Tell how you designed REST API for complex backend services and what approaches you used to ensure scalability and fault tolerance.

When designing REST APIs for complex backend services, I adhere to the following approaches:

• Clear separation of resources and their representations — each resource has its own URI, and proper HTTP methods are used (GET, POST, PUT, DELETE).
• API versioning — to ensure backward compatibility and enable evolution.
• Use of pagination, filtering, and sorting for handling large volumes of data.
• API documentation using OpenAPI/Swagger.

For scalability and fault tolerance:

• Horizontal scaling of services using containerization and orchestration (e.g., Kubernetes).
• Caching at the API Gateway or CDN level to reduce load.
• Use of message queues (RabbitMQ, Kafka) for asynchronous task processing.
• Distributed logging and monitoring for quick detection and resolution of issues.

Example: when designing an API for product orders, I allocated separate services for managing the catalog, orders, and payments, with interactions occurring through REST and asynchronous messages, which increased the system's fault tolerance and scalability.