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How-To: Publish a message and subscribe to a topic

Learn how to send messages to a topic with one service and subscribe to that topic in another service

Introduction

Pub/Sub is a common pattern in a distributed system with many services that want to utilize decoupled, asynchronous messaging. Using Pub/Sub, you can enable scenarios where event consumers are decoupled from event producers.

Dapr provides an extensible Pub/Sub system with At-Least-Once guarantees, allowing developers to publish and subscribe to topics. Dapr provides components for pub/sub, that enable operators to use their preferred infrastructure, for example Redis Streams, Kafka, etc.

Content Types

When publishing a message, it’s important to specify the content type of the data being sent. Unless specified, Dapr will assume text/plain. When using Dapr’s HTTP API, the content type can be set in a Content-Type header. gRPC clients and SDKs have a dedicated content type parameter.

Example:

The below code example loosely describes an application that processes orders. In the example, there are two services - an order processing service and a checkout service. Both services have Dapr sidecars. The order processing service uses Dapr to publish a message to RabbitMQ and the checkout service subscribes to the topic in the message queue.

Diagram showing state management of example service

Step 1: Setup the Pub/Sub component

The following example creates applications to publish and subscribe to a topic called orders.

The first step is to setup the Pub/Sub component:


The pubsub.yaml is created by default on your local machine when running dapr init. Verify by opening your components file under %UserProfile%\.dapr\components\pubsub.yaml on Windows or ~/.dapr/components/pubsub.yaml on Linux/MacOS.

In this example, RabbitMQ is used for publish and subscribe. Replace pubsub.yaml file contents with the below contents.

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: order_pub_sub
spec:
  type: pubsub.rabbitmq
  version: v1
  metadata:
  - name: host
    value: "amqp://localhost:5672"
  - name: durable
    value: "false"
  - name: deletedWhenUnused
    value: "false"
  - name: autoAck
    value: "false"
  - name: reconnectWait
    value: "0"
  - name: concurrency
    value: parallel
scopes:
  - orderprocessing
  - checkout

You can override this file with another Redis instance or another pubsub component by creating a components directory containing the file and using the flag --components-path with the dapr run CLI command.


To deploy this into a Kubernetes cluster, fill in the metadata connection details of your desired pubsub component in the yaml below, save as pubsub.yaml, and run kubectl apply -f pubsub.yaml.

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: order_pub_sub
  namespace: default
spec:
  type: pubsub.rabbitmq
  version: v1
  metadata:
  - name: host
    value: "amqp://localhost:5672"
  - name: durable
    value: "false"
  - name: deletedWhenUnused
    value: "false"
  - name: autoAck
    value: "false"
  - name: reconnectWait
    value: "0"
  - name: concurrency
    value: parallel
scopes:
  - orderprocessing
  - checkout

Step 2: Subscribe to topics

Dapr allows two methods by which you can subscribe to topics:

  • Declaratively, where subscriptions are defined in an external file.
  • Programmatically, where subscriptions are defined in user code.

Declarative subscriptions

You can subscribe to a topic using the following Custom Resources Definition (CRD). Create a file named subscription.yaml and paste the following:

apiVersion: dapr.io/v1alpha1
kind: Subscription
metadata:
  name: order_pub_sub
spec:
  topic: orders
  route: /checkout
  pubsubname: order_pub_sub
scopes:
- orderprocessing
- checkout

The example above shows an event subscription to topic orders, for the pubsub component order_pub_sub.

  • The route field tells Dapr to send all topic messages to the /checkout endpoint in the app.
  • The scopes field enables this subscription for apps with IDs orderprocessing and checkout.

Set the component with:

Place the CRD in your ./components directory. When Dapr starts up, it loads subscriptions along with components.

Note: By default, Dapr loads components from $HOME/.dapr/components on MacOS/Linux and %USERPROFILE%\.dapr\components on Windows.

You can also override the default directory by pointing the Dapr CLI to a components path:


dapr run --app-id myapp --components-path ./myComponents -- dotnet run

dapr run --app-id myapp --components-path ./myComponents -- mvn spring-boot:run

dapr run --app-id myapp --components-path ./myComponents -- python3 app.py

dapr run --app-id myapp --components-path ./myComponents -- go run app.go

dapr run --app-id myapp --components-path ./myComponents -- npm start

In Kubernetes, save the CRD to a file and apply it to the cluster:

kubectl apply -f subscription.yaml

Below are code examples that leverage Dapr SDKs to subscribe to a topic.


