How-To: Trigger your application with input bindings

Use Dapr input bindings to trigger event driven applications

Using bindings, your code can be triggered with incoming events from different resources which can be anything: a queue, messaging pipeline, cloud-service, filesystem etc.

This is ideal for event-driven processing, data pipelines or just generally reacting to events and doing further processing.

Dapr bindings allow you to:

  • Receive events without including specific SDKs or libraries
  • Replace bindings without changing your code
  • Focus on business logic and not the event resource implementation

For more info on bindings, read this overview.

For a quickstart sample showing bindings, visit this link.

1. Create a binding

An input binding represents an event resource that Dapr uses to read events from and push to your application.

For the purpose of this HowTo, we’ll use a Kafka binding. You can find a list of the different binding specs here.

Create the following YAML file, named binding.yaml, and save this to a components sub-folder in your application directory. (Use the --components-path flag with dapr run to point to your custom components dir)

Note: When running in Kubernetes, apply this file to your cluster using kubectl apply -f binding.yaml

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: myevent
  namespace: default
spec:
  type: bindings.kafka
  metadata:
  - name: topics
    value: topic1
  - name: brokers
    value: localhost:9092
  - name: consumerGroup
    value: group1

Here, you create a new binding component with the name of myevent.

Inside the metadata section, configure the Kafka related properties such as the topics to listen on, the brokers and more.

2. Listen for incoming events

Now configure your application to receive incoming events. If using HTTP, you need to listen on a POST endpoint with the name of the binding as specified in metadata.name in the file. In this example, this is myevent.

The following example shows how you would listen for the event in Node.js, but this is applicable to any programming language

const express = require('express')
const bodyParser = require('body-parser')
const app = express()
app.use(bodyParser.json())

const port = 3000

app.post('/myevent', (req, res) => {
    console.log(req.body)
    res.status(200).send()
})

app.listen(port, () => console.log(`Kafka consumer app listening on port ${port}!`))

ACK-ing an event

In order to tell Dapr that you successfully processed an event in your application, return a 200 OK response from your HTTP handler.

res.status(200).send()

Rejecting an event

In order to tell Dapr that the event wasn’t processed correctly in your application and schedule it for redelivery, return any response different from 200 OK. For example, a 500 Error.

res.status(500).send()

Event delivery Guarantees

Event delivery guarantees are controlled by the binding implementation. Depending on the binding implementation, the event delivery can be exactly once or at least once.

References