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Code generation from Aether Model API specification

Calling on the makefile target:

make oapi-codegen-aether-2.1.0

has the effect of first validating that the OpenAPI 3 specifications contains no errors.

It then proceeds to generate Go code that performs the following functions:

  • the types from the OpenAPI spec
  • the spec (so that the API specification can be retrieved by clients programmatically)
  • the paths and methods from the OpenAPI spec
  • conversion code to go from OpenAPI types to YANG model types
  • conversion code to go from YANG model types to OpenAPI types
  • code to call gNMI methods Get() and Set() from REST methods GET, POST, DELETE
  • code to integrate with the Echo Web Server built in to this project that serves the API

Breaking the steps down individually:

Generating Go types

The first step generates Go type definitions from the Schema definitions in the specification:

oapi-codegen -generate types -package types -o pkg/aether_2_1_0/types/aether-2.1.0-types.go api/aether-2.1.0-openapi3.yaml

Encoding specification in Go

The second run bundles up the API specification as a blob in base64, so that it may be extracted programmatically through the REST interface.

oapi-codegen -generate spec -package server -o pkg/aether_2_1_0/server/aether-2.1.0-spec.go api/aether-2.1.0-openapi3.yaml

Generating implementation of handlers

The third step generates an implementation of handlers for accessing the gNMI client.

    type ServerImpl struct {
        GnmiClient  southbound.GnmiClient
        GnmiTimeout time.Duration
    }

and creates a public and a private method for each of the paths in the specification. e.g.

func (i *ServerImpl) GetApplication(ctx echo.Context, enterpriseId externalRef1.EnterpriseId, applicationId string) error

These public functions are an implementation of the ServerInterface interface generated in the next step

func (i *ServerImpl) gnmiGetApplication(ctx context.Context,
	openApiPath string, enterpriseId externalRef1.EnterpriseId, args ...string) (*externalRef1.Application, error)

The public method is an implementation of a Handler from the 'server' file.

oapi-codegen \
    -generate types,server \
    -import-mapping imp1:"github.com/onosproject/aether-roc-api/pkg/aether_2_1_0/types",imp2:"github.com/onosproject/aether-models/models/aether-2.1.x/api" \
    -package server \
    -templates pkg/codegen/templates \
    -o pkg/aether_2_1_0/server/aether-2.1.0-impl.go \
    api/aether-2.1.0-openapi3.yaml

This call does not use the built in template of oapi-codegen but custom templates from the codegen/templates folder.

Also note the -import-mapping here - it is used to force the set of Go imports to include these paths.

Counterintuitively, the prefixes given here e.g. imp1 are discarded by oapi-codegen, and imports are ordered alphabetically, being given the prefixes externalRef0 etc inside the code

To correct an omission of an import, a sed statement is necessary to fix the file after generation.

The sed statement here is written with Posix compliance, so that it can run on linux and MacOS.

Generate Handlers

The fourth step generates the handlers for the Echo Web server.

The RegisterHandlers func gives a list of all the paths and their methods and their respective handler functions on the serverInterfaceWrapper struct.

These methods pass only an echo.Context argument (as all Echo handlers do) like:

func (w *serverInterfaceWrapper) GetApplication(ctx echo.Context) error

so this function extracts parameters like enterprise-id and application-id from this Context and passes to another level of function defined in the ServerInterface interface:

// ServerInterface represents all server handlers.
type ServerInterface interface {
    // GET /application Container
    // (GET /aether/v2.1.x/{enterprise-id}/application/{application-id})
    GetApplication(ctx echo.Context, enterpriseId externalRef0.EnterpriseId, applicationId string) error
...

It is this interface that the methods in the impl step above implement.

oapi-codegen \
    -generate server \
    -import-mapping externalRef0:"github.com/onosproject/aether-roc-api/pkg/aether_2_1_0/types" \
    -package server \
    -templates pkg/codegen/modified \
    -o pkg/aether_2_1_0/server/aether-2.1.0-server.go \
    api/aether-2.1.0-openapi3.yaml

Note that here again the templates in use are customized ones from codegen/modified

The same kind of sed command is used to correct some imports

Generate OpenAPI schema object structs to gNMI object structs mapping

The fifth step generates code that can transform structs generated from OpenAPI types (from the first steps above), to structs generated by YGOT from the YANG model, during the aether-models generation stage.

The YGOT structs can then be serialized to JSON and passed in gNMI calls as values.

It creates a function per OpenAPI type - e.g. for Application

func EncodeToGnmiApplication(
	jsonObj *types.Application, needKey bool, removeIndex bool, enterpriseId types.EnterpriseId, parentPath string, params ...string) (
	[]*gnmi.Update, error) {

In the template pkg/codegen/convert-oapi-gnmi/typedef.tmpl there is a variable $tnbase which may have white space padding at the left hand side - the template syntax {{- does not correctly get rid of this leading white space properly unless it is on a new line. To make it distinguishable we add in the marker > ! on the new line, so it can be removed easily later with sed.

oapi-codegen \
    -generate types \
    -import-mapping externalRef0:"github.com/onosproject/aether-models/models/aether-2.1.x/api" \
    -package server \
    -templates pkg/codegen/convert-oapi-gnmi \
    -o pkg/aether_2_1_0/server/aether-2.1.0-convert-oapi-gnmi.go \
    api/aether-2.1.0-openapi3.yaml

Note that here the templates in use are customized ones from codegen/convert-oapi-gnmi

The sed command used here is to format a particular hack that is used in the templates.

Specifically it is removing a > ! inserted in the code generation

Generate gNMI object structs to OpenAPI schema object structs mapping

The sixth step generates code that can transform structs created from YGOT (from the YANG model) to OpenAPI structs.

The YGOT structs are unmarshalled from the gNMI payloads (in JSON encoding) coming back in to the system in reply to gNMI GetRequests.

The template generates two different kinds of struct.

  1. with a pointer receiver to a YGOT parent object and
  2. a second that's a plain function that breaks apart the specific subsection of the YGOT object

An example of 1. is:

func (d *ModelPluginDevice) toApplication(params ...string) (*types.Application, error)

and an example of 2. is:

func toApplication(ygotObjValue *reflect.Value, params ...string) (*types.Application, error)

Here the generated code relies on utility functions that can walk the Go struct hierarchy e.g.

  • utils.FindModelPluginObject
oapi-codegen \
    -generate types \
    -import-mapping externalRef0:"github.com/onosproject/aether-models/models/aether-2.1.x/api" \
    -package server \
    -templates pkg/codegen/convert-gnmi-oapi \
    -o pkg/aether_2_1_0/server/aether-2.1.0-convert-gnmi-oapi.go \
    api/aether-2.1.0-openapi3.yaml

Note that here the templates in use are customized ones from codegen/convert-gnmi-oapi

The sed command used here is to format a particular hack that is used in the templates.

Specifically it is removing a > ! inserted in the code generation