Developing agents with NDK in Go#

This guide explains how to consume the NDK service when developers write the agents in a Go¹ programming language.

In addition to the publicly available protobuf files, which define the NDK Service, Nokia also provides generated Go bindings for data access classes of NDK in a nokia/srlinux-ndk-go repo.

The github.com/nokia/srlinux-ndk-go package provided in that repository enables developers of NDK agents to immediately start writing NDK applications without the need to generate the Go package themselves.

Establish gRPC channel with NDK manager and instantiate an NDK client#

Additional information

To call service methods, a developer first needs to create a gRPC channel to communicate with the NDK manager application running on SR Linux.

This is done by passing the NDK server address - localhost:50053 - to grpc.Dial() as follows:

import (
    "google.golang.org/grpc"
)

conn, err := grpc.Dial("localhost:50053", grpc.WithInsecure())
if err != nil {
  ...
}
defer conn.Close()

Once the gRPC channel is setup, we need to instantiate a client (often called stub) to perform RPCs. The client is obtained using the NewSdkMgrServiceClient method provided.

import "github.com/nokia/srlinux-ndk-go/v21/ndk"

client := ndk.NewSdkMgrServiceClient(conn)

Register the agent with the NDK manager#

Additional information

Agent must be first registered with SR Linux by calling the AgentRegister method available on the returned SdkMgrServiceClient interface. The initial agent state is created during the registration process.

Agent's context#

Go context is a required parameter for each RPC service method. Contexts provide the means of enforcing deadlines and cancellations as well as transmitting metadata within the request.

During registration, SR Linux will be expecting a key-value pair with the agent_name key and a value of the agent's name passed in the context of an RPC. The agent name is defined in the agent's YAML file.

ctx, cancel := context.WithCancel(context.Background())
defer cancel()
// appending agent's name to the context metadata
ctx = metadata.AppendToOutgoingContext(ctx, "agent_name", "ndkDemo")

Agent registration#

AgentRegister method takes in the context ctx that is by now has agent name as its metadata and an AgentRegistrationRequest.

AgentRegistrationRequest structure can be passed in with its default values for a basic registration request.

import "github.com/nokia/srlinux-ndk-go/v21/ndk"

r, err := client.AgentRegister(ctx, &ndk.AgentRegistrationRequest{})
if err != nil {
    log.Fatalf("agent registration failed: %v", err)
}

AgentRegister method returns AgentRegistrationResponse and an error. Response can be additionally checked for status and error description.

Register notification streams#

Additional information

Create subscription stream#

A subscription stream needs to be created first before any of the subscription types can be added.
SdkMgrServiceClient first creates the subscription stream by executing NotificationRegister method with a NotificationRegisterRequest only field Op set to a value of const NotificationRegisterRequest_Create. This effectively creates a stream which is identified with a StreamID returned inside the NotificationRegisterResponse.

StreamId must be associated when subscribing/unsubscribing to certain types of router notifications.

req := &ndk.NotificationRegisterRequest{
    Op: ndk.NotificationRegisterRequest_Create,
}

resp, err := client.NotificationRegister(ctx, req)
if err != nil {
    log.Fatalf("Notification Register failed with error: %v", err)
} else if resp.GetStatus() == ndk.SdkMgrStatus_kSdkMgrFailed {
    r.log.Fatalf("Notification Register failed with status %d", resp.GetStatus())
}

log.Debugf("Notification Register was successful: StreamID: %d SubscriptionID: %d", resp.GetStreamId(), resp.GetSubId())

Add notification subscriptions#

Once the StreamId is acquired, a client can register notifications of a particular type to be delivered over that stream.

Different types of notifications types can be subscribed to by calling the same NotificationRegister method with a NotificationRegisterRequest having Op field set to NotificationRegisterRequest_AddSubscription and certain SubscriptionType selected.

