Architecture & System Design
Design resilient services and integration boundaries that can grow with your product and team.
Tech Lead and Fullstack Software Engineer
I partner with teams that need senior technical leadership, cleaner architecture, and reliable shipping speed.
Hi, I am Julius Seltenheim, an Engineering Lead, Architect, and Senior Fullstack Developer with 10+ years of experience in high-impact environments. I work hands-on in code while aligning delivery, product, and stakeholders around a realistic execution plan.
From legacy modernization and cloud-native backends to frontend architecture and team enablement, I help organizations build software that performs under pressure and stays maintainable over time.
- 10+ years delivery experience
- Lead for startups and enterprises
- Backend specialist
How I Support Teams
Choose focused expert support or a broader technical lead engagement.
Delivery Acceleration
Fix planning friction, improve ticket quality, and establish engineering workflows that ship predictably.
Legacy Modernization
Refactor critical paths and replace brittle components without disrupting product delivery.
Lead Developer Enablement
Mentor developers and establish technical standards that compound quality over time.
Credentials
A few signals that matter when you need senior technical ownership quickly.
Leadership
Hands-on technical leadership
Led engineering teams of up to 5 developers, owned system architecture, improved planning quality, and helped teams ship with more clarity and momentum.
Specialization
Public sector and security-critical systems
Experience across regulated environments, cryptography and PKI-related systems, distributed cloud storage, and public-sector application delivery.
Background
International and interdisciplinary foundation
German native, English professional, Spanish intermediate. M.Sc. and B.Sc. in International Media and Computing from HTW Berlin.
Testimonials
Recent feedback from client-side leadership teams.
It was an absolute pleasure working with Julius over the past four months. He joined our cross-functional team, collaborating with our partner in the public sector to build a React and Python-based web application. Julius seamlessly stepped in to cover for a colleague on parental leave and quickly became an integral part of the team.
Julius took on the role of leading the development team, excelling in areas such as full-stack development, stakeholder communication, and feature preparation. His energy, initiative, and expertise were invaluable, driving significant improvements to the project's structure and code quality.
The team’s feedback was overwhelmingly positive, and I wouldn’t hesitate to work with him again in the future.
Thank you, Julius, for your outstanding contributions!
Julius worked with us on a project to improve our video rendering system architecture. He led the efforts as the technical lead of a small team of developers. Over the span of 4 months he was able to identify issues and bottlenecks of the previous system, design an new architecture, plan the project and implement many crucial parts of the new architecture.
Through his fullstack expertise he was able to both direct and execute the backend efforts as well as implement the frontend tasks himself, as he was the only developer on the team with relevant frontend knowledge.
It was a pleasure working with him and seeing the project coming to live in time and budget. The feedback from his team was extremely positive and his work will have a lasting effect on our product.
Julius, thank you so much, we hope to work again with you soon.
Projects
Representative engagements
Legacy Application Re-Boot
Freelance Consultant / Backend Engineer · Bundesdruckerei GmbHEnabled a production-ready backend for a legacy public-sector platform by guiding teams through domain constraints and leading migration of Couchbase services to PostgreSQL.
Impact: Helped move a constrained legacy backend toward production readiness in a regulated public-sector context.
Stack: Java / Spring Boot / PostgreSQL / Kubernetes / RabbitMQ / Cryptography & PKIPublic Sector Service Platform
Freelance Engineering Lead / Architect · neXenio GmbHLed full-stack delivery for a public-sector web platform, improving roadmap clarity and ticket quality to accelerate delivery while strengthening long-term maintainability.
Impact: Increased delivery clarity and execution quality by improving architecture decisions, ticket readiness, and team alignment.
Stack: Python / FastAPI / React / TypeScript / Azure / KubernetesRendering Engine Performance Initiative
Freelance Project Lead / Software Engineer · simpleshow GmbHImproved rendering performance by identifying scalability bottlenecks, translating findings into an actionable roadmap, and implementing high-impact architectural changes.
Impact: Delivered both the performance roadmap and key implementation pieces needed for future scalability.
Stack: Java / Spring Boot / TypeScript / Angular / AWS Lambda / ffmpeg / Video RenderingMein Cornelsen
Senior Backend Engineer · Cornelsen Verlag GmbHBuilt platform architecture and release processes from the ground up, introducing microfrontends and independent deployments to improve delivery speed and reliability across teams.
Impact: Established architecture and release practices that supported autonomous teams and more reliable deployment flow.
Stack: Java / Spring Boot / React / PostgreSQL / KubernetesDistributed Cloud Storage (BDrive)
Senior Software Engineer Backend · neXenio GmbHModernized a distributed cloud storage backend by splitting monolithic components into microservices, introducing relational persistence, and implementing SMS-based 2FA.
Impact: Reduced monolithic complexity while improving security and operational scalability with service decomposition and 2FA.
Stack: Java / Spring Boot / Couchbase / PostgreSQL / Kubernetes / RabbitMQ / Cryptography & PKIReady to accelerate your engineering delivery?
If you need senior technical leadership for a critical project, I can usually start with a focused discovery call within one week.