Boundaries before services
A modular monolith with real domain boundaries — enforced by the build, not by convention — is usually the cheaper place to start than a network of services. Split when the boundary has earned it.
About
James Summerill
I build and evolve backend systems: long-lived Ruby on Rails applications, version and dependency upgrades that ship in steps rather than as a rewrite, domain boundaries the build enforces, and integrations with the legacy systems those applications cannot leave behind. Ten years of backend work, most of it Rails in production. Since 2024 I have also built independent AI and agent tooling — some of it deployed as public services, some of it still research.
How I work
Evidence over assertion
Generated or hand-written, a change is only as good as the test that fails without it. I would rather record a reproducible result than a confident one.
AI, held to evidence
A model's output is a proposal, not a result. I build agent and AI tooling around tests, independent verification and recorded evidence, so a change is accepted because it passed something rather than because a model said it did.
Upgrades are maintenance, not projects
Keeping a long-lived Rails application upgradeable is continuous work: dependency hygiene, a suite that catches regressions, and CI that makes the change cheap to prove. Left alone, it becomes a rewrite.
Boring infrastructure
Containers that are pinned, unprivileged and read-only; one public path in through a tunnel; logs that are bounded. The goal is that nothing about running it is interesting.
Experience
Newest first. The City Electrical Factors entry is one employer across a sequence of contracts and then a full-time role, written up by phase rather than as four separate appointments.
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Career break & independent technical projects
Took a career break for personal reasons and continued independent development on selected projects: deployed developer tools, AI and agent infrastructure, and experimental systems research.
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Ruby on Rails Engineer → Senior Back End Developer
Maintained and evolved the primary cef.co.uk system — first on contracts that upgraded it off long-unsupported Rails versions, then full time owning cross-cutting technical health and helping steer engineering practice toward TDD, CI/CD and clearer domain boundaries.
The upgrade path, written up on the blog →
- Phase 1 · Rails 3.2 → 4.2, October 2019 — May 2020 (contract): upgraded a production retail application off a framework version that had been out of support for years, released in steps rather than as one cutover.
- Phase 2 · Rails 4.2 → 5.2, December 2020 — March 2021 (contract): the next hop, made smaller by having cleared the previous one's deprecations rather than deferring them.
- Phase 3 · March 2021 — October 2024 (full time): returned to own cross-cutting upgrades across Rails versions, gems and dependencies.
- Championed Docker containerisation and led the Bitbucket Pipelines implementation, so a green build meant a runnable environment.
- Built and maintained API integrations bridging modern applications and legacy Sybase systems.
- Advocated a modular-monolith direction using Packwerk and domain boundaries, without prematurely splitting the system into services.
- Promoted TDD and CI/CD, and shared approaches and emerging practices with the wider team.
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Senior Developer / Lead Developer
Led and supported Ruby on Rails developers in a lean team, providing hands-on guidance, feedback and technical direction.
- Analysed the existing platform and documented recommendations, supporting continuity across development work.
- Contributed to hiring, collaborative working practices and iterative delivery.
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Ruby on Rails Software Engineer
Maintained and extended the Rails platform and collaborated remotely with colleagues across functions.
- Worked with the Transcription Manager on an AI transcription service, reducing validation time for human reviewers and the cost of running it.
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Earlier roles
Four roles across agencies and product companies, in Ruby and the web stacks of the time.
- Evvnt · Ruby Developer, April — December 2015: maintained the Ruby/Heroku website and infrastructure, and developed Ruby, Watir and Sidekiq automation that reduced manual work.
- Arnold Clark · Ruby / Web Developer, July 2014 — March 2015: worked across legacy systems and new features, including Worldpay, AWS, Rails and Swift, and built an internal office dashboard on Raspberry Pi, Dashing and Debian.
- Appkick · Lead Backend Engineer, February — November 2014: led backend development, migrated the core application from Node.js to Ruby, and introduced CI practices and improved remote-team delivery workflows.
- Green Gorilla Apps · Full Stack Web Developer, June 2013 — May 2014: maintained and extended existing web applications and fixed defects, following Git-based review practices.
Toolkit
Languages & frameworks
- Ruby
- Ruby on Rails
- Python
- SQL
- JavaScript
- TypeScript
Architecture & design
- Modular monoliths
- Domain-driven design
- Packwerk
- Domain boundaries
- Transactional outbox
- Event-driven integration
- Legacy integration
Delivery & quality
- TDD
- CI/CD
- Bitbucket Pipelines
- Docker
- Dependency & framework upgrades
- Code review
- Developer mentoring
Data & platforms
- PostgreSQL
- MySQL
- Sybase
- Kafka
- Heroku
- AWS
- Linux
AI & systems
- Local LLM inference
- Agent systems
- Model routing
- Evaluation & verification harnesses
- PyTorch
Education
- BTEC National for IT Practitioners, Level 3 New College Durham · 2008 — 2010
Working on something that fits?
Available for senior backend and Rails engineering roles in the United Kingdom — remote or hybrid, permanent.