Senior Java Developer (AWS/RxJava), Madrid

Senior Java Developer (AWS/RxJava) We are still looking for the very Top Talent and we would be delighted if you were to join our team! More in detail, UST is a multinational company based in North America, certified as a Top Employer and Great Place to Work company with over 35,000 employees worldwide and presence in more than 35 countries. We are leaders in digital technology services, providing large-scale technological solutions to some of the worlds most renowned organizations. What we look for? We are seeking a highly skilled Senior Java Developer with extensive experience building scalable, high-performance distributed systems. You will join a talented engineering team responsible for designing, developing, and evolving mission-critical applications using modern Java, cloud-native architectures, and event-driven technologies. Key Responsibilities: - Design, develop, and maintain robust backend services using Java 17/21 and Spring Boot. - Build scalable and resilient microservices leveraging reactive programming principles. - Develop and maintain gRPC APIs and Protobuf schemas, ensuring backward compatibility and proper schema evolution. - Implement highly concurrent and performant solutions, applying advanced thread-safety and concurrency patterns. - Work with Kafka-based event-driven architectures and production-scale data stores. - Create and maintain automated testing suites, including unit, integration, and contract tests. - Collaborate with cross-functional teams on architecture decisions, design reviews, and technical documentation. - Participate in troubleshooting, root cause analysis, and postmortem activities. - Contribute to CI/CD pipelines, cloud infrastructure, and containerized deployments. - Produce clear technical documentation, design proposals, and pull request descriptions. Mandatory Requirements: - 8+ years of professional experience with Java development. - 3+ years of hands-on experience with Java 17 or Java 21, including modern language features such as records, sealed classes, pattern matching, and virtual threads when applicable. - Strong expertise in Spring Framework and Spring Boot in production environments. - Proven experience with reactive programming using Project Reactor or RxJava (Mono, Flux, backpressure, retry strategies, hot vs. cold publishers), or advanced asynchronous programming with CompletableFuture and structured concurrency. - Production experience with gRPC and Protocol Buffers, including streaming RPCs, deadlines, interceptors, and schema evolution. - Strong understanding of concurrency fundamentals, executor services, lock-free patterns, and thread lifecycle management. - Solid testing practices using JUnit 5, Mockito, and AssertJ. - Experience developing integration tests against real dependencies and contract/parity testing. - Strong knowledge of Kafka consumer and producer semantics. - Production experience with at least one of the following technologies: Redis, PostgreSQL, or MongoDB. - Cloud experience with AWS (or equivalent cloud platforms), including services such as S3, IAM, managed Kafka solutions, and container runtimes. - Excellent written communication skills with the ability to create design documents, postmortems, and technical documentation. - English level C1 or higher. Nice to Have: - Experience leading large-scale migrations or refactoring initiatives across multi-module codebases. - Knowledge of container packaging and multi-architecture image builds (Linux/AMD64 and Linux/ARM64). - Python knowledge with the ability to read, understand, and debug existing code. - Experience working with Apache Arrow or other columnar data formats. - CI/CD pipeline ownership using TeamCity, GitHub Actions, or similar technologies. - Experience contributing to architecture and technical strategy discussions. What can we offer? - 23 days of Annual Leave plus the 24th and 31st of December as discretionary days! -