So You Want to Build An Event Driven System? - James Eastham - CPH DevFest 2024

Event-driven architecture fundamentally reverses lines of integration and dependency between services, enabling more loosely coupled systems

Start small when adopting event-driven patterns - pick non-critical functionality first and gradually expand rather than trying to make the entire system event-driven overnight

Use thin events (containing minimal data like IDs) for public interfaces and thick events (containing full data) for private communications between services you control

Implement good governance practices including:

• Schema management
• Version control of events
• Deprecation policies
• Clean up mechanisms for old events
• Change management processes

Consider using patterns like:

• Outbox pattern for reliable event publishing
• Claim check pattern for handling large payloads
• Event-carried state transfer
• Asynchronous commands when needed

Plan for eventual consistency rather than strong consistency in most cases - embrace asynchronous patterns where possible

Control ingestion rates and protect systems with queues between event publishers and subscribers

Use event storming workshops to identify business events and establish shared language between technical and business teams

Keep events immutable - they represent facts that have happened in the past

Include proper observability, tracing and monitoring capabilities from the start as debugging event-driven systems can be complex