Intro (Go)

Intro in Services Framework

NATS services have always been straightforward to write. However, with the services framework, the NATS client library further simplifies the building, discovery and monitoring of services. The framework automatically places all subscriptions in a queue group and provides functionality for building subject hierarchies and their handlers.

Without any additional effort, the library enables automatic service discovery and status reporting. The NATS CLI nats micro command provides a simple way to query and report all the services using this framework.

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$ nbe run services/intro/go

View the source code or learn how to run this example yourself

Code

package main


import (
	"encoding/json"
	"fmt"
	"log"
	"os"
	"strings"
	"time"

Make sure that we have the NATS Go client imported.

	"github.com/nats-io/nats.go"
	"github.com/nats-io/nats.go/micro"


	"golang.org/x/exp/slices"
)


func main() {

Determine a suitable URL for a connection to a NATS server

	url, exists := os.LookupEnv("NATS_URL")
	if !exists {
		url = nats.DefaultURL
	} else {
		url = strings.TrimSpace(url)
	}


	if strings.TrimSpace(url) == "" {
		url = nats.DefaultURL
	}

Connect to the server

	nc, err := nats.Connect(url)
	if err != nil {
		log.Fatal(err)
		return
	}

Defining a Service

This will create a service definition. Service definitions are made up of the service name (which can’t have things like whitespace in it), a version, and a description. Even with no running endpoints, this service is discoverable via the micro protocol and by service discovery tools like nats micro. All of the default background handlers for discovery, PING, and stats are started at this point.

	srv, err := micro.AddService(nc, micro.Config{
		Name:        "minmax",
		Version:     "0.0.1",
		Description: "Returns the min/max number in a request",
	})

Each time we create a service, it will be given a new unique identifier. If multiple copies of the minmax service are running across a NATS subject space, then tools like nats micro will consider them like unique instances of the one service and the endpoint subscriptions are queue subscribed, so requests will only be sent to one endpoint instance at a time.

	fmt.Printf("Created service: %s (%s)\n", srv.Info().Name, srv.Info().ID)


	if err != nil {
		log.Fatal(err)
		return
	}

Adding endpoints

Groups serve as namespaces and are used as a subject prefix when endpoints don’t supply fixed subjects. In this case, all endpoints will be listening on a subject that starts with minmax.

	root := srv.AddGroup("minmax")

Adds two endpoints to the service, one for the min operation and one for the max operation. Each endpoint represents a subscription. The supplied handlers will respond to minmax.min and minmax.max, respectively.

	root.AddEndpoint("min", micro.HandlerFunc(handleMin))
	root.AddEndpoint("max", micro.HandlerFunc(handleMax))

Now we can use standard NATS requests to communicate with the service endpoints.

Here we create a slice of numbers that we’re going to pass to the minmax service.

	requestSlice := []int{-1, 2, 100, -2000}

Creating a raw set of bytes to send to the service endpoints

	requestData, _ := json.Marshal(requestSlice)

Make a request of the min endpoint of the minmax service, within the minmax group. Note that there’s nothing special about this request, it’s just a regular NATS request.

	msg, _ := nc.Request("minmax.min", requestData, 2*time.Second)

Decode is just a convenience method that unmarshals the JSON response into the ServiceResult type.

	result := decode(msg)
	fmt.Printf("Requested min value, got %d\n", result.Min)

Make a request of the max endpoint of the minmax service, within the minmax group.

	msg, _ = nc.Request("minmax.max", requestData, 2*time.Second)
	result = decode(msg)
	fmt.Printf("Requested max value, got %d\n", result.Max)

The statistics being managed by micro should now reflect the call made to each endpoint, and we didn’t have to write any code to manage that.

	fmt.Printf("Endpoint '%s' requests: %d\n", srv.Stats().Endpoints[0].Name, srv.Stats().Endpoints[0].NumRequests)
	fmt.Printf("Endpoint '%s' requests: %d\n", srv.Stats().Endpoints[1].Name, srv.Stats().Endpoints[1].NumRequests)
}


func handleMin(req micro.Request) {
	var arr []int
	_ = json.Unmarshal([]byte(req.Data()), &arr)
	slices.Sort(arr)


	res := ServiceResult{Min: arr[0]}
	req.RespondJSON(res)
}


func handleMax(req micro.Request) {
	var arr []int
	_ = json.Unmarshal([]byte(req.Data()), &arr)
	slices.Sort(arr)


	res := ServiceResult{Max: arr[len(arr)-1]}
	req.RespondJSON(res)
}


func decode(msg *nats.Msg) ServiceResult {
	var res ServiceResult
	json.Unmarshal(msg.Data, &res)
	return res
}


type ServiceResult struct {
	Min int `json:"min,omitempty"`
	Max int `json:"max,omitempty"`
}

Output

Created service: minmax (WQ4HFw887xmrfEnXCjewSr)
Requested min value, got -2000
Requested max value, got 100
Endpoint 'min' requests: 1
Endpoint 'max' requests: 1

Recording

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