//dependencies 
using System.Collections.Generic;
using System.Threading.Tasks;
using System;
using Microsoft.AspNetCore.Mvc;
using Dapr;
using Dapr.Client;

//code
namespace CheckoutService.controller
{
    [ApiController]
    public class CheckoutServiceController : Controller
    {
         //Subscribe to a topic 
        [Topic("order_pub_sub", "orders")]
        [HttpPost("checkout")]
        public void getCheckout([FromBody] int orderId)
        {
            Console.WriteLine("Subscriber received : " + orderId);
        }
    }
}

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id checkout --app-port 6002 --dapr-http-port 3602 --dapr-grpc-port 60002 --app-ssl dotnet run

//dependencies
import io.dapr.Topic;
import io.dapr.client.domain.CloudEvent;
import org.springframework.web.bind.annotation.*;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import reactor.core.publisher.Mono;

//code
@RestController
public class CheckoutServiceController {

    private static final Logger log = LoggerFactory.getLogger(CheckoutServiceController.class);
     //Subscribe to a topic
    @Topic(name = "orders", pubsubName = "order_pub_sub")
    @PostMapping(path = "/checkout")
    public Mono<Void> getCheckout(@RequestBody(required = false) CloudEvent<String> cloudEvent) {
        return Mono.fromRunnable(() -> {
            try {
                log.info("Subscriber received: " + cloudEvent.getData());
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        });
    }
}

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id checkout --app-port 6002 --dapr-http-port 3602 --dapr-grpc-port 60002 mvn spring-boot:run

#dependencies
from cloudevents.sdk.event import v1
from dapr.ext.grpc import App
import logging
import json

#code
app = App()
logging.basicConfig(level = logging.INFO)
#Subscribe to a topic 
@app.subscribe(pubsub_name='order_pub_sub', topic='orders')
def mytopic(event: v1.Event) -> None:
    data = json.loads(event.Data())
    logging.info('Subscriber received: ' + str(data))

app.run(6002)

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id checkout --app-port 6002 --dapr-http-port 3602 --app-protocol grpc -- python3 CheckoutService.py

//dependencies
import (
	"log"
	"net/http"
	"context"

	"github.com/dapr/go-sdk/service/common"
	daprd "github.com/dapr/go-sdk/service/http"
)

//code
var sub = &common.Subscription{
	PubsubName: "order_pub_sub",
	Topic:      "orders",
	Route:      "/checkout",
}

func main() {
	s := daprd.NewService(":6002")
   //Subscribe to a topic
	if err := s.AddTopicEventHandler(sub, eventHandler); err != nil {
		log.Fatalf("error adding topic subscription: %v", err)
	}
	if err := s.Start(); err != nil && err != http.ErrServerClosed {
		log.Fatalf("error listenning: %v", err)
	}
}

func eventHandler(ctx context.Context, e *common.TopicEvent) (retry bool, err error) {
	log.Printf("Subscriber received: %s", e.Data)
	return false, nil
}

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id checkout --app-port 6002 --dapr-http-port 3602 --dapr-grpc-port 60002 go run CheckoutService.go

//dependencies
import { DaprServer, CommunicationProtocolEnum } from 'dapr-client'; 

//code
const daprHost = "127.0.0.1"; 
const serverHost = "127.0.0.1";
const serverPort = "6002"; 

start().catch((e) => {
    console.error(e);
    process.exit(1);
});

async function start(orderId) {
    const server = new DaprServer(
        serverHost, 
        serverPort, 
        daprHost, 
        process.env.DAPR_HTTP_PORT, 
        CommunicationProtocolEnum.HTTP
    );
    //Subscribe to a topic
    await server.pubsub.subscribe("order_pub_sub", "orders", async (orderId) => {
        console.log(`Subscriber received: ${JSON.stringify(orderId)}`)
    });
    await server.startServer();
}

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id checkout --app-port 6002 --dapr-http-port 3602 --dapr-grpc-port 60002 npm start

The /checkout endpoint matches the route defined in the subscriptions and this is where Dapr will send all topic messages to.

