Kotlin Backend Developer (Middle+)

RDS DOO is hiring a Kotlin Backend Developer (Middle+) in Serbia (office) to work on a Smart City video-surveillance platform with event-driven architecture.

About the project: The Project is a Smart City video-surveillance platform. The platform handles video conversion, integrates with storage, and uses an event-driven architecture. Deployed across several countries, it focuses on scalability, observability, and reliability. Responsibilities: Develop and maintain backend services in Kotlin / Spring Boot; design and implement REST APIs and business logic; work with PostgreSQL / ClickHouse, JPA / Hibernate and optimize database access; identify and resolve N+1, lazy-loading and transaction-related issues; design and implement RBAC and permission-based authorization; implement tenant- and resource-level access control; work with JWT authentication and Spring Security; deploy and troubleshoot services in Docker / Kubernetes environments; refactor and optimize existing Kotlin code; write unit and integration tests; participate in code reviews and backend architecture decisions.

Must-have: 2+ years of commercial backend development experience with Kotlin / Spring Boot (Middle+ level); strong Kotlin knowledge and good understanding of Java ecosystem; production experience with Spring Boot / Spring MVC; solid understanding of Spring IoC, DI, proxies and @Transactional; practical experience with PostgreSQL and JPA / Hibernate; understanding of persistence context, lazy loading, N+1 and dirty checking; experience with Spring Security and JWT; good understanding of roles, permissions and RBAC; understanding of resource-level authorization and multi-tenant access control; hands-on experience with Docker, Kubernetes and Linux; ability to analyze and optimize existing code and database queries; solid understanding of collections, algorithmic complexity and basic performance trade-offs. Nice-to-have: ClickHouse production experience; Kotlin Coroutines / Flow; experience with high-load or distributed backend systems; ABAC / policy-based authorization experience; Testcontainers; Kafka / RabbitMQ; Kubernetes operations and troubleshooting; Prometheus / Grafana / distributed tracing; OAuth2 / OpenID Connect; experience designing multi-tenant systems.

Offer: Strong engineering team and real impact on backend architecture; modern Kotlin / Spring stack, CI/CD, Kubernetes and code reviews; opportunity to work on security- and performance-critical backend components; flexible schedule; competitive compensation based on experience and interview performance.