Assignment 2 - Using Dapr for pub/sub with Kafka

Table of contents

Instructions
Step 1: Publish messages in the TrafficControlService
Step 2: Receive messages in the FineCollectionService
Step 3: Test the application
Step 4: Debug Dapr applications in Eclipse

In this assignment, you’re going to replace direct Spring Kafka producer and consumer implementation with Dapr publish/subscribe messaging to send messages from the TrafficControlService to the FineCollectionService.

With the Dapr pub/sub building block, you use a topic to send and receive messages. The producer sends messages to the topic and one or more consumers subscribe to this topic to receive those messages. First you are going to prepare the TrafficControlService so it can send messages using Dapr pub/sub.

Dapr provides two methods by which you can subscribe to topics:

Declaratively, where subscriptions are defined in an external file.
Programmatically, where subscriptions are defined in user code, using language specific SDK’s.

This example demonstrates a programmatic approach using Dapr’s Java SDK.

If you want to get more detailed information, read the overview of this building block in the Dapr documentation.

To complete this assignment, you must reach the following goals:

The TrafficControlService sends SpeedingViolation messages using the Dapr pub/sub building block.
The FineCollectionService receives SpeedingViolation messages using the Dapr pub/sub building block.
Kafka is used as pub/sub message broker that runs as part of the solution, either in a Docker container, on directly on laptop.

Instructions

Open the file dapr/kafka-pubsub.yaml in your code editor.

Inspect this file. As you can see, it specifies the type of the message broker to use (pubsub.kafka) and specifies information on how to connect to the Kafka server you started in step 1 (running on localhost on port 9092) in the metadata section.

 apiVersion: dapr.io/v1alpha1
 kind: Component
 metadata:
   name: pubsub
   namespace: default
 spec:
   type: pubsub.kafka
   version: v1
   metadata:
   - name: brokers # Required. Kafka broker connection setting
     value: "localhost:9092"
   - name: consumerGroup # Optional. Used for input bindings.
     value: "test"
   - name: clientID # Optional. Used as client tracing ID by Kafka brokers.
     value: "my-dapr-app-id"
   - name: authType # Required.
   - name: authRequired
     value: "false"
   - name: maxMessageBytes # Optional.
     value: 1024
   - name: consumeRetryInterval # Optional.
     value: 200ms
   - name: version # Optional.
     value: 0.10.2.0
   - name: disableTls # Optional. Disable TLS. This is not safe for production!! You should read the `Mutual TLS` section for how to use TLS.
     value: "true"
   scopes:
   - trafficcontrolservice
   - finecollectionservice

In the scopes section, you specify that only the TrafficControlService and FineCollectionService should use the pub/sub building block.

Copy or Move this file dapr/kafka-pubsub.yaml to dapr/components/ folder (when starting Dapr applications from command line, you specify a folder dapr/components/ where Dapr component manifests are located). From the root folder, run the following command:
```
 mkdir dapr/components
 cp dapr/kafka-pubsub.yaml dapr/components/
```

Step 1: Publish messages in the TrafficControlService

Open the file, TrafficControlService/src/main/java/dapr/traffic/fines/DaprFineCollectionClient.java in your code editor, and inspect it.

It implements the FineCollectionClient interface.

 public class DaprFineCollectionClient implements FineCollectionClient{
   private final DaprClient daprClient;

   public DaprFineCollectionClient(final DaprClient daprClient) {
     this.daprClient = daprClient;
   }
      
   @Override
   public void submitForFine(SpeedingViolation speedingViolation) {
        
        
     daprClient.publishEvent("pubsub",  "test", speedingViolation).block();
   }

 }

Open the file TrafficControlService/src/main/java/dapr/traffic/TrafficControlConfiguration.java in your code editor.

