Aggregates.MSSP 1.0.0-preview.5

Aggregates

⚠️ Preview: This is a pre-release version (1.0.0-preview.4). The API is stabilising but may still change before 1.0.0. For the previous stable version, see the legacy branch.

This library provides all the boilerplate code needed to do event sourcing, leaving you with just the core functionality of your domain to write.

Note: This is a ground-up rewrite of the existing Aggregates library. Both versions will continue to exist alongside each other for the time being.

Packages

Storage integration packages are provided separately and are required to wire everything up:

Getting started

Functional event sourcing

This library is based on two core functions from functional event sourcing.

The first takes a state and an event, and produces the next state:

state → event → state

Since an event is an irrejectable fact that has already happened, this function must always produce a new state — no exceptions allowed.

The second takes a state and a command, and produces a sequence of events:

state → command → events[]

A command is an intent, not a guarantee. This function is allowed to fail — this is where you validate input and enforce your domain rules. The produced sequence may contain one event, multiple events, or none at all.

IState

Implement IState<TState, TEvent> on your state object:

record ShoppingCartState(ImmutableDictionary<string, int> Items)
    : IState<ShoppingCartState, IShoppingCartEvent> {

    public static ShoppingCartState Initial => new(ImmutableDictionary<string, int>.Empty);

    public ShoppingCartState Apply(IShoppingCartEvent @event) => @event switch {
        ItemAdded e   => this with { Items = Items.SetItem(e.ItemId, Items.GetValueOrDefault(e.ItemId) + e.Quantity) },
        ItemRemoved e => this with { Items = Items.SetItem(e.ItemId, Items[e.ItemId] - e.Quantity) },
        _             => this
    };
}

ICommand

Implement ICommand<TState, TEvent> on your commands. The Id property identifies the target aggregate; ProgressAsync validates the current state and yields the resulting events:

record AddItem(AggregateIdentifier Id, string ItemId, int Quantity)
    : ICommand<ShoppingCartState, IShoppingCartEvent> {

    public async IAsyncEnumerable<IShoppingCartEvent> ProgressAsync(
        ShoppingCartState state,
        [EnumeratorCancellation] CancellationToken cancellationToken = default) {

        if (Quantity <= 0) throw new ArgumentOutOfRangeException(nameof(Quantity));
        yield return new ItemAdded(ItemId, Quantity);
    }
}

Handling commands

Inject ICommandHandler<TCommand> wherever you need it:

class ShoppingCartController(ICommandHandler<AddItem> handler) : ControllerBase {
    [HttpPost("{id:guid}")]
    public async Task<IActionResult> Post(Guid id, [FromBody] AddItemRequest request, CancellationToken ct) {
        await handler.HandleAsync(new AddItem(id.ToString(), request.ItemId, request.Quantity), ct);
        return Ok();
    }
}

Wiring up

KurrentDB:

services
    .AddAggregates(o => o.ScanAssemblies(typeof(AddItem).Assembly))
    .AddKurrentDb(o => {
        o.Serialize   = e => new SerializedEvent(e.GetType().Name, JsonSerializer.SerializeToUtf8Bytes(e));
        o.Deserialize = (type, data) => /* your deserializer */;
    });

MSSP:

services
    .AddAggregates(o => o.ScanAssemblies(typeof(AddItem).Assembly))
    .AddMssp(o => {
        o.Serialize   = e => new EventData(e.GetType().Name, JsonSerializer.SerializeToUtf8Bytes(e));
        o.Deserialize = (type, data) => /* your deserializer */;
    });

ScanAssemblies discovers all ICommand<TState, TEvent> implementations in the given assemblies and registers a handler for each.

Sagas

A saga is a stateful process manager that reacts to a stream of domain events by dispatching commands. Unlike a policy, a saga maintains its own state across multiple events.

ISaga

Implement ISaga<TSagaState, TEvent> to define the reaction logic. The saga class is pure — it receives the current state and the triggering event, and yields zero or more commands:

[SagaContract("OrderFulfillment")]
class OrderFulfillmentSaga : ISaga<OrderFulfillmentState, IOrderEvent> {

    public async IAsyncEnumerable<ICommand> ReactAsync(
        OrderFulfillmentState state,
        IOrderEvent @event,
        [EnumeratorCancellation] CancellationToken cancellationToken = default) {

        if (@event is OrderPlaced placed && !state.IsShipped)
            yield return new ShipOrder(placed.OrderId);
    }
}

The saga state uses the same IState<TState, TEvent> pattern as aggregates:

record OrderFulfillmentState(bool IsShipped) : IState<OrderFulfillmentState, IOrderEvent> {
    public static OrderFulfillmentState Initial => new(false);

    public OrderFulfillmentState Apply(IOrderEvent @event) => @event switch {
        OrderShipped => this with { IsShipped = true },
        _            => this
    };
}

SagaContractAttribute

Decorate your saga class with [SagaContract] to assign it a stable identity for checkpointing and versioning:

[SagaContract("OrderFulfillment", version: 1, namespace: "Orders", startFromEnd: false)]
class OrderFulfillmentSaga : ISaga<OrderFulfillmentState, IOrderEvent> { ... }

startFromEnd: true causes the saga to start consuming only new events on its first run, skipping the existing history.

