Sebastian Fastner

For a product startup, I built a web-based appointment booking system for freelancers and companies as a Product Engineer, moving scheduling coordination out of email threads and into a shareable, multilingual booking link. Visitors can view available time slots without registration, book in their own time zone, and cancel appointments through a personal link. I owned the product from the public booking page to the administration area, with a strong focus on GDPR-oriented workflows, a cookie-free booking page, and reliable availability calculation.

React Router 7, TypeScript, and Convex form the technical foundation. The application integrates Google Calendar via OAuth as well as CalDAV-compatible calendars such as iCloud and Fastmail, generates iCal feeds, sends customizable email templates through customer-owned SMTP servers, and protects sensitive credentials through encryption. Better Auth with Pocket ID, Bunny Static Hosting with Edge Middleware, security headers, rate limits, health checks, and an extensive Vitest-/Playwright-oriented test base support operations and ongoing development.

For a product startup, I was responsible as Product Architect for a developer-first product for AI-powered localization designed to move translation work out of the release bottleneck. Instead of optimizing traditional translation tickets, manual exports, and downstream review loops, it focuses on a CLI-first workflow for PO- and MessageFormat-based projects. I brought product strategy, positioning, the open-core split, website, newsletter backend, and API/pipeline infrastructure into a cohesive product approach.

The product core separates an open, local i18n foundation from a commercial translation and control layer. Delta translations, terminology and protected terms, context enrichment, QA reports, targeted retries, and traceable PO metadata form the central product functionality. The website was built with React Router 7, Vanilla Extract, Lingui, and self-hosted Convex; newsletter flows, a preference center, double opt-in, unsubscribe mechanics, Relanto email delivery, Pocket ID admin authentication, and a Convex-based API key and pipeline structure support the product architecture.

Regrello Corp. – an AI-focused startup in supply chain management, acquired by Salesforce in late 2025 – needed a complete internationalization effort leveraging large language models. I led this initiative and evaluated the quality of translations from English into six target languages: French, German, Chinese, Spanish, Korean, and Thai. The developed plugin system enables automatic translation of all user inputs, significantly simplifying the product's international scaling.

A plugin system for the Go backend's GraphQL resolvers and mutations formed the core of the solution. New user inputs are automatically translated via LLM (Google Gemini and gemma3) and cached in Redis. Translations are orchestrated through Temporal workflows, for which I implemented corresponding queues and workers in Go. Since plaintext user content could not be stored in the Temporal queue for security reasons, I developed a performant encryption system for Temporal. The backend was structurally improved and decoupled, making it more maintainable and comprehensible.

As Senior Frontend Architect, I led the ongoing development of the DWS Robo-Advisor WISE, which from 2022 was continued under the spin-off MorgenFund GmbH. The migration to a cloud system modernized the infrastructure and enabled more flexible scaling. Porting the dashboard from React to React Native opened mobile devices as an additional channel for retail investors. The successful rollout to further European and Asian markets demonstrated the platform's international scalability.

Azure served as the new cloud platform for the system. Implementing a continuous deployment strategy accelerated release cycles and increased deployment reliability. The React Native port enabled code sharing between web and mobile. The multi-market architecture supported varying regulatory requirements and localizations.

17 online shops of the Witt Group – a subsidiary of the Otto Group – were to be replaced by a complete redevelopment. I led this initiative with a focus on scalability, multi-brand support across international markets, and ease of maintenance. The integration of Contentful empowered the SEO team with dynamic pages and customizable elements on the shop pages. The new architecture enabled faster time-to-market for new brands and markets.

Next.js, React, and Styled Components formed the frontend foundation, with product data connected to the ABOUT YOU backbone. Apollo GraphQL encapsulated the backend systems, creating a unified API layer. The deployment infrastructure with Kubernetes and Vault on AWS enabled secure continuous deployment. An automated test infrastructure with GraphQL query tests ensured release quality.

As Senior Frontend Architect, I developed a white-label investment platform (WISE) for DWS – from prototype phase to production system. The prototype served as a sales pitch for internal funding and presented a wizard for surveying end customers about their investment profile. The complete platform – comprising risk profiling, onboarding with digital contract signing via DocuSign, and a dashboard – was rolled out to multiple European and Asian countries.

Vue.js and MobX initially formed the basis of the prototype; after cross-project alignment, the frontend was ported to React and TypeScript. I created an enterprise-wide UI component library based on React and was responsible for the complete tooling stack with Webpack, Babel, and ESLint. Spring Boot served as middleware to the production investment and CRM systems, while Keycloak with OpenID/OAuth handled identity management.