LaunchDarkly's engineers, Mike Zorn and Justin Caballero, presented at AWS re:Invent 2021 on achieving a seamless database migration with zero downtime using feature flags. Their approach involved converting a streaming event architecture and migrating production databases gradually, reversibly, and verifiably without affecting user experience. Faced with the challenges of database migrations, LaunchDarkly aimed to improve disaster recovery by transitioning from Mongo and PostgreSQL to CockroachDB. To ensure a smooth transition, they employed a dual-mode operation allowing the application to interact with both old and new databases simultaneously, with the new database initially in "shadow mode." Feature flags provided immediate control over the migration process, enabling gradual changes and easy reversals if needed. This method allowed for extensive testing and optimization in production without user disruption, ensuring that their customers remained unaware of the migration.