The role of API:

1. Remote procedure call (RPC): Communication between programs is realized through the process (or task) acting on the shared data cache.

2. Standard Query Language (SQL): It is a standard query language for accessing data, and realizes data sharing between applications through a universal database.

3. File transfer: File transfer realizes data sharing between applications by sending formatted files.

4. Information delivery: refers to small formatted information between loosely coupled or tightly coupled applications, and data sharing is achieved through direct communication between programs. Principle: API (Application Programming Interface, Application Programming Interface) is a number of pre-defined functions, the purpose is to provide applications and developers with the ability to access a set of routines based on certain software or hardware without having to access the source code or understand the inner workings Details of the mechanism.