Declarative and programmatic subscription methods

Learn more about the methods by which Dapr allows you to subscribe to topics.

Pub/sub API subscription methods

Dapr applications can subscribe to published topics via two methods that support the same features: declarative and programmatic.

The examples below demonstrate pub/sub messaging between a checkout app and an orderprocessing app via the orders topic. The examples demonstrate the same Dapr pub/sub component used first declaratively, then programmatically.

Declarative subscriptions

You can subscribe declaratively to a topic using an external component file. This example uses a YAML component file named subscription.yaml:

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

Here the subscription called order:

• Uses the pub/sub component called pubsub to subscribes to the topic called orders.
• Sets the route field to send all topic messages to the /checkout endpoint in the app.
• Sets scopes field to scope this subscription for access only by apps with IDs orderprocessing and checkout.

When running Dapr, set the YAML component file path to point Dapr to the component.

• .NET
• Java
• Python
• JavaScript
• Go
• Kubernetes

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

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

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

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

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

In Kubernetes, apply the component to the cluster:

kubectl apply -f subscription.yaml

In your application code, subscribe to the topic specified in the Dapr pub/sub component.

• .NET
• Java
• Python
• JavaScript
• Go

 //Subscribe to a topic 
[HttpPost("checkout")]
public void getCheckout([FromBody] int orderId)
{
    Console.WriteLine("Subscriber received : " + orderId);
}

import io.dapr.client.domain.CloudEvent;
 //Subscribe to a topic
@PostMapping(path = "/checkout")
public Mono<Void> getCheckout(@RequestBody(required = false) CloudEvent<String> cloudEvent) {
    return Mono.fromRunnable(() -> {
        try {
            log.info("Subscriber received: " + cloudEvent.getData());
        } 
    });
}

from cloudevents.sdk.event import v1
#Subscribe to a topic 
@app.route('/checkout', methods=['POST'])
def checkout(event: v1.Event) -> None:
    data = json.loads(event.Data())
    logging.info('Subscriber received: ' + str(data))

const express = require('express')
const bodyParser = require('body-parser')
const app = express()
app.use(bodyParser.json({ type: 'application/*+json' }));
// listen to the declarative route
app.post('/checkout', (req, res) => {
  console.log(req.body);
  res.sendStatus(200);
});

//Subscribe to a topic
var sub = &common.Subscription{
    PubsubName: "pubsub",
    Topic:      "orders",
    Route:      "/checkout",
}
func eventHandler(ctx context.Context, e *common.TopicEvent) (retry bool, err error) {
    log.Printf("Subscriber received: %s", e.Data)
    return false, nil
}

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

Programmatic subscriptions

The programmatic approach returns the routes JSON structure within the code, unlike the declarative approach’s route YAML structure. In the example below, you define the values found in the declarative YAML subscription above within the application code.

• .NET
• Java
• Python
• JavaScript
• Go

[Topic("pubsub", "orders")]
[HttpPost("/checkout")]
public async Task<ActionResult<Order>>Checkout(Order order, [FromServices] DaprClient daprClient)
{
    // Logic
    return order;
}

or

// Dapr subscription in [Topic] routes orders topic to this route
app.MapPost("/checkout", [Topic("pubsub", "orders")] (Order order) => {
    Console.WriteLine("Subscriber received : " + order);
    return Results.Ok(order);
});

private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
@Topic(name = "checkout", pubsubName = "pubsub")
@PostMapping(path = "/orders")
public Mono<Void> handleMessage(@RequestBody(required = false) CloudEvent<String> cloudEvent) {
  return Mono.fromRunnable(() -> {
    try {
      System.out.println("Subscriber received: " + cloudEvent.getData());
      System.out.println("Subscriber received: " + OBJECT_MAPPER.writeValueAsString(cloudEvent));
    } catch (Exception e) {
      throw new RuntimeException(e);
    }
  });

@app.route('/dapr/subscribe', methods=['GET'])
def subscribe():
    subscriptions = [
      {
        'pubsubname': 'pubsub',
        'topic': 'checkout',
        'routes': {
          'rules': [
            {
              'match': 'event.type == "order"',
              'path': '/orders'
            },
          ],
          'default': '/orders'
        }
      }]
    return jsonify(subscriptions)
@app.route('/orders', methods=['POST'])
def ds_subscriber():
    print(request.json, flush=True)
    return json.dumps({'success':True}), 200, {'ContentType':'application/json'}
app.run()

const express = require('express')
const bodyParser = require('body-parser')
const app = express()
app.use(bodyParser.json({ type: 'application/*+json' }));
const port = 3000
app.get('/dapr/subscribe', (req, res) => {
  res.json([
    {
      pubsubname: "pubsub",
      topic: "checkout",
      routes: {
        rules: [
          {
            match: 'event.type == "order"',
            path: '/orders'
          },
        ],
        default: '/products'
      }
    }
  ]);
})
app.post('/orders', (req, res) => {
  console.log(req.body);
  res.sendStatus(200);
});
app.listen(port, () => console.log(`consumer app listening on port ${port}!`))

package main
    "encoding/json"
    "fmt"
    "log"
    "net/http"
    "github.com/gorilla/mux"
)
const appPort = 3000
type subscription struct {
    PubsubName string            `json:"pubsubname"`
    Topic      string            `json:"topic"`
    Metadata   map[string]string `json:"metadata,omitempty"`
    Routes     routes            `json:"routes"`
}
type routes struct {
    Rules   []rule `json:"rules,omitempty"`
    Default string `json:"default,omitempty"`
}
type rule struct {
    Match string `json:"match"`
    Path  string `json:"path"`
}
// This handles /dapr/subscribe
func configureSubscribeHandler(w http.ResponseWriter, _ *http.Request) {
    t := []subscription{
        {
            PubsubName: "pubsub",
            Topic:      "checkout",
            Routes: routes{
                Rules: []rule{
                    {
                        Match: `event.type == "order"`,
                        Path:  "/orders",
                    },
                },
                Default: "/orders",
            },
        },
    }
    w.WriteHeader(http.StatusOK)
    json.NewEncoder(w).Encode(t)
}
func main() {
    router := mux.NewRouter().StrictSlash(true)
    router.HandleFunc("/dapr/subscribe", configureSubscribeHandler).Methods("GET")
    log.Fatal(http.ListenAndServe(fmt.Sprintf(":%d", appPort), router))
}

Next Steps

• Try out the pub/sub Quickstart
• Follow: How-To: Configure pub/sub components with multiple namespaces
• Learn more about declarative and programmatic subscription methods.
• Learn about topic scoping
• Learn about message TTL
• Learn more about pub/sub with and without CloudEvent
• List of pub/sub components
• Read the pub/sub API reference

Last modified January 18, 2023: `components-path` —> `resources-path` in how-tos and quickstarts (#3016) (235626fa)