11/12/2026 updated
100 % available
Freelance Backend Engineer | Java & Kotlin | Spring Boot | Kafka | Data Migrations & Integrations
Manresa, Spain
Only remote
BSc Computer Engineering, Software Engineering specializationAbout me
Backend engineer (Java, Kotlin, Spring Boot) with 4+ years on production systems. I take on scoped work packages end to end: live data migrations, platform upgrades, SSO and system integrations. Also co-founder and CTO of a fintech whose platform I built from scratch. Remote, EU-based.
KeycloakAI AgentsJava (Programming Language)HibernateJUnitPostgreSQLSpring BootBackendKotlinKubernetesApache KafkaRestful ApiDockerMicroservices
Java & Kotlin Backend Development
Advanced proficiency in Java and Kotlin for building production-grade backend systems using Spring Boot and Quarkus, with experience in REST and gRPC APIs, Hibernate/JPA, and multi-tenancy architectures.
Data Migrations & System Integrations
Expertise in live data-model migrations using Flyway, platform upgrades across multiple services, and integrations with ERP systems, Jira Service Management API, and banking-as-a-service providers.
Messaging & Distributed Architecture
Deep knowledge of Apache Kafka, microservices design, saga orchestration, transactional outbox patterns, and multi-tenant distributed workflow orchestration.
Security & Identity Management
Implementation of centralized SSO solutions using Keycloak and JWT, including RBAC configuration for multiple products and thousands of users.
Database Management & Optimization
Hands-on experience with PostgreSQL, MariaDB, and Redis, including query optimization, schema design, and significant response time improvements on critical endpoints.
Testing & Quality Assurance
Comprehensive testing practices using JUnit, Testcontainers, integration tests, and end-to-end tests, with demonstrated improvement of test coverage from ~40% to 75%+.
Tooling & Observability
Proficiency with Docker, Kubernetes, Jenkins, Git, Maven, Prometheus, Grafana, and Tempo for CI/CD pipelines and full system observability.
Frontend & Full-Stack Technologies
Experience with Next.js, TypeScript, React Native (Expo) for building web and mobile applications, including iOS and Android apps.
AI-Assisted Development
Use of Claude Code with maintained agent context files within client policies to support and accelerate development workflows.
Advanced proficiency in Java and Kotlin for building production-grade backend systems using Spring Boot and Quarkus, with experience in REST and gRPC APIs, Hibernate/JPA, and multi-tenancy architectures.
Data Migrations & System Integrations
Expertise in live data-model migrations using Flyway, platform upgrades across multiple services, and integrations with ERP systems, Jira Service Management API, and banking-as-a-service providers.
Messaging & Distributed Architecture
Deep knowledge of Apache Kafka, microservices design, saga orchestration, transactional outbox patterns, and multi-tenant distributed workflow orchestration.
Security & Identity Management
Implementation of centralized SSO solutions using Keycloak and JWT, including RBAC configuration for multiple products and thousands of users.
Database Management & Optimization
Hands-on experience with PostgreSQL, MariaDB, and Redis, including query optimization, schema design, and significant response time improvements on critical endpoints.
Testing & Quality Assurance
Comprehensive testing practices using JUnit, Testcontainers, integration tests, and end-to-end tests, with demonstrated improvement of test coverage from ~40% to 75%+.
Tooling & Observability
Proficiency with Docker, Kubernetes, Jenkins, Git, Maven, Prometheus, Grafana, and Tempo for CI/CD pipelines and full system observability.
Frontend & Full-Stack Technologies
Experience with Next.js, TypeScript, React Native (Expo) for building web and mobile applications, including iOS and Android apps.
AI-Assisted Development
Use of Claude Code with maintained agent context files within client policies to support and accelerate development workflows.
Experience
Professional experience0 - 5 years
Freelancer since2026
Languages
GermanBasic knowledgeEnglishFluentSpanishNative speaker
Project history
Pre-launch P2P lending fintech for credit-invisible consumers in Germany. As CTO, responsible for the entire technical side: architecture, technology decisions, and hands-on development. Built the platform from scratch using Kotlin/Spring Boot backend, Next.js/TypeScript web app, and React Native (Expo) iOS and Android apps. Led product design, banking integration evaluation, engineering setup, and developer onboarding. Stack: Kotlin, Spring Boot, Next.js, TypeScript, React Native, Expo.
German manufacturer of optical transceivers and networking hardware. Delivering Java backend work packages for the product development department, each scoped with deliverables and an hour budget. Work includes compatibility data model replacements, firmware tracking, data-integrity fixes, ERP integrations, and new network-device service implementation. Stack: Java, Spring Boot, Hibernate/JPA, MariaDB, Flyway, REST, SOAP, Jira Service Management API.
Sole backend owner of core platform components serving 10,000+ customers, including Kafka messaging, Keycloak SSO, and multi-tenancy. Led platform-wide migration to Java 25 and Spring Boot 4 across 15+ services, designed and built centralized SSO for 3 products and ~8,000 users, resolved 20+ long-standing production bugs, optimized critical queries, raised integration test coverage from ~40% to 75%+, and contributed to hiring by designing technical interview pipelines. Stack: Java, Spring Boot, Quarkus, Hibernate/JPA, MariaDB, Kafka, Keycloak.