Introduction

ygot (YANG Go Tools) is a collection of Go utilities that can be used to:

• Generate a set of Go structures and enumerated values for a set of YANG modules, with associated helper methods.
• Validate the contents of the Go structures against the YANG schema (e.g., validating range and regular expression constraints).
• Render the Go structures to an output format - such as JSON, or a set of gNMI Notifications for use in a deployment of streaming telemetry.

Whilst ygot is designed to work with any YANG module, for OpenConfig modules, it can provide transformations of the schema to optimise the data structures that are produced for use in systems that generate data instances of the models for configuration purposes. These helper methods require that the OpenConfig style guide patterns are implemented, a model can be verified to conform with these requirements using the OpenConfig linter.

Note: This is not an official Google product.

Getting Started with ygot

ygot consists of a number of parts, generator which is a binary using the ygen library to generate Go code from a set of YANG modules. ygot which provides helper methods for the ygen-produced structs - for example, rendering to JSON, or gNMI notifications - and ytypes which provides validation of the contents of ygen structs against the YANG schema.

The basic workflow for working with ygot is as follows:

• Generate Go code from a set of YANG files.
• Write code that populates the Go structures.
• Validate the contents of the Go structures.
• Output the contents of the structures as JSON or gNMI Notifications.

The demo/getting_started directory walks through this process for a simple implementation of openconfig-interfaces.

Generating Go Structures from YANG

The generator binary takes a set of YANG modules as input and outputs generated code. For example:

generator -output_file=<outputpath> -package_name=<pkg> [yangfiles]

Will output generated Go code for yangfiles (a space separated list of YANG files) to a file at <outputpath> with the Go package named <pkg>.

Most YANG modules include other modules. If these included modules are not within the current working directory, the path argument is used. The argument to path is a comma-separated list of directories which will be recursively searched for included files.

By default, ygot does not output an entity for the root of the schema tree - such that there is not a root entity to consider in code. If one is desired then it can be produced by using the generate_fakeroot argument. If specified an element with the name specified by fakeroot_name will be created in the output code. By default the fake root element is called device, since the root is often considered to be a device within the OpenConfig use case.

If schema transformations for OpenConfig are desired, these are enabled using the compress_paths argument.

Putting this all together, a command line to generate OpenConfig interfaces from the contents of the demo/getting_started/yang directory is:

go run $GOPATH/src/github.com/openconfig/ygot/generator/generator.go -path=yang -output_file=pkg/ocdemo/oc.go -package_name=ocdemo -generate_fakeroot -fakeroot_name=device -compress_paths=true  -exclude_modules=ietf-interfaces yang/openconfig-interfaces.yang

To allow this file to be auto-created, you can place a command which allows this code generation to be done automatically, either by creating a file within the YANG directory, or directly embedding this command within the source file that populates the structures. For an example, see the demo/getting_started/main.go file which includes:

//go:generate go run ../../generator/generator.go -path=yang -output_file=pkg/ocdemo/oc.go -package_name=ocdemo -generate_fakeroot -fakeroot_name=device -compress_paths=true  -exclude_modules=ietf-interfaces yang/openconfig-interfaces.yang

This means that we can simply type go generate within demo/getting_started - and the demo/getting_started/pkg/ocdemo/oc.go is created with the code bindings for the OpenConfig interfaces module.

Writing Code that Populates the Go Structures

Once we have generated the Go bindings for the YANG module, we're ready to use them in an application.

First, let's take a look at what the demo/getting_started/pkg/ocdemo/oc.go file contains. Particularly, looking at the fake root entity that we created (named device):

// Device represents the /device YANG schema element.
type Device struct {
        Interface       map[string]*Interface   `path:"interfaces/interface" rootname:"interface" module:"openconfig-interfaces"`
}

Since we enabled compress_paths, then the /interfaces/interface element in OpenConfig was represented as Interface at the root (called Device). We can see that since interface is a list, keyed by the name element, then the Interface map is keyed by a string.

Looking further down the tree at Interface:

// Interface represents the /openconfig-interfaces/interfaces/interface YANG schema element.
type Interface struct {
        AdminStatus  E_OpenconfigInterfaces_Interface_AdminStatus `path:"state/admin-status" module:"openconfig-interfaces"`
        Counters     *Interface_Counters                          `path:"state/counters" module:"openconfig-interfaces"`
        Description  *string                                      `path:"config/description" module:"openconfig-interfaces"`
        Enabled      *bool                                        `path:"config/enabled" module:"openconfig-interfaces"`
        HoldTime     *Interface_HoldTime                          `path:"hold-time" module:"openconfig-interfaces"`
        Ifindex      *uint32                                      `path:"state/ifindex" module:"openconfig-interfaces"`
        LastChange   *uint32                                      `path:"state/last-change" module:"openconfig-interfaces"`
        Mtu          *uint16                                      `path:"config/mtu" module:"openconfig-interfaces"`
        Name         *string                                      `path:"config/name|name" module:"openconfig-interfaces"`
        OperStatus   E_OpenconfigInterfaces_Interface_AdminStatus `path:"state/oper-status" module:"openconfig-interfaces"`
        Subinterface map[uint32]*Interface_Subinterface           `path:"subinterfaces/subinterface" module:"openconfig-interfaces"`
        Type         E_IETFInterfaces_InterfaceType               `path:"config/type" module:"openconfig-interfaces"`
}

