In what cases can we not ensure synchronous interaction through REST API?

Synchronous interaction via REST API is impossible or inefficient in the following cases:

• Long operations: if the server takes a long time to process a request (e.g., complex calculations or database operations), the client will wait for a response, which can lead to timeouts.
• Unreliable networks: with unstable connections, synchronous calls can frequently be interrupted.
• Scalability: with a large number of clients, synchronous requests can block server resources.
• Asynchronous processes: if a task requires background execution (e.g., report generation), it is better to use asynchronous mechanisms (queues, WebSocket, callback).

In such cases, asynchronous approaches are used: the client sends a request, receives confirmation of task acceptance, and later requests the result or receives a notification.