In the example below we would like to receive notifications from the Config service, hence we specify NotificationRegisterRequest_Config subscription type.

subType := &ndk.NotificationRegisterRequest_Config{ // This is unique to each notification type (Config, Intf, etc.).
    Config: &ndk.ConfigSubscriptionRequest{},
}
req := &ndk.NotificationRegisterRequest{
    StreamId:          resp.GetStreamId(), // StreamId is retrieved from the NotificationRegisterResponse
    Op:                ndk.NotificationRegisterRequest_AddSubscription,
    SubscriptionTypes: subType,
}
resp, err := r.mgrStub.NotificationRegister(r.ctx, req)
if err != nil {
    log.Fatalf("Agent could not subscribe for config notification")
} else if resp.GetStatus() == ndk.SdkMgrStatus_kSdkMgrFailed {
    log.Fatalf("Agent could not subscribe for config notification with status  %d", resp.GetStatus())
}
log.Infof("Agent was able to subscribe for config notification with status %d", resp.GetStatus())

Streaming notifications#

Additional information

Actual streaming of notifications is a task for another service - SdkNotificationService. This service requires developers to create its own client, which is done with NewSdkNotificationServiceClient function.

The returned SdkNotificationServiceClient interface has a single method NotificationStream that is used to start streaming notifications.

NotificationsStream is a server-side streaming RPC which means that SR Linux (server) will send back multiple event notification responses after getting the agent's (client) request.

To tell the server to start streaming notifications that were subscribed to before the NewSdkNotificationServiceClient executes NotificationsStream method where NotificationStreamRequest struct has its StreamId field set to the value that was obtained at subscription stage.

req := &ndk.NotificationStreamRequest{
    StreamId: resp.GetStreamId(),
}
streamResp, err := notifClient.NotificationStream(ctx, req)
if err != nil {
    log.Fatal("Agent failed to create stream client with error: ", err)
}

Handle the streamed notifications#

Additional information

Handling notifications starts with reading the incoming notification messages and detecting which type this notification is exactly. When the type is known the client reads the fields of a certain notification. Here is the pseudocode that illustrates the flow:

func HandleNotifications(stream ndk.SdkNotificationService_NotificationStreamClient) {
    for { // loop until stream returns io.EoF
        notification stream response (nsr) := stream.Recv()
        for notif in nsr.Notification { // nsr.Notification is a slice of `Notification`
            if notif.GetConfig() is not nil {
                1. config notif = notif.GetConfig()
                2. handle config notif
            } else if notif.GetIntf() is not nil {
                1. intf notif = notif.GetIntf()
                2. handle intf notif
            } ... // Do this if statement for every notification type the agent is subscribed to
        }
    }
}

NotificationStream method of the SdkNotificationServiceClient interface will return a stream client SdkNotificationService_NotificationStreamClient.

SdkNotificationService_NotificationStreamClient contains a Recv() to retrieve notifications one by one. At the end of a stream Rev() will return io.EOF.

Recv() returns a *NotificationStreamResponse which contains a slice of Notification.

Notification struct has GetXXX() methods defined which retrieve the notification of a specific type. For example: GetConfig returns ConfigNotification.

Once the specific XXXNotification has been extracted using the GetXXX() method, users can access the fields of the notification and process the data contained within the notification using GetKey() and GetData() methods.

Exiting gracefully#

Agent needs to handle SIGTERM signal that is sent when a user invokes stop command via SR Linux CLI. The following is the required steps to cleanly stop the agent:

1. Remove any agent's state if it was set using TelemetryDelete method of a Telemetry client.
2. Delete notification subscriptions stream NotificationRegister method with Op set to NotificationRegisterRequest_Delete.
3. Invoke use AgentUnRegister() method of a SdkMgrServiceClient interface.
4. Close gRPC channel with the sdk_mgr.

Logging#

To debug an agent, the developers can analyze the log messages that the agent produced. If the agent's logging facility used stdout/stderr to write log messages, then these messages will be found at /var/log/srlinux/stdout/ directory.

The default SR Linux debug messages are found in the messages directory /var/log/srlinux/buffer/messages; check them when something went wrong within the SR Linux system (agent registration failed, IDB server warning messages, etc.).

Logrus is a popular structured logger for Go that can log messages of different levels of importance, but developers are free to choose whatever logging package they see fit.