Use Pub/Sub to publish a message to a topic

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 different implementation of the underlying system, and allows operators to bring in their preferred infrastructure, for example Redis Streams, Kafka, etc.

Setup the Pub/Sub component

The first step is to setup the Pub/Sub component. For this guide, we’ll use Redis Streams, which is also installed by default on a local machine when running dapr init.

Note: When running Dapr locally, a pub/sub component YAML will automatically be created if it doesn’t exist in a directory called components in your current working directory.

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: messagebus
spec:
  type: pubsub.redis
  metadata:
  - name: redisHost
    value: localhost:6379
  - name: redisPassword
    value: ""

To deploy this into a Kubernetes cluster, fill in the metadata connection details in the yaml, and run kubectl apply -f pubsub.yaml.

Publish a topic

To publish a message to a topic, invoke the following endpoint on a Dapr instance:

curl -X POST http://localhost:3500/v1.0/publish/deathStarStatus \
	-H "Content-Type: application/json" \
	-d '{
       	     "status": "completed"
      	}'

The above example publishes a JSON payload to a deathStartStatus topic. Dapr will wrap the user payload in a Cloud Events v0.3 compliant envelope.