Freelance Backend Engineer – Java, Kotlin, Spring Boot, Kafka

9/2026 – offen

German manufacturer of optical transceivers and networking hardware. Independent contractor delivering Java backend work packages for the product development department, each scoped with deliverables and an hour budget. Working remotely from Spain.

- Backend services: design and evolve core services and data models in Java and Spring Boot, end to end from design to production.
- Data migrations: lead data-model migrations, backfills and production rollouts on live systems.
- Integrations: build and maintain integrations with ERP and service-desk platforms.
- Quality: raise reliability and code quality through CI safeguards, testing and code reviews.

Stack: Java, Spring Boot, Hibernate/JPA, MariaDB, Flyway, REST, SOAP, Jira Service Management API

Mariadb, Java-Entwickler, Hibernate (Java), Jira, Spring Framework, SOAP, Backend Entwickler

3/2026 – offen

Pre-launch P2P lending fintech for credit-invisible consumers in Germany, designed to operate through a licensed partner bank. As co-founder and CTO (part-time), I own the entire technical side: architecture, every technology decision and hands-on development across the codebase.

- Built the platform from scratch: Kotlin/Spring Boot backend, Next.js/TypeScript web app and React Native (Expo) iOS and Android apps.
- Architecture and technical decisions: all architecture, data-model, technology and tooling choices.
- Product design: designed the MVP end to end: borrower KYC, lender marketplace, SEPA payments, admin panel.
- Banking integration: led the technical evaluation of partner banks and banking-as-a-service providers.
- Engineering setup and team: set up the development workflow, project tooling and company IT; onboarded and lead a developer.

Stack: Kotlin, Spring Boot, Next.js, TypeScript, React Native, Expo

Kotlin, Spring Framework, Typescript, React Native, Backend Entwickler

4/2023 – 8/2026

Sole backend owner of core platform components serving 10,000+ customers, including Kafka messaging, Keycloak SSO and multi-tenancy.

- Led a platform-wide migration to Java 25 and Spring Boot 4 across 15+ services, improving build performance by about 30% and removing a critical class of security vulnerabilities.
- Designed and built centralized SSO for 3 products and about 8,000 users with Keycloak and JWT, together with the backend, webshop and IT teams; login-related support tickets fell by about 40%.
- Resolved 20+ long-standing production bugs in legacy systems, some open for 2-3 years.
- Optimized critical queries and schema design, cutting response time on key endpoints by about 25%.
- Raised integration test coverage on critical components from about 40% to 75%+.
- Contributed to hiring by designing technical interview pipelines and evaluating 10+ candidates.

Stack: Java, Spring Boot, Quarkus, Hibernate/JPA, MariaDB, Kafka, Keycloak

Apache Kafka, Mariadb, Java-Entwickler, Docker, Hibernate (Java), Spring Framework, Kubernetes, Backend Entwickler