Available for new engagements
Fractional CTO · Software Architect · Data Engineering
I'm Harmen van der Spek, a hands-on engineering leader with 25 years of experience and a PhD in computer science. Organisations bring me in for software that cannot fail: complex migrations, platforms that must keep up with growth, high-performance data infrastructure. I set the technical direction and build alongside the team until it runs reliably.
Services
A platform that needs to be ready for its next growth phase, or a monolith due for replacement. I design and build architecture that grows with you, from new microservices to complex migrations where customers keep working as usual.
Dashboards that slow down, pipelines with runaway costs, numbers nobody trusts anymore. I build data pipelines, query engines, and visualizations that stay fast at scale (on Databricks, PostgreSQL, and DuckDB, for example) and the data foundation that makes AI applications production-ready.
A team growing fast but lacking guidance, or a founder losing grip on the technology. As a fractional CTO I bring strategic direction, architecture, security, and engineering discipline: senior leadership without a full-time hire.
Approach
Maintainability, test coverage, coupling: quality is measurable, not a matter of taste. I build the guardrails into the development process itself (type safety, automated tests, CI/CD), so quality doesn't depend on good intentions but lives in the system.
Setting direction from a slide deck doesn't work. I'm in the code, so every architectural decision proves itself in working software. And because we build together, the team can carry on independently afterwards.
The goal isn't exciting technology, but software that does its job: predictable, secure, maintainable. Incidents should be rare, and a well-designed system absorbs them on its own.
I take responsibility for the end result and stay until the software runs calmly in production, not until the report is finished.
Selected work
Platform engineering · RIVM
Deploying on OpenShift / Kubernetes is specialist work that takes real time, and not every team has that knowledge in-house. I designed and built a GitOps-driven internal developer platform (type-safe FastAPI backend, React/TypeScript frontend, OIDC) that automates the manual work and puts the development teams themselves in control: deploying independently and safely.
Public health · RIVM
Monitoring antibiotic resistance requires data you can trust: submissions from laboratories across the country, numbers that policy relies on. I led the rebuild of the ISIS-AR reporting platform, with Databricks powering the heavy analyses. The result: new visualizations and a platform that stays fast and reliable as data volumes grow.
Construction SaaS · Gappless
The low-code platform that got Gappless off the ground quickly eventually couldn't handle the pace of growth, and you don't replace a monolith like that piece by piece. As CTO I directed the rebuild and built alongside the team: Java and Go microservices, API-compatible with the old one. The transition itself got the same care as the product: testable migration strategies, guardrails, and a phased changeover for customers. Hosting and support costs dropped sharply and incidents became a rarity.
Financial software · IBANC
With the transition to SEPA, organisations had to phase out their old payment formats. Flawlessly, because money is involved. I built the first version of IBANC, a Swiss army knife for editing transactions in ISO 20022 formats: parsing, generation, a complex validation engine, and import of legacy file formats from across Europe. Around it I automated registration, licensing, and invoicing. The product grew into the market leader in its segment.
Background
I hold a PhD in computer science from Leiden University, following a cum laude MSc, and have spent 25 years building software across public health, construction SaaS, payments software, laboratory systems, and FPGA compiler tooling.
The common thread: I take ownership of systems that are complex, business-critical, or growing fast, and make them reliable. My conviction: quality is a practice, not a feature of your language, framework, or managed service. The less the technology enforces for you, the more you have to secure with extra tests, strict conventions, and tooling. That discipline works in any stack, from FPGA to cloud.
Work with me
Currently available for new engagements. Let's talk about your next project. Feel free to email or call; Dutch or English both work.