What is YANG, and what problem does it solve?

YANG (Yet Another Next Generation) is a data-modelling language that describes what can be configured, monitored or executed on a device, as a typed tree. Data models give one uniform way to describe the data across vendor platforms.

Without a model, every vendor's CLI and output looks different, so automation has to be written per platform. YANG describes the data itself:

• Tree structure. The model shows the path by which each element is reached, for example interfaces → interface → name.
• Configurable or not. Each element is either configuration (you can set it) or state (you can only read it, like a counter).
• Typed. Every element has a defined type, such as string, boolean or IP address, so for example an interface's enabled can only be true or false.

Because the model is machine-readable, tools can check data against it before sending it to a device. Protocols such as NETCONF and RESTCONF use YANG models as the contract for what they carry.

Go deeper:

• YANG — Wikipedia — the language, its history and where it is used.
• YangModels/yang on GitHub — real YANG modules from the IETF and the vendors.