Step 3: Publish a topic

Start an instance of Dapr with an app-id called orderprocessing:

dapr run --app-id orderprocessing --dapr-http-port 3601

Then publish a message to the orders topic:

dapr publish --publish-app-id orderprocessing --pubsub order_pub_sub --topic orders --data '{"orderId": "100"}'

Then publish a message to the orders topic:

curl -X POST http://localhost:3601/v1.0/publish/order_pub_sub/orders -H "Content-Type: application/json" -d '{"orderId": "100"}'

Then publish a message to the orders topic:

Invoke-RestMethod -Method Post -ContentType 'application/json' -Body '{"orderId": "100"}' -Uri 'http://localhost:3601/v1.0/publish/order_pub_sub/orders'

Dapr automatically wraps the user payload in a Cloud Events v1.0 compliant envelope, using Content-Type header value for datacontenttype attribute.

Below are code examples that leverage Dapr SDKs to publish a topic.


//dependencies
using System;
using System.Collections.Generic;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;
using Dapr.Client;
using Microsoft.AspNetCore.Mvc;
using System.Threading;

//code
namespace EventService
{
    class Program
    {
        static async Task Main(string[] args)
        {
           string PUBSUB_NAME = "order_pub_sub";
           string TOPIC_NAME = "orders";
           while(true) {
                System.Threading.Thread.Sleep(5000);
                Random random = new Random();
                int orderId = random.Next(1,1000);
                CancellationTokenSource source = new CancellationTokenSource();
                CancellationToken cancellationToken = source.Token;
                using var client = new DaprClientBuilder().Build();
                //Using Dapr SDK to publish a topic
                await client.PublishEventAsync(PUBSUB_NAME, TOPIC_NAME, orderId, cancellationToken);
                Console.WriteLine("Published data: " + orderId);
		        }
        }
    }
}

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id orderprocessing --app-port 6001 --dapr-http-port 3601 --dapr-grpc-port 60001 --app-ssl dotnet run

//dependencies
import io.dapr.client.DaprClient;
import io.dapr.client.DaprClientBuilder;
import io.dapr.client.domain.Metadata;
import static java.util.Collections.singletonMap;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.Random;
import java.util.concurrent.TimeUnit;

//code
@SpringBootApplication
public class OrderProcessingServiceApplication {

	private static final Logger log = LoggerFactory.getLogger(OrderProcessingServiceApplication.class);

	public static void main(String[] args) throws InterruptedException{
		String MESSAGE_TTL_IN_SECONDS = "1000";
		String TOPIC_NAME = "orders";
		String PUBSUB_NAME = "order_pub_sub";

		while(true) {
			TimeUnit.MILLISECONDS.sleep(5000);
			Random random = new Random();
			int orderId = random.nextInt(1000-1) + 1;
			DaprClient client = new DaprClientBuilder().build();
      //Using Dapr SDK to publish a topic
			client.publishEvent(
					PUBSUB_NAME,
					TOPIC_NAME,
					orderId,
					singletonMap(Metadata.TTL_IN_SECONDS, MESSAGE_TTL_IN_SECONDS)).block();
			log.info("Published data:" + orderId);
		}
	}
}

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id orderprocessing --app-port 6001 --dapr-http-port 3601 --dapr-grpc-port 60001 mvn spring-boot:run

#dependencies  
import random
from time import sleep    
import requests
import logging
import json
from dapr.clients import DaprClient

#code
logging.basicConfig(level = logging.INFO)
while True:
    sleep(random.randrange(50, 5000) / 1000)
    orderId = random.randint(1, 1000)
    PUBSUB_NAME = 'order_pub_sub'
    TOPIC_NAME = 'orders'
    with DaprClient() as client:
        #Using Dapr SDK to publish a topic
        result = client.publish_event(
            pubsub_name=PUBSUB_NAME,
            topic_name=TOPIC_NAME,
            data=json.dumps(orderId),
            data_content_type='application/json',
        )
    logging.info('Published data: ' + str(orderId))

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id orderprocessing --app-port 6001 --dapr-http-port 3601 --app-protocol grpc python3 OrderProcessingService.py