Since OpenConfig path compression was enabled, then this Interface struct contains both direct descendants of /interfaces/interface - such as hold-time (in the Hold-Time field), along with those that were within the config and state fields. The path information is retained in the path struct tag -- but this isn't of interest to most developers working directly with the structs!

We can populate an interface by using a mixture of the helper methods, and directly setting fields of the struct. To create a new interface within the device, we can use the NewInterface method. A New... method is created for all lists within the YANG schema, and takes an argument of the key that is used for the list. It creates a new entry in the map with the specified key, returning an error if the key is already defined.

An example is shown below:

// Create a new interface called "eth0"
i, err := d.NewInterface("eth0")

// Set the fields that are within the struct.
i.AdminStatus = oc.OpenconfigInterfaces_Interface_AdminStatus_UP
i.Mtu = ygot.Uint16(1500)
i.Description = ygot.String("An Interface")

The ygot package provides helpers that allow an input type to returned as a pointer to be populated within the structs. For example, ygot.String returns a string pointer to the argument supplied.

Equally, we can define a new interface directly and add it to the map, without using the NewInterface method:

d.Interface["eth1"] = &oc.Interface{
	Name:        ygot.String("eth1"),
	Description: ygot.String("Another Interface"),
	Enabled:     ygot.Bool(false),
	Type:        oc.IETFInterfaces_InterfaceType_ethernetCsmacd,
}

Validating the Struct Contents

For some fields of the structures, enumerated values for example, values of fields are restricted such that they cannot have invalid values specified. In other cases, such as an IPv4 addresses, a string may not match a regular expression, but the Go structure does not restrict the contents of the struct being populated with this data.

By default each struct has a Validate method, this can be used to validate the struct's contents against the schema. Validate can be called against each structure, for example:

if err := d.Interface["eth0"].Validate(); err != nil {
	panic(fmt.Sprintf("Interface validation failed: %v", err))
}

In the case that the struct does not contain valid contents, Validate returns an error, containing a list of errors encountered during validation of the struct contents. Whilst the error can be directly handled as a comma-separated list of strings containing validation errors, casting it to the ytypes.Errors type allows handling of individual errors more cleanly. For example:

_, err = subif.Ipv4.NewAddress("Not a valid address")
if err := invalidIf.Validate(); err == nil {
	panic(fmt.Sprintf("Did not find invalid address, got nil err: %v", err))
} else {
	errs := err.(ytypes.Errors)
	for _, err := range errs {
		fmt.Printf("Got expected error: %v\n", err)	}
}

Outputting JSON from GoStructs

To serialise the structures to JSON, the ygot package provides an EmitJSON method which can be called with an arbitrary structure. In the example below, the fake root (Device) struct is called:

json, err := ygot.EmitJSON(d, &ygot.EmitJSONConfig{
	Format: ygot.RFC7951,
	Indent: "  ",
	RFC7951Config: &ygot.RFC7951JSONConfig{
		AppendModuleName: true,
 	},
})

if err != nil {
	panic(fmt.Sprintf("JSON demo error: %v", err))
}
fmt.Println(json)

EmitJSON performs both Validate and outputs the structure to JSON. The format can be an internal JSON format, or that described by RFC7951. Validation or JSON marshalling errors are directly returned.

Unmarshalling JSON to a GoStruct

ygot includes a function to unmarshal data from RFC7951-encoded JSON to a GoStruct. Since this function relies on the schema of the generated code, it us output within the generated code package - and named Unmarshal. The function takes an argument of a []byte (byte slice) containing the JSON document to be unmarshalled, and a pointer to the struct into which it should be unmarshalled. Any struct can be unmarshalled into. If data cannot be unmarshalled, an error is returned.

To unmarshal the example created in this guide, we call Unmarshal with the oc.Device struct pointer, and the JSON document:

// Device struct to unmarshal into.
loadd := &oc.Device{}
if err := oc.Unmarshal([]byte(json), loadd); err != nil {
  panic(fmt.Sprintf("Cannot unmarshal JSON: %v", err))
}

Currently, only the RFC7951 format of JSON is supported for unmarshalling, the Internal format supported by ygot is not yet supported.

For Developers

• Contributing - how to contribute to ygot.
• Contributors - Folks who have contributed to ygot, thanks very much!
• Design Choices - This document provides information pertaining to design choices made within the library itself; and should be reviewed in conjunction with the comments in the library code.

Licensing

Copyright 2017 Google Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.