C (librdkafka): Code Example for Apache Kafka®¶
In this tutorial, you will run a C 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¶
- librdkafka installed on your machine. See the librdkafka installation instructions.
Configure SSL trust store¶
Depending on your operating system or Linux distribution you may need to take extra
steps to set up the SSL CA root certificates. If your system doesn’t have the
SSL CA root certificates properly set up, you may receive a SSL handshake failed
error message similar to the following:
%3|1605776788.619|FAIL|rdkafka#producer-1| [thrd:sasl_ssl://...confluent.cloud:9092/bootstr]: sasl_ssl://...confluent.cloud:9092/bootstrap: SSL handshake failed: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed: broker certificate could not be verified, verify that ssl.ca.location is correctly configured or root CA certificates are installed (brew install openssl) (after 258ms in state CONNECT)
In this case, you need to manually install a bundle of validated CA root certificates and potentially modify the client code to set the ssl.ca.location
configuration property.
(For more information, see the documentation for librdkafka on which this client is built)
macOS¶
On newer versions of macOS (for example, 10.15), you may need to add an additional dependency.
For the Python client:
pip install certifi
For other clients:
brew install openssl
Once you install the CA root certificates, set the ssl.ca.location
property in the client code.
Edit both the producer and consumer code files, and add the ssl.ca.location
configuration parameter into the producer and consumer properties.
The value should correspond to the location of the appropriate CA root certificates file on your host.
For the Python client, use certifi.where()
to determine the location of the certificate files:
ssl.ca.location: certifi.where()
For other clients, check the install path and provide it in the code:
ssl.ca.location: '/usr/local/etc/openssl@1.1/cert.pem'
CentOS¶
You may need to install CA root certificates in the following way:
sudo yum reinstall ca-certificates
This should be sufficient for the Kafka clients to find the certificates.
However, if you still get the same error, you can set the ssl.ca.location
property in the client code.
Edit both the producer and consumer code files, and add the ssl.ca.location
configuration parameter into the producer and consumer properties.
The value should correspond to the location of the appropriate CA root certificates file on your host, for example:
ssl.ca.location: '/etc/ssl/certs/ca-bundle.crt'
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.
- The first 20 users to sign up for Confluent Cloud and use promo code
C50INTEG
will receive an additional $50 free usage (details) - For an automated way to create a Kafka cluster, credentials, and ACLs in Confluent Cloud, see ccloud-stack Utility for Confluent Cloud.
- The first 20 users to sign up for Confluent Cloud and use promo code
Setup¶
Clone the confluentinc/examples GitHub repository and check out the
6.1.0-post
branch.git clone https://github.com/confluentinc/examples cd examples git checkout 6.1.0-post
Change directory to the example for C.
cd clients/cloud/c/
Create a local file (for example, at
$HOME/.confluent/librdkafka.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
# Kafka bootstrap.servers={{ BROKER_ENDPOINT }} security.protocol=SASL_SSL sasl.mechanisms=PLAIN sasl.username={{ CLUSTER_API_KEY }} sasl.password={{ CLUSTER_API_SECRET }}
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 example
producer
andconsumer
applications.make
Verify the build worked. You should see:
cc consumer.c common.c json.c -o consumer -lrdkafka -lm cc producer.c common.c json.c -o producer -lrdkafka -lm
Run the producer, passing in arguments for:
- the local file with configuration parameters to connect to your Kafka cluster
- the topic name
./producer test1 $HOME/.confluent/librdkafka.config
Verify the producer sent all the messages. You should see:
Creating topic test1 Topic test1 successfully created Producing message #0 to test1: alice={ "count": 1 } Producing message #1 to test1: alice={ "count": 2 } Producing message #2 to test1: alice={ "count": 3 } Producing message #3 to test1: alice={ "count": 4 } Producing message #4 to test1: alice={ "count": 5 } Producing message #5 to test1: alice={ "count": 6 } Producing message #6 to test1: alice={ "count": 7 } Producing message #7 to test1: alice={ "count": 8 } Producing message #8 to test1: alice={ "count": 9 } Producing message #9 to test1: alice={ "count": 10 } Waiting for 10 more delivery results Message delivered to test1 [0] at offset 0 in 22.75ms: { "count": 1 } Message delivered to test1 [0] at offset 1 in 22.77ms: { "count": 2 } Message delivered to test1 [0] at offset 2 in 22.77ms: { "count": 3 } Message delivered to test1 [0] at offset 3 in 22.78ms: { "count": 4 } Message delivered to test1 [0] at offset 4 in 22.78ms: { "count": 5 } Message delivered to test1 [0] at offset 5 in 22.78ms: { "count": 6 } Message delivered to test1 [0] at offset 6 in 22.78ms: { "count": 7 } Message delivered to test1 [0] at offset 7 in 22.79ms: { "count": 8 } Message delivered to test1 [0] at offset 8 in 22.80ms: { "count": 9 } Message delivered to test1 [0] at offset 9 in 22.81ms: { "count": 10 } 10/10 messages delivered
View the producer code.
Consume Records¶
Run the consumer, passing in arguments for:
- the topic name you used earlier
- the local file with configuration parameters to connect to your Kafka cluster.
./consumer test1 $HOME/.confluent/librdkafka.config
Verify the consumer received all the messages. You should see:
Subscribing to test1, waiting for assignment and messages... Press Ctrl-C to exit. Received message on test1 [0] at offset 0: { "count": 1 } User alice sum 1 Received message on test1 [0] at offset 1: { "count": 2 } User alice sum 3 Received message on test1 [0] at offset 2: { "count": 3 } User alice sum 6 Received message on test1 [0] at offset 3: { "count": 4 } User alice sum 10 Received message on test1 [0] at offset 4: { "count": 5 } User alice sum 15 Received message on test1 [0] at offset 5: { "count": 6 } User alice sum 21 Received message on test1 [0] at offset 6: { "count": 7 } User alice sum 28 Received message on test1 [0] at offset 7: { "count": 8 } User alice sum 36 Received message on test1 [0] at offset 8: { "count": 9 } User alice sum 45 Received message on test1 [0] at offset 9: { "count": 10 } User alice sum 55
Press
Ctrl-C
to exit.View the consumer code.