API Definition
An Application Protocol Interface (API) Definition as a Contract
- Operations and Payloads: use APIs to outline operations (like “login” or “transfer”) and the data payloads a server must process.
- Application-Specific Protocols: By defining these operations and payloads, the API is essentially used to create an application-specific protocol.
- Layer Placement: In the network stack, these protocols and their APIs reside in the Application Layer, which handles high-level protocols like HTTP and FTP
Application-Specific Protocols
An application-specific protocol is essentially a set of custom-defined request and response rules, usually represented using text formats (such as JSON) or binary formats.
Definition: An Application Protocol Interface (API) is used to define a specific protocol for an application by outlining the various operations a server supports (such as “login,” “transfer,” “balance,” or “withdraw”) and the payload the server must process to fulfill those requests
Evolution from RPC API to HTTP API
RMI/RPC APIs are often tightly coupled, meaning any change to the API requires all clients to be recompiled.
HTTP APIs (which have largely replaced RMI in practice) are loosely coupled and language-independent. Changing an HTTP API does not require recompiling all clients, as the client is not directly invoking an internal server method.
• Level of Abstraction: While APIs for standard transport protocols like TCP and UDP are widely available in programming languages, they are frequently hidden from applications by higher-level abstractions such as HTTP or WebSockets.