APIs
REST, gRPC and GraphQL for request-response, and WebSockets, long polling and SSE for pushing data to clients.
What an API is
An API (Application Programming Interface) is the contract that lets one program call another: what you may ask for, the shape of the request, the shape of the answer. In system design you meet two kinds:
- Request-response: the client asks, the server answers (REST, gRPC, GraphQL).
- Push: the server sends data without being asked each time (WebSockets, SSE, long polling).
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