Save Me, Holmes! Rebuilding a Home Service Handling 30 Million Traffic at a Bank

KakaoBank's Home Service Development Team carried out a project to split a massive legacy monolithic server into microservices (MSA). This was to resolve technical debt accumulated from rapid growth and to build an efficient collaboration environment aligned with changes in organizational structure.

The main technical challenge was to simultaneously resolve structural problems and performance problems. In particular, the existing system had the channel system excessively depending on the account system, which is core data, causing external models to penetrate into the domain layer and reducing cohesion.

To address this, the following strategies were introduced.

• Structural improvement: Introduced Hexagonal Architecture to protect domain logic from external dependencies and lower coupling
• Performance improvement: Leveraged coroutines (Kotlin Coroutine) to achieve performance optimization in response to environmental changes
• Stable migration: Carried out the migration while ensuring the stability of the live service through response comparison, sample inspection, A/B ratio adjustment, and a Fallback strategy