Full-stack software engineer with 6 years of experience building and operating production systems used daily by thousands of people. I work across the stack—Python (Flask) and REST APIs on the backend, React on the frontend—using PostgreSQL, Elasticsearch, and asynchronous processing to deliver reliable features from design to operations. I particularly enjoy backend work involving concurrency, performance, and systems under load. I also take ownership of deployments (Kubernetes/OpenShift), monitoring, and incident response, and I’ve led migrations and mentored teammates while integrating external services and ensuring data consistency at scale.

Javier Romero Castro

Full-stack software engineer with 6 years of experience building and operating production systems used daily by thousands of people. I work across the stack—Python (Flask) and REST APIs on the backend, React on the frontend—using PostgreSQL, Elasticsearch, and asynchronous processing to deliver reliable features from design to operations. I particularly enjoy backend work involving concurrency, performance, and systems under load. I also take ownership of deployments (Kubernetes/OpenShift), monitoring, and incident response, and I’ve led migrations and mentored teammates while integrating external services and ensuring data consistency at scale.

Available to hire

Full-stack software engineer with 6 years of experience building and operating production systems used daily by thousands of people. I work across the stack—Python (Flask) and REST APIs on the backend, React on the frontend—using PostgreSQL, Elasticsearch, and asynchronous processing to deliver reliable features from design to operations.

I particularly enjoy backend work involving concurrency, performance, and systems under load. I also take ownership of deployments (Kubernetes/OpenShift), monitoring, and incident response, and I’ve led migrations and mentored teammates while integrating external services and ensuring data consistency at scale.

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Experience Level

Expert
Expert
Expert
Expert
Expert

Language

English
Fluent
Spanish; Castilian
Fluent
Galician
Fluent
French
Beginner
German
Beginner

Work Experience

Software Engineer (Contractor) at CERN
December 1, 2025 - March 31, 2026
Built back end and full-stack features for CDS (CERN Document Server), a large open-source platform, working independently from design through delivery. Implemented notification workflows, collections, and community management across a Python back end and React front end.
Software Engineer (Fellow) at CERN
October 1, 2022 - October 1, 2025
Backend development for CDS, a platform with 1M+ records used daily by thousands of people, using Python (Flask), REST APIs, PostgreSQL, Elasticsearch, and React. Diagnosed and fixed a critical PostgreSQL performance issue at framework level (missing foreign key indexes), improving processing up to 20x and merging upstream to benefit all instances including Zenodo. Built asynchronous and background-processing systems with Celery, Redis, and RabbitMQ for external API integrations, search indexing, and data synchronization, with retry logic, failure alerting, and full job lifecycle handling. Built data synchronization pipelines integrating external systems (IAM service APIs and LDAP), keeping user and group data consistent at scale. Responded to production incidents including repeated abuse from bots and crawlers that degraded service: identified offenders using request patterns and user-agent fingerprints and blocked them at the load balancer without impacting legitimate traffic. Owned
Software Engineer (Project Associate) at CERN
July 1, 2020 - September 1, 2022
Built back end services and React front-end features across 150+ repositories with CI/CD pipelines (GitHub Actions, GitLab CI). Designed REST APIs for integrating external services and handling complex metadata structures. Part of the team that deployed the CERN Library Catalogue to production in 2021. Automated testing with PyTest, performed code reviews, and supported deployment workflows.
Full-Stack Developer at Quindel Group
August 1, 2019 - June 30, 2020
Built enterprise applications for large clients: back end integrations, business logic, and front-end work using Java, JavaScript, and SQL.

Education

Master’s Degree in Computer Science Engineering at University of Vigo
January 1, 2020 - January 1, 2022
Bachelor’s Degree in Computer Science Engineering at University of Vigo
January 1, 2014 - January 1, 2018

Qualifications

Add your qualifications or awards here.

Industry Experience

Software & Internet, Government, Non-Profit Organization, Professional Services, Computers & Electronics