key.fit manages tenants, studios, members, subscriptions, access permissions and billing in one shared SaaS application.
Multi-tenant SaaS
One platform for several independent gym companies.
Each tenant manages its studios, staff, members and contracts in a shared system. Business rules stay cleanly separated, while operations and infrastructure are provided centrally.
Domain and technology
Membership management, contract logic, access and billing work together.
The project connects backend, frontend, PostgreSQL, authentication, VPN transport, device communication, deployments and backups into one productive system.
Access and locking system
key.fit connects the web application with physical access control.
RFID and Raspberry Pi controllers
RFID cards, writer devices and door controllers are securely connected to the platform.
On-premises devices communicate with the backend over secured connections. They synchronise permissions, report events back and enable controlled access to studios.
Offline-capable permissions
Doors must work reliably even during temporary connection problems.
Lock controllers keep local permission lists. When access rights change, the lock version is increased so devices update their data selectively.
Billing is an area where experience and maintainability matter most.
Subscription and payment logic
Memberships, booking days, payment start and billing groups are modelled cleanly.
The platform supports subscription-based flows with traceable start dates, billing periods and banking-related processes. Such rules must not work approximately — they must stay verifiable.
SEPA-related processes
Bank data and billing data need clear domain boundaries.
IBAN, account holder, booking runs and payment times are part of a domain model that must stay maintainable long term. This is exactly where classic engineering experience pays off.
Application service providing
Aberger Software does not just deliver code, it operates the application as a productive service.
Infrastructure and operations
Backend, database, authentication, reverse proxy, VPN and backups are part of the overall responsibility.
Ongoing operation requires reproducible deployments, controlled database changes, monitoring, backup strategies and stable recovery processes.
Business-critical responsibility
When access and billing are affected, software quality is not an add-on.
key.fit shows how domain knowledge, infrastructure understanding and robust delivery must come together so a system works reliably in everyday use.
key.fit shows how spec-driven development is used in real, complex systems: not as prompt play, but as a controlled process from specification through implementation to operations.
OpenSpec as a tool
OpenSpec structures requirements, design decisions and tasks for human and agent.
The method behind it is spec-driven development: specifications are not decoration, but a steering part of delivery. That makes agentic development reviewable, testable and maintainable.
Iterative agent workflows
Specification, implementation, tests and feedback are brought together in controlled loops.
The agents do not work free-floating, but against defined requirements, review criteria and tests. This creates speed without losing control, traceability and maintainability.