//dependencies
import (
	"context"
	"log"
	"math/rand"
	"time"
	"strconv"
	dapr "github.com/dapr/go-sdk/client"
)

//code
var (
	PUBSUB_NAME = "order_pub_sub"
	TOPIC_NAME  = "orders"
)

func main() {
	for i := 0; i < 10; i++ {
		time.Sleep(5000)
		orderId := rand.Intn(1000-1) + 1
		client, err := dapr.NewClient()
		if err != nil {
			panic(err)
		}
		defer client.Close()
		ctx := context.Background()
    //Using Dapr SDK to publish a topic
		if err := client.PublishEvent(ctx, PUBSUB_NAME, TOPIC_NAME, []byte(strconv.Itoa(orderId))); 
		err != nil {
			panic(err)
		}

		log.Println("Published data: " + strconv.Itoa(orderId))
	}
}

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id orderprocessing --app-port 6001 --dapr-http-port 3601 --dapr-grpc-port 60001 go run OrderProcessingService.go

//dependencies
import { DaprServer, DaprClient, CommunicationProtocolEnum } from 'dapr-client'; 

const daprHost = "127.0.0.1"; 

var main = function() {
    for(var i=0;i<10;i++) {
        sleep(5000);
        var orderId = Math.floor(Math.random() * (1000 - 1) + 1);
        start(orderId).catch((e) => {
            console.error(e);
            process.exit(1);
        });
    }
}

async function start(orderId) {
    const PUBSUB_NAME = "order_pub_sub"
    const TOPIC_NAME  = "orders"
    const client = new DaprClient(daprHost, process.env.DAPR_HTTP_PORT, CommunicationProtocolEnum.HTTP);
    console.log("Published data:" + orderId)
    //Using Dapr SDK to publish a topic
    await client.pubsub.publish(PUBSUB_NAME, TOPIC_NAME, orderId);
}

function sleep(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
}

main();

Navigate to the directory containing the above code, then run the following command to launch a Dapr sidecar and run the application:

dapr run --app-id orderprocessing --app-port 6001 --dapr-http-port 3601 --dapr-grpc-port 60001 npm start

Step 4: ACK-ing a message

In order to tell Dapr that a message was processed successfully, return a 200 OK response. If Dapr receives any other return status code than 200, or if your app crashes, Dapr will attempt to redeliver the message following at-least-once semantics.

Sending a custom CloudEvent

Dapr automatically takes the data sent on the publish request and wraps it in a CloudEvent 1.0 envelope. If you want to use your own custom CloudEvent, make sure to specify the content type as application/cloudevents+json.

Read about content types here, and about the Cloud Events message format.

Example


Publish a custom CloudEvent to the orders topic:

dapr publish --publish-app-id orderprocessing --pubsub order_pub_sub --topic orders --data '{"specversion" : "1.0", "type" : "com.dapr.cloudevent.sent", "source" : "testcloudeventspubsub", "subject" : "Cloud Events Test", "id" : "someCloudEventId", "time" : "2021-08-02T09:00:00Z", "datacontenttype" : "application/cloudevents+json", "data" : {"orderId": "100"}}'

Publish a custom CloudEvent to the orders topic:

curl -X POST http://localhost:3601/v1.0/publish/order_pub_sub/orders -H "Content-Type: application/cloudevents+json" -d '{"specversion" : "1.0", "type" : "com.dapr.cloudevent.sent", "source" : "testcloudeventspubsub", "subject" : "Cloud Events Test", "id" : "someCloudEventId", "time" : "2021-08-02T09:00:00Z", "datacontenttype" : "application/cloudevents+json", "data" : {"orderId": "100"}}'

Publish a custom CloudEvent to the orders topic:

Invoke-RestMethod -Method Post -ContentType 'application/cloudevents+json' -Body '{"specversion" : "1.0", "type" : "com.dapr.cloudevent.sent", "source" : "testcloudeventspubsub", "subject" : "Cloud Events Test", "id" : "someCloudEventId", "time" : "2021-08-02T09:00:00Z", "datacontenttype" : "application/cloudevents+json", "data" : {"orderId": "100"}}' -Uri 'http://localhost:3601/v1.0/publish/order_pub_sub/orders'

Next steps


Last modified July 26, 2022: Update translations for v1.6 (#2665) (e18b8c6)