Projections Overview

Projections are the mechanism for building read models from events. Polecat supports several projection types and lifecycle strategies.

Projection Lifecycle

Every projection runs with one of three lifecycle strategies:

Inline

Projections run in the same transaction as the event append. This provides strong consistency -- the read model is always up to date:

opts.Projections.Snapshot<OrderSummary>(SnapshotLifecycle.Inline);

Async

Projections run in the background via the async daemon. The read model is eventually consistent:

opts.Projections.Snapshot<OrderSummary>(SnapshotLifecycle.Async);

Live

Projections are built on demand by replaying events each time. No read model is persisted:

var order = await session.Events.AggregateStreamAsync<OrderSummary>(streamId);

Projection Types

Type	Description	Use Case
Single Stream	One aggregate per stream	Order, Invoice, Account
Multi Stream	Aggregate across multiple streams	Dashboard, Report
Event Projection	Per-event document creation	Audit log, Search index
Flat Table	Direct SQL table writes	Reporting, Analytics
Composite	Multi-stage orchestration	Complex pipelines

Conventional Projection Methods

Polecat discovers projection methods by convention:

Create

Creates the initial aggregate from a stream-starting event:

public static OrderSummary Create(OrderCreated e) =>
    new() { Status = "Created", Amount = e.Amount };

Apply

Applies an event to an existing aggregate:

public void Apply(OrderShipped e)
{
    Status = "Shipped";
    ShippedDate = e.ShippedAt;
}

ShouldDelete

Signals that the aggregate should be deleted:

public bool ShouldDelete(OrderCancelled e) => true;

Registration

var store = DocumentStore.For(opts =>
{
    opts.Connection("...");

    // Single stream projection as inline snapshot
    opts.Projections.Snapshot<OrderSummary>(SnapshotLifecycle.Inline);

    // Multi stream projection as async
    opts.Projections.Add<DashboardProjection>(ProjectionLifecycle.Async);

    // Event projection
    opts.Projections.Add<AuditLogProjection>(ProjectionLifecycle.Inline);
});