Inline Projections

Inline projections run in the same transaction as the event append, providing strong consistency between events and their read models.

How It Works

When SaveChangesAsync() is called:

1. Events are inserted into pc_events
2. Stream versions are updated in pc_streams
3. Inline projections process the new events
4. Projection documents are upserted into their tables
5. All operations commit in a single transaction

If any step fails, the entire transaction rolls back -- events and projections are always consistent.

Registration

Register a projection for inline processing:

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

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

// Multi stream projection
opts.Projections.Add<CustomerDashboardProjection>(ProjectionLifecycle.Inline);

When to Use Inline Projections

Inline projections are ideal when:

• Read models must always be consistent with events
• Queries immediately follow writes in the same request
• The projection is simple and fast (doesn't slow down writes)

Trade-offs

Advantage	Disadvantage
Strong consistency	Adds latency to writes
No staleness window	Transaction grows larger
Simpler architecture	No async daemon needed, but more write contention

Inline vs Async

If your projections are complex or slow, consider async projections instead. The async daemon processes events in the background, decoupling write performance from projection processing.

TIP

Start with inline projections for simplicity. Move to async if write performance becomes a concern.