Feature Flag Development Story: A Journey Toward Real-Time Data Synchronization

Feature flags are a technology that allows specific features to be controlled at runtime without additional code changes or deployments. This enables the separation of deployment schedules from release schedules, and allows flexible execution of user targeting or rollout ratio adjustments.

For efficient operation, an architecture separating the admin server and serving server was adopted. The admin server uses Spring MVC and MySQL to manage complex configurations and data persistence, while the serving server handles high traffic using an asynchronous approach based on Spring WebFlux and Coroutine.

As the servers were separated, data synchronization between the two physically distant servers emerged as a key challenge. If the serving server directly calls the admin server, traffic bottlenecks occur, and a periodic Polling approach has the drawback of delayed reflection of the latest data.

To solve this, the design was carried out with trade-offs in mind, using Redis Pub/Sub to secure real-time performance, while introducing Polling as a backup plan to ensure stability.