How-To: Route messages to different event handlers

Learn how to route messages from a topic to different event handlers based on CloudEvent fields

Pub/sub routing is an implementation of content-based routing, a messaging pattern that utilizes a DSL instead of imperative application code. With pub/sub routing, you use expressions to route CloudEvents (based on their contents) to different URIs/paths and event handlers in your application. If no route matches, then an optional default route is used. This proves useful as your applications expand to support multiple event versions or special cases.

While routing can be implemented with code, keeping routing rules external from the application can improve portability.

This feature is available to both the declarative and programmatic subscription approaches.

Declarative subscription

For declarative subscriptions, use dapr.io/v2alpha1 as the apiVersion. Here is an example of subscriptions.yaml using routing:

apiVersion: dapr.io/v2alpha1
kind: Subscription
metadata:
  name: myevent-subscription
spec:
  pubsubname: pubsub
  topic: inventory
  routes:
    rules:
      - match: event.type == "widget"
        path: /widgets
      - match: event.type == "gadget"
        path: /gadgets
    default: /products
scopes:
  - app1
  - app2

Programmatic subscription

In the programmatic approach, the routes structure is returned instead of route. The JSON structure matches the declarative YAML:


import flask
from flask import request, jsonify
from flask_cors import CORS
import json
import sys

app = flask.Flask(__name__)
CORS(app)

@app.route('/dapr/subscribe', methods=['GET'])
def subscribe():
    subscriptions = [
      {
        'pubsubname': 'pubsub',
        'topic': 'inventory',
        'routes': {
          'rules': [
            {
              'match': 'event.type == "widget"',
              'path': '/widgets'
            },
            {
              'match': 'event.type == "gadget"',
              'path': '/gadgets'
            },
          ],
          'default': '/products'
        }
      }]
    return jsonify(subscriptions)

@app.route('/products', 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: "inventory",
      routes: {
        rules: [
          {
            match: 'event.type == "widget"',
            path: '/widgets'
          },
          {
            match: 'event.type == "gadget"',
            path: '/gadgets'
          },
        ],
        default: '/products'
      }
    }
  ]);
})

app.post('/products', (req, res) => {
  console.log(req.body);
  res.sendStatus(200);
});

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

        [Topic("pubsub", "inventory", "event.type ==\"widget\"", 1)]
        [HttpPost("widgets")]
        public async Task<ActionResult<Stock>> HandleWidget(Widget widget, [FromServices] DaprClient daprClient)
        {
            // Logic
            return stock;
        }

        [Topic("pubsub", "inventory", "event.type ==\"gadget\"", 2)]
        [HttpPost("gadgets")]
        public async Task<ActionResult<Stock>> HandleGadget(Gadget gadget, [FromServices] DaprClient daprClient)
        {
            // Logic
            return stock;
        }

        [Topic("pubsub", "inventory")]
        [HttpPost("products")]
        public async Task<ActionResult<Stock>> HandleProduct(Product product, [FromServices] DaprClient daprClient)
        {
            // Logic
            return stock;
        }

package main

import (
	"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:      "inventory",
			Routes: routes{
				Rules: []rule{
					{
						Match: `event.type == "widget"`,
						Path:  "/widgets",
					},
					{
						Match: `event.type == "gadget"`,
						Path:  "/gadgets",
					},
				},
				Default: "/products",
			},
		},
	}

	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))
}

<?php

require_once __DIR__.'/vendor/autoload.php';

$app = \Dapr\App::create(configure: fn(\DI\ContainerBuilder $builder) => $builder->addDefinitions(['dapr.subscriptions' => [
    new \Dapr\PubSub\Subscription(pubsubname: 'pubsub', topic: 'inventory', routes: (
      rules: => [
        ('match': 'event.type == "widget"', path: '/widgets'),
        ('match': 'event.type == "gadget"', path: '/gadgets'),
      ]
      default: '/products')),
]]));
$app->post('/products', function(
    #[\Dapr\Attributes\FromBody]
    \Dapr\PubSub\CloudEvent $cloudEvent,
    \Psr\Log\LoggerInterface $logger
    ) {
        $logger->alert('Received event: {event}', ['event' => $cloudEvent]);
        return ['status' => 'SUCCESS'];
    }
);
$app->start();

Common Expression Language (CEL)

In these examples, depending on the event.type, the application will be called on:

  • /widgets
  • /gadgets
  • /products

The expressions are written as Common Expression Language (CEL) where event represents the cloud event. Any of the attributes from the CloudEvents core specification can be referenced in the expression.

Example expressions

Match “important” messages:

has(event.data.important) && event.data.important == true

Match deposits greater than $10,000:

event.type == "deposit" && event.data.amount > 10000

Match multiple versions of a message:

event.type == "mymessage.v1"
event.type == "mymessage.v2"

CloudEvent attributes

For reference, the following attributes are from the CloudEvents specification.

Event Data

data

As defined by the term data, CloudEvents may include domain-specific information about the occurrence. When present, this information will be encapsulated within data.

  • Description: The event payload. This specification places no restriction on the information type. It is encoded into a media format, specified by the datacontenttype attribute (e.g. application/json), and adheres to the dataschema format when those respective attributes are present.
  • Constraints:
    • OPTIONAL

REQUIRED Attributes

The following attributes are required in all CloudEvents:

id

  • Type: String
  • Description: Identifies the event. Producers must ensure that source + id are unique for each distinct event. If a duplicate event is re-sent (e.g. due to a network error), it may have the same id. Consumers may assume that events with identical source and id are duplicates.
  • Constraints:
    • REQUIRED
    • Must be a non-empty string
    • Must be unique within the scope of the producer
  • Examples:
    • An event counter maintained by the producer
    • A UUID

source

  • Type: URI-reference

  • Description: Identifies the context in which an event happened. Often this includes information such as:

    • The type of the event source
    • The organization publishing the event
    • The process that produced the event

    The exact syntax and semantics behind the data encoded in the URI is defined by the event producer.

    Producers must ensure that source + id are unique for each distinct event.

    An application may:

    • Assign a unique source to each distinct producer, making it easier to produce unique IDs and preventing other producers from having the same source.
    • Use UUIDs, URNs, DNS authorities, or an application-specific scheme to create unique source identifiers.

    A source may include more than one producer. In this case, the producers must collaborate to ensure that source + id are unique for each distinct event.

  • Constraints:

    • REQUIRED
    • Must be a non-empty URI-reference
    • An absolute URI is RECOMMENDED
  • Examples:

    • Internet-wide unique URI with a DNS authority:
    • Universally-unique URN with a UUID:
      • urn:uuid:6e8bc430-9c3a-11d9-9669-0800200c9a66
    • Application-specific identifiers:
      • /cloudevents/spec/pull/123
      • /sensors/tn-1234567/alerts
      • 1-555-123-4567

specversion

  • Type: String

  • Description: The version of the CloudEvents specification used by the event. This enables the interpretation of the context. Compliant event producers must use a value of 1.0 when referring to this version of the specification.

    Currently, this attribute only includes the ‘major’ and ‘minor’ version numbers. This allows patch changes to the specification to be made without changing this property’s value in the serialization.

    Note: for ‘release candidate’ releases, a suffix might be used for testing purposes.

  • Constraints:

    • REQUIRED
    • Must be a non-empty string

type

  • Type: String
  • Description: Contains a value describing the event type related to the originating occurrence. Often, this attribute is used for routing, observability, policy enforcement, etc. The format is producer-defined and might include information like the version of the type. See Versioning of CloudEvents in the Primer for more information.
  • Constraints:
    • REQUIRED
    • Must be a non-empty string
    • Should be prefixed with a reverse-DNS name. The prefixed domain dictates the organization, which defines the semantics of this event type.
  • Examples:
    • com.github.pull_request.opened
    • com.example.object.deleted.v2

OPTIONAL Attributes

The following attributes are optional to appear in CloudEvents. See the Notational Conventions section for more information on the definition of OPTIONAL.

datacontenttype

  • Type: String per RFC 2046

  • Description: Content type of data value. This attribute enables data to carry any type of content, whereby format and encoding might differ from that of the chosen event format.

    For example, an event rendered using the JSON envelope format might carry an XML payload in data. The consumer is informed by this attribute being set to "application/xml".

    The rules for how data content is rendered for different datacontenttype values are defined in the event format specifications. For example, the JSON event format defines the relationship in section 3.1.

    For some binary mode protocol bindings, this field is directly mapped to the respective protocol’s content-type metadata property. You can find normative rules for the binary mode and the content-type metadata mapping in the respective protocol.

    In some event formats, you may omit the datacontenttype attribute. For example, if a JSON format event has no datacontenttype attribute, it’s implied that the data is a JSON value conforming to the "application/json" media type. In other words: a JSON-format event with no datacontenttype is exactly equivalent to one with datacontenttype="application/json".

    When translating an event message with no datacontenttype attribute to a different format or protocol binding, the target datacontenttype should be set explicitly to the implied datacontenttype of the source.

  • Constraints:

    • OPTIONAL
    • If present, must adhere to the format specified in RFC 2046
  • For Media Type examples, see IANA Media Types

dataschema

  • Type: URI
  • Description: Identifies the schema that data adheres to. Incompatible changes to the schema should be reflected by a different URI. See Versioning of CloudEvents in the Primer for more information.
  • Constraints:
    • OPTIONAL
    • If present, must be a non-empty URI

subject

  • Type: String

  • Description: This describes the event subject in the context of the event producer (identified by source). In publish-subscribe scenarios, a subscriber will typically subscribe to events emitted by a source. The source identifier alone might not be sufficient as a qualifier for any specific event if the source context has internal sub-structure.

    Identifying the subject of the event in context metadata (opposed to only in the data payload) is helpful in generic subscription filtering scenarios, where middleware is unable to interpret the data content. In the above example, the subscriber might only be interested in blobs with names ending with ‘.jpg’ or ‘.jpeg’. With the subject attribute, you can construct a simple and efficient string-suffix filter for that subset of events.

  • Constraints:

    • OPTIONAL
    • If present, must be a non-empty string
  • Example:
    A subscriber might register interest for when new blobs are created inside a blob-storage container. In this case:

    • The event source identifies the subscription scope (storage container)
    • The event type identifies the “blob created” event
    • The event id uniquely identifies the event instance to distinguish separately created occurrences of a same-named blob.

    The name of the newly created blob is carried in subject:

time

  • Type: Timestamp
  • Description: Timestamp of when the occurrence happened. If the time of the occurrence cannot be determined, then this attribute may be set to some other time (such as the current time) by the CloudEvents producer. However, all producers for the same source must be consistent in this respect. In other words, either they all use the actual time of the occurrence or they all use the same algorithm to determine the value used.
  • Constraints:
    • OPTIONAL
    • If present, must adhere to the format specified in RFC 3339

Community call demo

Watch this video on how to use message routing with pub/sub:

Next steps