Diego Barrio Hortigüela
Senior Backend Engineer · Java, Spring & Applied AI
Payments and banking platforms · Event-driven systems · Applied AI
Alicante, Spain diegobarrioh@gmail.com LinkedIn diegobarrioh.dev backendtothefuture.com
About
Senior Backend Engineer with over 15 years in Java, most of them in banking and payments. I currently work on the payment and billing APIs of Europcar Mobility Group, which handle around 10,000 transactions and millions of euros a day, on Java 21, Spring Boot, Kafka and Kubernetes. Before that I spent eight years on Banco Sabadell systems: mortgage simulation and amortization in core banking, ATM services, and later the security of the document platform behind its online banking. I work with DDD, hexagonal architecture and TDD. In my own projects I build LLM features where the model's output is validated against a domain contract before the system accepts it.
Core Competencies
Backend Engineering
Software Architecture
Domains
Security
Platform & Observability
Data & Messaging
AI Engineering
Professional Experience
Senior Backend Engineer
- Build and maintain the payment and billing APIs, with ~10,000 payment transactions and millions of euros processed per day, averaging ~1,000 transactions an hour with higher peaks in high season.
- Daily billing runs issue tens of thousands of invoices with sub-second generation time, monitored through Splunk dashboards.
- Event-driven integration with Apache Kafka for asynchronous processing between services.
- OAuth 2.0 authentication for the payment API's clients, with Spring Cloud OpenFeign clients wired through Spring Boot auto-configuration.
- Services on Java 21, with the first microservices already migrated to Java 25.
- Drive DDD, hexagonal architecture and TDD in the team; GitLab CI/CD to Kubernetes on AWS.
Technologies: Java 21, Java 25, Spring Boot, OpenFeign, OAuth 2.0, Kafka, Kubernetes, AWS, Oracle, Splunk, GitLab CI/CD
Senior Analyst
- Responsible for the Document Management System (DMS) of Banco Sabadell's online banking, the platform that shares sensitive documents with customers.
- Security-focused work: authentication and access control with Spring Security, SAML and JWT.
- Tuned complex Oracle and SQL Server queries and IBM Tivoli batch workflows.
Technologies: Java, Spring Boot, Spring Security, SAML, JWT, Oracle, SQL Server, IBM Tivoli
Senior Specialist
- Domain expert for Seneca, the core-banking application that produces mortgage payment simulations and amortization schedules.
- Application Manager for Seneca: defined the work and assigned it to an NTT DATA team.
- Development and maintenance of ATM channel services.
- Java 8 on Proteo, the bank's in-house enterprise framework, alongside Spring MVC and Hibernate.
Technologies: Java 8, Proteo, Spring MVC, Hibernate, Oracle
Analyst
- Internal business applications on the Spring ecosystem.
Technologies: Java, Spring, Thymeleaf, Oracle
Analyst & Developer
- Online banking applications, ATM services and core banking migration projects.
Technologies: Java EE, Oracle
Web Developer
- Dynamic web applications.
Technologies: PHP, MySQL, JavaScript
Personal Projects
Forma
Personal Fitness Operating System forma.backendtothefuture.com
A health and fitness planning system that connects body composition, training, nutrition and shopping cost into one weekly decision loop.
- Hexagonal backend with a framework-free domain: JDBC and Flyway over PostgreSQL, no ORM, decided and recorded in an ADR.
- LLM-generated nutrition plans are fenced by a validated contract: a plan may only reference food identifiers the API publishes, and anything the model invents is rejected on import.
- Spec-driven delivery with 13 ADRs, Vitest and Playwright suites gated in CI.
Technologies: Java 21, Spring Boot 3, PostgreSQL, Flyway, React 19, Playwright
Akademia
RAG-Powered Learning Platform akademia.backendtothefuture.com
Study platform whose core is a RAG pipeline that turns the material a user uploads into tests and questions grounded in the source. Includes spaced-repetition flashcards and a Stripe store.
Technologies: Java, Spring Boot, PostgreSQL, RAG, OpenRouter, Stripe
TokenMeter
Tokenization & LLM Cost Exploration tokenmeter.backendtothefuture.com
Measures what LLM code generation really costs, comparing agentic loops with single-shot flows, using the jtokkit o200k_base encoder instead of a character-based estimate.
Technologies: Java, Spring Boot, Docker, Cloudflare
Education
Master's Degree in Web Applications and Services Development
Certificate in Java Enterprise Development
Computer Engineering Degree
Equivalent to MSc level under the Bologna Process.
Community
- Author of Backend to the Future, a blog on software architecture, backend engineering and applied AI.
- Hands-on experimentation with local LLMs (Ollama, LM Studio) and agentic development workflows.
- Former mentor for Computer Science students through Fundación Banco Sabadell.
Learning Focus
Languages
Spanish: Native · English: Full Professional Proficiency (C1)