How-To: Trigger your application with input bindings

Use Dapr input bindings to trigger event driven applications

With input bindings, you can trigger your application when an event from an external resource occurs. An external resource could be a queue, messaging pipeline, cloud-service, filesystem, etc. An optional payload and metadata may be sent with the request.

Input bindings are ideal for event-driven processing, data pipelines, or generally reacting to events and performing further processing. Dapr input 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

This guide uses a Kafka binding as an example. You can find your preferred binding spec from the list of bindings components. In this guide:

1. The example invokes the /binding endpoint with checkout, the name of the binding to invoke.
2. The payload goes inside the mandatory data field, and can be any JSON serializable value.
3. The operation field tells the binding what action it needs to take. For example, the Kafka binding supports the create operation.
   • You can check which operations (specific to each component) are supported for every output binding.

Note

If you haven’t already, try out the bindings quickstart for a quick walk-through on how to use the bindings API.

Create a binding

Create a binding.yaml file and save to a components sub-folder in your application directory.

Create a new binding component named checkout. Within the metadata section, configure the following Kafka-related properties:

• The topic to which you’ll publish the message

• The broker

• Self-Hosted (CLI)

• Kubernetes

Use the --resources-path flag with the dapr run command to point to your custom resources directory.

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: checkout
spec:
  type: bindings.kafka
  version: v1
  metadata:
  # Kafka broker connection setting
  - name: brokers
    value: localhost:9092
  # consumer configuration: topic and consumer group
  - name: topics
    value: sample
  - name: consumerGroup
    value: group1
  # publisher configuration: topic
  - name: publishTopic
    value: sample
  - name: authRequired
    value: "false"

To deploy into a Kubernetes cluster, run kubectl apply -f binding.yaml.

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: checkout
spec:
  type: bindings.kafka
  version: v1
  metadata:
  # Kafka broker connection setting
  - name: brokers
    value: localhost:9092
  # consumer configuration: topic and consumer group
  - name: topics
    value: sample
  - name: consumerGroup
    value: group1
  # publisher configuration: topic
  - name: publishTopic
    value: sample
  - name: authRequired
    value: "false"

Listen for incoming events (input binding)

Configure your application to receive incoming events. If you’re using HTTP, you need to:

• Listen on a POST endpoint with the name of the binding, as specified in metadata.name in the binding.yaml file.
• Verify your application allows Dapr to make an OPTIONS request for this endpoint.

Below are code examples that leverage Dapr SDKs to demonstrate an output binding.

• Dotnet
• Java
• Python
• Go
• JavaScript

//dependencies
using System.Collections.Generic;
using System.Threading.Tasks;
using System;
using Microsoft.AspNetCore.Mvc;
//code
namespace CheckoutService.controller
{
    [ApiController]
    public class CheckoutServiceController : Controller
    {
        [HttpPost("/checkout")]
        public ActionResult<string> getCheckout([FromBody] int orderId)
        {
            Console.WriteLine("Received Message: " + orderId);
            return "CID" + orderId;
        }
    }
}

//dependencies
import org.springframework.web.bind.annotation.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import reactor.core.publisher.Mono;
//code
@RestController
@RequestMapping("/")
public class CheckoutServiceController {
    private static final Logger log = LoggerFactory.getLogger(CheckoutServiceController.class);
        @PostMapping(path = "/checkout")
        public Mono<String> getCheckout(@RequestBody(required = false) byte[] body) {
            return Mono.fromRunnable(() ->
                    log.info("Received Message: " + new String(body)));
        }
}

#dependencies
import logging
from dapr.ext.grpc import App, BindingRequest
#code
app = App()
@app.binding('checkout')
def getCheckout(request: BindingRequest):
    logging.basicConfig(level = logging.INFO)
    logging.info('Received Message : ' + request.text())
app.run(6002)

//dependencies
import (
    "encoding/json"
    "log"
    "net/http"
    "github.com/gorilla/mux"
)
//code
func getCheckout(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/json")
    var orderId int
    err := json.NewDecoder(r.Body).Decode(&orderId)
    log.Println("Received Message: ", orderId)
    if err != nil {
        log.Printf("error parsing checkout input binding payload: %s", err)
        w.WriteHeader(http.StatusOK)
        return
    }
}
func main() {
    r := mux.NewRouter()
    r.HandleFunc("/checkout", getCheckout).Methods("POST", "OPTIONS")
    http.ListenAndServe(":6002", r)
}

//dependencies 
import { DaprServer, CommunicationProtocolEnum } from '@dapr/dapr'; 
//code
const daprHost = "127.0.0.1"; 
const serverHost = "127.0.0.1";
const serverPort = "6002"; 
const daprPort = "3602"; 
start().catch((e) => {
    console.error(e);
    process.exit(1);
});
async function start() {
    const server = new DaprServer(serverHost, serverPort, daprHost, daprPort, CommunicationProtocolEnum.HTTP);
    await server.binding.receive('checkout', async (orderId) => console.log(`Received Message: ${JSON.stringify(orderId)}`));
    await server.startServer();
}

ACK-ing an event

Tell Dapr you’ve successfully processed an event in your application by returning a 200 OK response from your HTTP handler.

Rejecting an event

Tell Dapr the event was not processed correctly in your application and schedule it for redelivery by returning any response other than 200 OK. For example, a 500 Error.

Specifying a custom route

By default, incoming events will be sent to an HTTP endpoint that corresponds to the name of the input binding. You can override this by setting the following metadata property in binding.yaml:

name: mybinding
spec:
  type: binding.rabbitmq
  metadata:
  - name: route
    value: /onevent

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

• Bindings building block
• Bindings API
• Components concept
• Supported bindings

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