There are four big components of JET
1. Python: Python 2.7.8 is supported on all JUNOS devices. By using Python users can develop and execute Python scripts on JUNOS.
2. JSON: JSON is popular data exchange program because of its simplicity, light weight and rich native support in several programming languages. Most of the python programmers use JSON as data exchange program. With this help now JUNOS operational show commands can display data in JSON format also along with XML format. JUNOS configuration can be provided directly in JSON format which can help programmer to write code in python and push configuration on box without having worry of XML.
3. Fast Programmatic Configuration Database: It is fast programmatic configuration database used by JUNOS and gives access to controllers applications which are pushing fast state changes onto JUNOS. SDN applications can make the changes at very high rate with 1000 of configuration changes per second with no configuration validation. The onus of configuration validation will be taken care by the external SDN controllers and applications.
4. JET API: SDN requires programmable interface for fast rate of configuration changes with multiple instance of databases without the contention of database resources. JET APIs are AVATAR of JUNOS SDK which enables APP developer to program the JUNOS control and data plane. JET APIs constitutes of 5 things as mentioned below a. Route b. Interface c. Firewall d. Management APIs e. Notifications JET APIs framework is language and Operating System agnostic which means APP developer can use any of the language and OS of their own choice. The APPS which are written will be binary compatible and decoupled from the JUNOS releases. All the APIs exposed externally by JET will be used internally as well. Source: