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Chapter 5. Using the Topic Operator

When you create, modify or delete a topic using the KafkaTopic resource, the Topic Operator ensures those changes are reflected in the Kafka cluster.

5.1. Kafka topic resource
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The KafkaTopic resource is used to configure topics, including the number of partitions and replicas.

The full schema for KafkaTopic is described in KafkaTopic schema reference.

5.1.1. Kafka topic usage recommendations
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When working with topics, be consistent. Always operate on either KafkaTopic resources or topics directly in OpenShift. Avoid routinely switching between both methods for a given topic.

Use topic names that reflect the nature of the topic, and remember that names cannot be changed later.

If creating a topic in Kafka, use a name that is a valid OpenShift resource name, otherwise the Topic Operator will need to create the corresponding KafkaTopic with a name that conforms to the OpenShift rules.

Note

Recommendations for identifiers and names in OpenShift are outlined in Identifiers and Names in OpenShift community article.

5.1.2. Kafka topic naming conventions
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Kafka and OpenShift impose their own validation rules for the naming of topics in Kafka and KafkaTopic.metadata.name respectively. There are valid names for each which are invalid in the other.

Using the spec.topicName property, it is possible to create a valid topic in Kafka with a name that would be invalid for the Kafka topic in OpenShift.

The spec.topicName property inherits Kafka naming validation rules:

  • The name must not be longer than 249 characters.
  • Valid characters for Kafka topics are ASCII alphanumerics, ., _, and -.
  • The name cannot be . or .., though . can be used in a name, such as exampleTopic. or .exampleTopic.

spec.topicName must not be changed.

For example: 

kind: KafkaTopic
metadata:
  name: topic-name-1
spec:
  topicName: topicName-1
1 

  # ...
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1
Upper case is invalid in OpenShift.

cannot be changed to: 

kind: KafkaTopic
metadata:
  name: topic-name-1
spec:
  topicName: name-2
  # ...
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Note

Some Kafka client applications, such as Kafka Streams, can create topics in Kafka programmatically. If those topics have names that are invalid OpenShift resource names, the Topic Operator gives them valid names based on the Kafka names. Invalid characters are replaced and a hash is appended to the name.

5.2. Configuring a Kafka topic
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Use the properties of the KafkaTopic resource to configure a Kafka topic.

You can use oc apply to create or modify topics, and oc delete to delete existing topics.

For example:

  • oc apply -f <topic-config-file>
  • oc delete KafkaTopic <topic-name>

This procedure shows how to create a topic with 10 partitions and 2 replicas.

Before you start

It is important that you consider the following before making your changes:

  • Kafka does not support making the following changes through the KafkaTopic resource:

    • Changing topic names using spec.topicName
    • Decreasing partition size using spec.partitions
  • You cannot use spec.replicas to change the number of replicas that were initially specified.
  • Increasing spec.partitions for topics with keys will change how records are partitioned, which can be particularly problematic when the topic uses semantic partitioning.

Prerequisites

Procedure

  1. Prepare a file containing the KafkaTopic to be created.

    An example KafkaTopic

    apiVersion: kafka.strimzi.io/v1beta1
kind: KafkaTopic
metadata:
  name: orders
  labels:
    strimzi.io/cluster: my-cluster
spec:
  partitions: 10
  replicas: 2
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    Tip

    When modifying a topic, you can get the current version of the resource using oc get kafkatopic orders -o yaml.

  2. Create the KafkaTopic resource in OpenShift. 

    oc apply -f <topic-config-file>
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