AsyncAPI + Overlays: Testing Event Flows with Triggers and Side-Effects

Testing event-driven systems is fundamentally harder than testing REST APIs. Validating schemas is straightforward, but validating behaviour in terms of what the system actually does when it receives or produces an event is still a major challenge. Teams often struggle to trigger their systems reliably, observe side-effects, and automate end-to-end event flow testing without custom scripts or brittle harnesses. In this demo, I will introduce a new capability in Specmatic that brings executable behaviour into the AsyncAPI ecosystem. Using a simple Overlay file, engineers can define:

• Triggers that stimulate the system (for example, an HTTP call that causes an event to be published), and
• Side-effects that must occur when the system consumes an event (such as an emitted message, state change, or downstream API call).

This approach turns an AsyncAPI specification into a fully executable contract test that verifies real event flows, not just schemas. I will walk through a practical example of how overlays allow teams to test end-to-end asynchronous workflows in a declarative, CI-friendly manner without relying on mocks, glue code, or environment-specific hacks. Attendees will leave with a clear understanding of how AsyncAPI can work together to close a long-standing gap in event-driven testing, making it possible to ship reliable, behaviourally correct event-based systems with confidence.