If the server fails, the queue persist the message (optionally, even if the machine shutdown).
When the server is working again, it receives the pending message.
If the server gives a response to the call and the client fails, if the client didn't acknowledge the response the message is persisted.
You have contention, you can decide how many requests are handled by the server (call it worker instead).
You don't expect an immediate synchronous response, but you can implement/simulate synchronous calls.