Designed and developed backend APIs using NestJS and TypeScript for a tenant administration system based on OAuth2, integrated with Keycloak. Managed the tenant registration flow, account creation, and access/session management through Keycloak API integration. Built endpoints for user authentication and authorization, including token handling, role and permission management, and session lifecycle control. Developed an activity logging module to capture critical tenant events such as login, logout, profile updates, and administrative actions for auditing and security purposes. Designed the database schema and relationships between tenants, users, sessions, and activity logs using MongoDB. Implemented middleware and guards to secure endpoints, including JWT validation and role-based access control (RBAC). Documented all API endpoints using Swagger and performed unit and integration testing with Jest.