Challenges of EDA

Adopting EDA patterns for our application brings along some challenges that must be overcome for the application to succeed.

Eventual consistency

Eventual consistency is a challenge for any distributed application. Changes in the application state may not be immediately available. Queries may produce stale results until the change has been fully recorded. An asynchronous application might have to deal with eventual consistency issues, but without a doubt, an event-driven application certainly will.

Dual writes

Not entirely a challenge of event-driven applications alone, dual write refers to any time we’re changing the application state in two or more places during an operation. For an event-driven application, this means we are making a change locally to a database, and then we’re publishing an event either about the event or the event itself. If the events we intend to publish never make it to the event broker, then our state changes cannot be shared, and post-operation operations will never happen.

For this challenge, we have a solution that will have us publish our events into the database alongside the rest of the changes to keep the state change atomic.

This allows a second record of to-be-published events to be created and even adds additional resiliency on top of what we got from using an event broker between components, as illustrated in the following diagram: