStatus.Rejected);
+
+        // Assert Payment 4: Pending
+        var payment4 = database.Get<PaymentVerification>(payment4Id)!;
+        payment4.Should().NotBeNull();
+        payment4.Id.Should().Be(payment4Id);
+        payment4.Status.Should().Be(PaymentStatus.Pending);
+    }
+}

Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/README.md

@@ -0,0 +1,29 @@
+# Exercise 15 - Multi-Stream Projections
+
+In exercises 12-14 you built projections from a single event stream. Now you'll combine events from **multiple streams** into a single read model.
+
+## Scenario: Payment Verification
+
+A payment verification requires data from three independent checks, each producing events on its own stream:
+
+1. **Payment recorded** — from the payment service (amount, order reference)
+2. **Merchant limits checked** — from the merchant service (within daily limits?)
+3. **Fraud score calculated** — from the fraud detection service (risk score, acceptable?)
+4. **Verification completed** — final decision event (approved or rejected)
+
+All events share a `PaymentId` that ties them to the same payment verification read model.
+
+## What to implement
+
+With the [Database](./Tools/Database.cs) interface representing the sample database, implement a `PaymentVerification` read model and projection:
+
+1. Define the `PaymentVerification` read model properties — the test assertions tell you what shape it needs.
+2. Create a `PaymentVerificationProjection` class with typed `Handle` methods for each event.
+3. Register handlers in the test using `eventStore.Register`.
+
+The key difference from single-stream projections: each event arrives on a **different stream ID**, but they all reference the same `PaymentId`. Your projection must use `PaymentId` (not the stream ID) as the read model key.
+
+## Reference
+
+Read more about multi-stream projections and handling events from multiple sources:
+- [Handling Events Coming in an Unknown Order](https://www.architecture-weekly.com/p/handling-events-coming-in-an-unknown)

Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/Tools/Database.cs

@@ -0,0 +1,22 @@
+using System.Text.Json;
+
+namespace IntroductionToEventSourcing.Projections.MultiStream.Tools;
+
+public class Database
+{
+    private readonly Dictionary<string, object> storage = new();
+
+    public void Store<T>(Guid id, T obj) where T: class =>
+        storage[GetId<T>(id)] = obj;
+
+    public void Delete<T>(Guid id) =>
+        storage.Remove(GetId<T>(id));
+
+    public T? Get<T>(Guid id) where T: class =>
+        storage.TryGetValue(GetId<T>(id), out var result) ?
+            // Clone to simulate getting new instance on loading
+            JsonSerializer.Deserialize<T>(JsonSerializer.Serialize((T)result))
+            : null;
+
+    private static string GetId<T>(Guid id) => $"{typeof(T).Name}-{id}";
+}

Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/Tools/EventEnvelope.cs

@@ -0,0 +1,24 @@
+namespace IntroductionToEventSourcing.Projections.MultiStream.Tools;
+
+public record EventMetadata(
+    string EventId,
+    ulong StreamPosition,
+    ulong LogPosition
+)
+{
+    public static EventMetadata For(ulong streamPosition, ulong logPosition) =>
+        new(Guid.CreateVersion7().ToString(), streamPosition, logPosition);
+}
+
+public record EventEnvelope(
+    object Data,
+    EventMetadata Metadata
+);
+
+public record EventEnvelope<T>(
+    T Data,
+    EventMetadata Metadata
+): EventEnvelope(Data, Metadata) where T : notnull
+{
+    public new T Data => (T)base.Data;
+}

Workshops/IntroductionToEventSourcing/15-Projections.MultiStream/Tools/EventStore.cs

@@ -0,0 +1,41 @@
+namespace IntroductionToEventSourcing.Projections.MultiStream.Tools;
+
+public class EventStore
+{
+    private readonly Dictionary<Type, List<Action<EventEnvelope>>> handlers = new();
+    private readonly Dictionary<Guid, List<EventEnvelope>> events = new();
+
+    public void Register<TEvent>(Action<EventEnvelope<TEvent>> handler) where TEvent : notnull
+    {
+        var eventType = typeof(TEvent);
+
+        void WrappedHandler(object @event) => handler((EventEnvelope<TEvent>)@event);
+
+        if (handlers.TryGetValue(eventType, out var handler1))
+            handler1.Add(WrappedHandler);
+        else
+            handlers.Add(eventType, [WrappedHandler]);
+    }
+
+    public void Append<TEvent>(Guid streamId, TEvent @event) where TEvent : notnull
+    {
+        if (!events.ContainsKey(streamId))
+            events[streamId] = [];
+
+        var eventEnvelope = new EventEnvelope<TEvent>(@event,
+            EventMetadata.For(
+                (ulong)events[streamId].Count + 1,
+                (ulong)events.Values.Sum(s => s.Count)
+            )
+        );
+
+        events[streamId].Add(eventEnvelope);
+
+        if (!handlers.TryGetValue(eventEnvelope.Data.GetType(), out var eventHandlers)) return;
+
+        foreach (var handle in eventHandlers)
+        {
+            handle(eventEnvelope);
+        }
+    }
+}

Workshops/IntroductionToEventSourcing/16-Projections.MultiStream.OutOfOrder/16-Projections.MultiStream.OutOfOrder.csproj

@@ -0,0 +1,26 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <RootNamespace>IntroductionToEventSourcing.Projections.MultiStream.OutOfOrder</RootNamespace>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="FluentAssertions" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" />
+        <PackageReference Include="GitHubActionsTestLogger">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
+        <PackageReference Include="xunit" />
+        <PackageReference Include="xunit.runner.visualstudio">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+        </PackageReference>
+        <PackageReference Include="coverlet.collector">
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+            <PrivateAssets>all</PrivateAssets>
+        </PackageReference>
+    </ItemGroup>
+
+</Project>