02. What is YANG?

OpenConfig adopted YANG data modeling language (RFC 6020) to define vendor-neutral configuration model and operational state model.
So what is YANG?

YANG super basic overview:

YANG is an IETF standard described in RFC 6020.
YANG is a data modeling language used to model both configuration data and state data manipulated by the Network Configuration Protocol (NETCONF).
This allows a complete description of all data sent between a NETCONF client and server.
YANG can model configuration data (writable data), as well as state data (read-only status), based on the "config" statement:

• When a node is tagged with config true, it is flagged as configuration data. it can be returned using NETCONF's <get-config> operation.
• When a node is tagged with config false, it is flagged as state data. it can be returned using NETCONF's <get> operation, not the <get-config> operation. it cannot be manipulated using <edit-config> netconf operation.

YANG modules can be translated into an equivalent XML syntax called YANG Independent Notation (YIN)

leaf:

A leaf:

• Has exactly one value of a particular type.
• Has no children

YANG Example:

leaf host-name {
  type string;
  mandatory true;
  config true;
  description "Hostname for this system";
}
leaf cpu-temp {
  type int32
  units degrees-celsius;
  config false;
  description ”Current temperature in CPU";
}

NETCONF XML Example:

<host-name>my.example.com</host-name>

host-name is a leaf. It has no children. It has one value of type string. It is a configuration data: it can be manipulated using <edit-config> netconf operation, it is returned in NETCONF <get‐config> operation.

cpu-temp is a leaf. It has no children. It has one value of type int32 (32-bit signed integer). It is a state data. it is returned in NETCONF <get> operation. it is not returned in NETCONF <get‐config> operation. it can not be manipulated using <edit-config> netconf operation.

leaf-list

Like the leaf node but defines a set of nodes rather than a single node. Each node has a value but no child nodes.
So a leaf-list:

• has no children
• holds multiple values of (the same) particular type.

YANG Example:

leaf-list domain-search {
  type string;
  ordered-by user;
  description "List of domain names to search";
}

NETCONF XML Example:

<domain-search>high.example.com</domain-search>
<domain-search>low.example.com</domain-search>

container

A container:

• has no value
• has only a set of child nodes.

YANG Example:

container system {
  container login {
    leaf message {
      type string;
      description
        "Message given at start of login session";
    }
  }
}

NETCONF XML Example:

<system>
  <login>
    <message>Good morning</message>
  </login>
</system>

YANG Example:

container system {
  leaf host-name {
    type string;
    description "Hostname for this system";
  }
  leaf-list domain-search {
    type string;
    description "List of domain names to search";
  }
  container login {
    leaf message {
      type string;
      description
        "Message given at start of login session";
    }
  }
}