Freelancer profile translated to English.
Back to original languageAbout Yohann
Modernization of .NET / SaaS legacy applications, progressive migration, APIs, IT integration, observability, and simplification of business workflows.
I help companies and software vendors regain more stable, maintainable applications capable of sustainably supporting their growth.
Tech Lead / Fullstack .NET / React Developer with 20+ years of experience, I work to secure foundations from the design stage or unblock existing projects to stabilize, accelerate, and evolve your applications – without starting from scratch.
I don't limit myself to code: I work on architecture, reliability, delivery, and overall IT system consistency.
🔧 Expertise
👉 Refactoring of legacy applications
👉 Progressive migration (.NET Framework → .NET / React)
👉 Modernization & reduction of technical debt
👉 SaaS development & business applications
👉 Integration (ERP, CRM, WMS, APIs)
👉 Workflow automation & data reliability
👉 Monitoring, observability & production stability
👉 DevOps, CI/CD & environment optimization
👉 Security (SSO, IAM, Keycloak)
🎯 I intervene when
✔ Evolutions become slow or risky
✔ Your application is difficult to maintain
✔ You need to modernize without breaking the existing system
✔ Your systems don't communicate well with each other
✔ Your production lacks visibility or stability
✔ A project is blocked or no longer progressing
✔ You need quickly operational support
🚀 Goal
Help you regain a reliable, scalable system that supports your business without becoming a bottleneck. A stable application and an architecture capable of supporting your growth.
30+ projects — SaaS, ERP, logistics, business portals
Environment & technologies: .NET 5 to 10, ASP.NET Core, C#, React, TypeScript, SQL Server, PostgreSQL, Azure, Docker, microservices, REST APIs, event-driven, CI/CD, Git.
English
Conversational
French
Fluent
Can work on-site
Rennes (up to 50km), Nantes (up to 50km), Paris (up to 30km), Lyon (up to 10km)
Experience
- FF Randonnée PédestreMigration of a legacy IT system to a modern .NET / APIs / SSO architectureCIVIC AND SOCIAL ORGANIZATIONSSeptember 2025 - Today (11 months)Rennes, FranceMission as Tech Lead & Fullstack .NET / React Developer on the complete modernization of a legacy web application and SaaS.👉 Progressive migration from ASP.NET WebForms to .NET Core / React, refactoring of the application architecture, modernization of APIs, and SSO / IAM integration (Keycloak).Objective: Make the IT system reliable, accelerate development, and modernize the platform without interrupting business activities.🔹 Main missions✅ Fullstack .NET / React DevelopmentDesign and development of web applications in C#, .NET Core, React, and REST APIs, with a focus on performance and maintainability.✅ Architecture & APIsApplication structuring, service decomposition, design of robust and scalable APIs.✅ Application & Database MigrationMigration of a legacy application (.NET Framework → .NET Core / React) and SQL Server → PostgreSQL migration, with performance optimization and data reliability.✅ DevOps, CI/CD & CloudSetup of AWS infrastructure, CI/CD pipelines, deployment automation, and reliability improvement.✅ Security & IAM (Keycloak)SSO integration, access management, and workflow security. Keycloak customization (templates, emails) and plugin development.#
- FF Randonnée PédestreDesign and development of a member portal connected to the business IT systemCIVIC AND SOCIAL ORGANIZATIONSMay 2026 - Today (3 months)Rennes, FranceMission as Front-end Next.js / React / TypeScript Developer on the design and evolution of a secure member area connected to the business IT system.👉 Development of a secure "My Account" portal allowing the centralization of various user journeys: profile, membership, renewal, licenses, donations, volunteering, subscription services, and access to specific content.Objective: Offer a modern, reliable, and maintainable interface, capable of unifying access to member services while integrating seamlessly with existing business APIs.🔹 Main missions✅ Front-end Next.js / React DevelopmentDesign and development of a web application in Next.js, React, and TypeScript, with particular attention to code quality, maintainability, and user experience fluidity.✅ API Integration & Business Data StructuringConnecting the front-end to business REST APIs, managing typed models, dynamic data rendering, and handling different use cases based on user journeys.✅ Authentication & Access SecurityImplementation of authentication via OIDC / Keycloak, session management, protection of private routes, and navigation security within the connected area.✅ Complex Forms & User JourneysDevelopment of multi-step forms, data validation, interface state management, and design of clear journeys for memberships, licenses, and personal information updates.✅ Front-end Architecture & Reusable ComponentsCode organization, feature-based decomposition, creation of reusable UI components, server-side state management, and establishment of a scalable technical foundation.✅ Responsive UI & User ExperienceDesign of desktop/mobile adaptive interfaces, overall ergonomics improvement, and consistency of experience across the entire portal.
- DIMOTRANSFullstack .NET / React Developer – Logistics extranet & business application connected to WMSLOGISTICS AND SUPPLY CHAINApril 2025 - Today (1 year and 4 months)Rennes, FranceMission as Fullstack Developer on the design and development of a SaaS business application (logistics extranet) connected to a WMS.👉 Objective: Make logistics workflows reliable and simple, improve data visibility, and build a performant, scalable, and integrated web platform with the IT system.---🔹 Main missions✅ Fullstack Development (.NET / React / Next.js)Design of a business web application in C#, .NET Core, React, Next.js, and APIs, with a focus on performance, maintainability, and user experience.✅ System Integration & Workflow AutomationConnection to the WMS and integration into the IT system, automation and simplification of stock, order, and shipping workflows, reliability of data exchange between systems.✅ Architecture & ScalabilityStructuring the application to handle large volumes, performance optimization, and improvement of scalability.✅ Modern Frontend & User ExperienceOptimized web interfaces (SSR / React), reduction of loading times, and improvement of user experience.📌 Logistics Extranet – Real-time management of stocks, orders, and flows📌 Client & Partner Portal – Secure access to business data📌 Traceability Module – WMS-connected shipment tracking💡 Results✔ Better visibility on logistics flows and stocks✔ Reliability of data between WMS and business tools✔ Simplification of exchanges between systems and business processes✔ Reduction of errors and manual data entry✔ Improved performance and user experience✔ More scalable, stable, and maintainable application✔ Smoother and better-controlled IT system
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Education
- DUT Computer ScienceIUT Lannion2002Analyse BDD