The default JSON serialization is not suitable for todays goal, so you need to customize the Jackson ObjectMapper that it uses. You do so by adding a static inner class to configure the JSON serialization:
```
 static class JsonObjectSerializer extends DefaultObjectSerializer {
     public JsonObjectSerializer() {
         OBJECT_MAPPER.registerModule(new JavaTimeModule());
         OBJECT_MAPPER.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
     }
 }
```

Comment out following @Bean method:

 @Bean
 public FineCollectionClient fineCollectionClient() {
     return new KafkaFineCollectionClient();
 }

Uncomment following @Bean method:

 //    @Bean
 //    public FineCollectionClient fineCollectionClient(final DaprClient daprClient) {
 //        return new DaprFineCollectionClient(daprClient);
 //    }

Uncomment following @Bean method:

 //    @Bean
 //    public DaprClient daprClient() {
 //        return new DaprClientBuilder()
 //                .withObjectSerializer(new JsonObjectSerializer())
 //                .build();
 //    }

Check all your code changes are correct by building the code. Execute the following command in the terminal window:
```
mvn package
```

Step 2: Receive messages in the FineCollectionService

Dapr will call your service on a POST endpoint /collectfine to retrieve the subscriptions for that service. You will implement this endpoint and return the subscription for the test topic.

Open the file FineCollectionService/src/main/java/dapr/fines/violation/ViolationController.java in your code editor.
Uncomment the code line below:
```
 //@RestController
```

Uncomment the code snippet below:

 // @PostMapping(path = "/collectfine")
 // @Topic(name = "test", pubsubName = "pubsub")
 // public ResponseEntity<Void> registerViolation(@RequestBody final CloudEvent<SpeedingViolation> event) {
 // 	var violation = event.getData();
 // 	violationProcessor.processSpeedingViolation(violation);
 //     return ResponseEntity.ok().build();
 // }

Open the file FineCollectionService/src/main/java/dapr/fines/violation/KafkaViolationConsumer.java in your code editor.

Comment out @KafkaLister annotation line:

 @KafkaListener(topics = "test", groupId = "test", containerFactory = "kafkaListenerContainerFactory")

Check all your code changes are correct by building the code. Execute the following command in the terminal window:
```
mvn package
```

Now you can test the application.

Step 3: Test the application

You’re going to start all the services now.

Make sure no services from previous tests are running (close the command-shell windows).
Open the terminal window and make sure the current folder is VehicleRegistrationService.
Enter the following command to run the VehicleRegistrationService:
```
mvn spring-boot:run
```
Open a new terminal window and change the current folder to FineCollectionService.

Enter the following command to run the FineCollectionService with a Dapr sidecar:

Ensure you have run dapr init command prior to running the below command.

 dapr run --app-id finecollectionservice --app-port 6001 --dapr-http-port 3601 --dapr-grpc-port 60001 --resources-path ../dapr/components mvn spring-boot:run

Open a new terminal window and change the current folder to TrafficControlService.

Enter the following command to run the TrafficControlService with a Dapr sidecar:

 dapr run --app-id trafficcontrolservice --app-port 6000 --dapr-http-port 3600 --dapr-grpc-port 60000 --resources-path ../dapr/components mvn spring-boot:run

Open a new terminal window and change the current folder to Simulation.
Start the simulation:
```
 mvn spring-boot:run
```

You should see the same logs as Assignment 1. Obviously, the behavior of the application is exactly the same as before.

Step 4: Debug Dapr applications in Eclipse

The steps below are tailored to debug TrafficControlService, but would be the same for debugging any Dapr application in Eclipse.

Click Run > External Tools > External Tools Configuration...
Click New Launch Configuration icon.
- Name = trafficcontrolservice-dapr-debug
- Location = c:\dapr\dapr.exe
- Working Directory = ${workspace_loc:/TrafficControlService}
- Arguments = run –app-id finecollectionservice –app-port 6001 –dapr-http-port 3601 –dapr-grpc-port 60001 –resources-path ../dapr/components
Apply.
Run.
Set breakpoints in your code as you normally would in Eclipse.
From Debug menu start the application either as a Java Application or as a Spring Boot App.

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