Create an Apache Kafka Client App for Kotlin¶

In this tutorial, you will run a Kotlin client application that produces messages to and consumes messages from an Apache Kafka® cluster.

After you run the tutorial, use the provided source code as a reference to develop your own Kafka client application.

Prerequisites¶

Client¶

Java 1.8 or higher to run the demo application.

Kafka Cluster¶

You can use this tutorial with a Kafka cluster in any environment:
- In Confluent Cloud
- On your local host
- Any remote Kafka cluster
If you are running on Confluent Cloud, you must have access to a Confluent Cloud cluster with an API key and secret.
- For an automated way to create a Kafka cluster, credentials, and ACLs in Confluent Cloud, see ccloud-stack Utility for Confluent Cloud.

Setup¶

Clone the confluentinc/examples GitHub repository and check out the 6.1.15-post branch.

git clone https://github.com/confluentinc/examples
cd examples
git checkout 6.1.15-post

Change directory to the example for Kotlin.
```
cd clients/cloud/kotlin/
```
Create a local file (for example, at $HOME/.confluent/java.config) with configuration parameters to connect to your Kafka cluster. Starting with one of the templates below, customize the file with connection information to your cluster. Substitute your values for {{ BROKER_ENDPOINT }}, {{CLUSTER_API_KEY }}, and {{ CLUSTER_API_SECRET }} (see Configure Confluent Cloud Clients for instructions on how to manually find these values, or use the ccloud-stack Utility for Confluent Cloud to automatically create them).
- Template configuration file for Confluent Cloud
```
# Required connection configs for Kafka producer, consumer, and admin
bootstrap.servers={{ BROKER_ENDPOINT }}
security.protocol=SASL_SSL
sasl.jaas.config=org.apache.kafka.common.security.plain.PlainLoginModule required username='{{ CLUSTER_API_KEY }}' password='{{ CLUSTER_API_SECRET }}';
sasl.mechanism=PLAIN
# Required for correctness in Apache Kafka clients prior to 2.6
client.dns.lookup=use_all_dns_ips

# Best practice for Kafka producer to prevent data loss 
acks=all
```
- Template configuration file for local host
```
# Kafka
bootstrap.servers=localhost:9092
```

Basic Producer and Consumer¶

In this example, the producer application writes Kafka data to a topic in your Kafka cluster. If the topic does not already exist in your Kafka cluster, the producer application will use the Kafka Admin Client API to create the topic. Each record written to Kafka has a key representing a username (for example, alice) and a value of a count, formatted as json (for example, {"count": 0}). The consumer application reads the same Kafka topic and keeps a rolling sum of the count as it processes each record.

Produce Records¶

Build the client examples:
```
./gradlew clean build
```

Run the producer, passing in arguments for:

the local file with configuration parameters to connect to your Kafka Cluster
the topic name

./gradlew runApp -PmainClass="io.confluent.examples.clients.cloud.ProducerExample" \
     -PconfigPath="$HOME/.confluent/java.config" \
     -Ptopic="test1"

Verify the producer sent all the messages. You should see:

...
Producing record: alice        {"count":0}
Producing record: alice        {"count":1}
Producing record: alice        {"count":2}
Producing record: alice        {"count":3}
Producing record: alice        {"count":4}
Producing record: alice        {"count":5}
Producing record: alice        {"count":6}
Producing record: alice        {"count":7}
Producing record: alice        {"count":8}
Producing record: alice        {"count":9}
Produced record to topic test1 partition [0] @ offset 0
Produced record to topic test1 partition [0] @ offset 1
Produced record to topic test1 partition [0] @ offset 2
Produced record to topic test1 partition [0] @ offset 3
Produced record to topic test1 partition [0] @ offset 4
Produced record to topic test1 partition [0] @ offset 5
Produced record to topic test1 partition [0] @ offset 6
Produced record to topic test1 partition [0] @ offset 7
Produced record to topic test1 partition [0] @ offset 8
Produced record to topic test1 partition [0] @ offset 9
10 messages were produced to topic test1
...

View the producer code.

Consume Records¶

Run the consumer, passing in arguments for:

the local file with configuration parameters to connect to your Kafka cluster
the same topic name you used earlier.

./gradlew runApp -PmainClass="io.confluent.examples.clients.cloud.ConsumerExample"\
     -PconfigPath="$HOME/.confluent/java.config"\
     -Ptopic="test1"

Verify that the consumer received all the messages. You should see:

...
Consumed record with key alice and value {"count":0}, and updated total count to 0
Consumed record with key alice and value {"count":1}, and updated total count to 1
Consumed record with key alice and value {"count":2}, and updated total count to 3
Consumed record with key alice and value {"count":3}, and updated total count to 6
Consumed record with key alice and value {"count":4}, and updated total count to 10
Consumed record with key alice and value {"count":5}, and updated total count to 15
Consumed record with key alice and value {"count":6}, and updated total count to 21
Consumed record with key alice and value {"count":7}, and updated total count to 28
Consumed record with key alice and value {"count":8}, and updated total count to 36
Consumed record with key alice and value {"count":9}, and updated total count to 45

When you are done, press CTRL-C.
View the consumer code.

Kafka Streams¶

Run the Kafka Streams application, passing in arguments for:

the local file with configuration parameters to connect to your Kafka cluster
the same topic name you used earlier

./gradlew runApp -PmainClass="io.confluent.examples.clients.cloud.StreamsExample" \
     -PconfigPath="$HOME/.confluent/java.config" \
     -Ptopic="test1"

Verify the consumer received all the messages. You should see:

...
[Consumed record]: alice, 0
[Consumed record]: alice, 1
[Consumed record]: alice, 2
[Consumed record]: alice, 3
[Consumed record]: alice, 4
[Consumed record]: alice, 5
[Consumed record]: alice, 6
[Consumed record]: alice, 7
[Consumed record]: alice, 8
[Consumed record]: alice, 9
...
[Running count]: alice, 0
[Running count]: alice, 1
[Running count]: alice, 3
[Running count]: alice, 6
[Running count]: alice, 10
[Running count]: alice, 15
[Running count]: alice, 21
[Running count]: alice, 28
[Running count]: alice, 36
[Running count]: alice, 45
...

When you are done, press CTRL-C.
View the Kafka Streams code.