Wiring up sagas

KurrentDB:

services
    .AddAggregates(o => o.ScanAssemblies(typeof(ShipOrder).Assembly))
    .AddKurrentDb(o => { /* serialization */ });

services
    .AddAggregates()
    .AddSagas(o => o
        .ScanAssemblies(typeof(OrderFulfillmentSaga).Assembly)
        .WithResolver<IOrderEvent>(e => e is OrderPlaced p
            ? [new AggregateIdentifier(p.OrderId)]
            : []))
    .AddKurrentDb();

MSSP:

services
    .AddAggregates(o => o.ScanAssemblies(typeof(ShipOrder).Assembly))
    .AddMssp(o => { /* serialization */ });

services
    .AddAggregates()
    .AddSagas(o => o
        .ScanAssemblies(typeof(OrderFulfillmentSaga).Assembly)
        .WithResolver<IOrderEvent>(e => e is OrderPlaced p
            ? [new AggregateIdentifier(p.OrderId)]
            : []))
    .AddMssp();

WithResolver determines which saga instance(s) should handle a given event. The subscription hosted service is registered automatically for every saga that has a resolver.

Policies

A policy is a stateless event-driven reaction: it receives an event and dispatches zero or more commands, with no state of its own.

IPolicy

Implement IPolicy<TEvent> to define the reaction logic:

[PolicyContract("SendWelcomeEmail")]
class SendWelcomeEmailPolicy : IPolicy<UserRegistered> {

    public async IAsyncEnumerable<ICommand> ReactAsync(
        UserRegistered @event,
        [EnumeratorCancellation] CancellationToken cancellationToken = default) {

        yield return new SendEmail(@event.UserId, "Welcome!");
    }
}

Wiring up policies

KurrentDB:

services
    .AddAggregates(o => o.ScanAssemblies(typeof(SendEmail).Assembly))
    .AddKurrentDb(o => { /* serialization */ });

services
    .AddAggregates()
    .AddPolicies(o => o.ScanAssemblies(typeof(SendWelcomeEmailPolicy).Assembly))
    .AddKurrentDb();

MSSP:

services
    .AddAggregates(o => o.ScanAssemblies(typeof(SendEmail).Assembly))
    .AddMssp(o => { /* serialization */ });

services
    .AddAggregates()
    .AddPolicies(o => o.ScanAssemblies(typeof(SendWelcomeEmailPolicy).Assembly))
    .AddMssp();

The subscription hosted service is registered automatically for every scanned policy.

Projections

A projection listens to a stream of domain events and builds a read model by writing to an external store (e.g. a SQL database). Unlike sagas and policies, projections do not dispatch commands — they produce writes.

IProjection

Implement IProjection<TEvent> to define the projection logic. The method returns an ICommit that describes the pending writes; the infrastructure executes it after the method returns:

[ProjectionContract("Orders")]
class OrdersProjection(IDbConnectionFactory db) : IProjection<IOrdersProjectionEvent> {

    public ValueTask<ICommit> ProjectAsync(
        IOrdersProjectionEvent @event,
        CancellationToken cancellationToken = default) =>

        ValueTask.FromResult<ICommit>(@event switch {
            OrderPlaced e  => Commit.Create().UseSql(db)
                                .Query("INSERT INTO orders (id, date) VALUES (@Id, @Date)",
                                       new { e.Id, e.Date }),
            OrderShipped e => Commit.Create().UseSql(db)
                                .Query("UPDATE orders SET shipped = 1 WHERE id = @Id",
                                       new { e.Id }),
            _              => Commit.Create()
        });
}

Wiring up projections

KurrentDB:

services
    .AddProjections(o => o.ScanAssemblies(typeof(OrdersProjection).Assembly))
    .AddKurrentDb();

MSSP:

services
    .AddProjections(o => o.ScanAssemblies(typeof(OrdersProjection).Assembly))
    .AddMssp();

ScanAssemblies discovers all IProjection<TEvent> implementations and registers a subscription hosted service for each. AddKurrentDb/AddMssp supplies the ISubscriptionFactory and ICheckpointStore.