Composite Projections

Composite projections orchestrate multiple projection stages that must run in a specific order. Within each stage, projections can run in parallel.

How It Works

A composite projection defines stages:

1. Stage 1: Projections A and B run in parallel
2. Stage 2: Projection C runs after Stage 1 completes (it depends on A and B)
3. Stage 3: Projections D and E run in parallel after Stage 2

Defining a Composite Projection

public class MyCompositeProjection : PolecatCompositeProjection
{
    public MyCompositeProjection()
    {
        // Stage 1: Run these in parallel
        AddProjection<OrderSummaryProjection>();
        AddProjection<InventoryProjection>();

        // Stage 2: Runs after stage 1
        AddProjection<DashboardProjection>(dependsOn: 1);
    }
}

Registration

Composite projections are always async:

opts.Projections.Add<MyCompositeProjection>(ProjectionLifecycle.Async);

Thread Safety

The PolecatProjectionBatch used by composite projections uses ConcurrentBag and ConcurrentQueue internally to safely handle parallel stage execution.

Use Cases

• Dependent read models -- Dashboard that depends on individual aggregates
• Multi-stage processing -- Transform data through a pipeline
• Performance optimization -- Parallelize independent projections within a stage