Declarative Topics

Karafka allows you to manage your topics in two ways:

Using the built-in Declarative Topics routing + CLI functionality (recommended)
Directly via the Admin API

Karafka considers your topics setup (retention, partitions, etc.) as part of your business logic. You can describe them in the routing and make Karafka ensure their consistency across all the environments using the appropriate CLI commands. Thanks to that, you can make sure that everything is described as code.

Admin actions will always be applied to the default cluster defined in the configuration.

Keeping Kafka topics configuration as code has several benefits:

Version Control: By keeping the topic settings as code, you can track changes over time and easily understand historical changes related to the topics. This is particularly important in a production environment where changes need to be carefully managed.
Reproducibility: When you define Kafka topics settings as code, you can easily recreate the same topic with the same settings in multiple environments. This ensures that your development, staging, and production environments are consistent, which can help prevent unexpected issues and bugs.
Automation: If you use code to define Kafka topics settings, you can automate the process of creating and updating topics. This can save time and reduce the risk of human error.
Collaboration: When you keep Kafka topics settings as code, you can collaborate with other developers on the configuration. You can use tools like Git to manage changes and merge different configurations.
Documentation: Code is self-documenting, meaning anyone can look at the configuration and understand what is happening. This can make it easier for new team members to get up to speed and help troubleshoot issues.

Overall, keeping Kafka topics settings as code can make it easier to manage, automate, and collaborate on Kafka topics, saving time and reducing the risk of errors.

Karafka routing allows you to do that via per topic #config method that you can use to describe your Kafka topic configuration.

This configuration is used by a set of Karafka CLI commands you can invoke to operate on your application's topics.

There are the following commands supported:

Command	Description
`karafka topics create`	creates topics with appropriate settings.
`karafka topics delete`	deletes all the topics defined in the routes.
`karafka topics repartition`	adds additional partitions to topics with fewer partitions than expected.
`karafka topics reset`	deletes and re-creates all the topics.
`karafka topics plan`	plans the migration process and prints what changes are going to be applied if migration runs.
`karafka topics align`	aligns configuration of all the declarative topics that exist based on the declarative topics definitions.
`karafka topics migrate`	creates missing topics, repartitions existing to match expected partitions count and aligns the configuration.

The below example illustrates the usage of the migrate command to align the number of partitions and to add one additional topic:

Defining topic configuration

All the configuration for a given topic needs to be defined using the topic scope #config method.

class KarafkaApp < Karafka::App
  routes.draw do
    topic :a do
      config(
        partitions: 6,
        replication_factor: 3,
        'retention.ms': 86_400_000, # 1 day in ms
        'cleanup.policy': 'delete'
      )

      consumer ConsumerA
    end

    topic :b do
      config(
        partitions: 2,
        replication_factor: 3
        # The rest will be according to the cluster defaults
      )

      consumer ConsumerB
    end
  end
end

If not invoked, the default config looks as followed:

config(
  partitions: 1,
  replication_factor: 1
)

Excluding topics from the topics management

If you want to manage only part of your topics using Karafka, you can set the active flag for a given topic configuration to false.

class KarafkaApp < Karafka::App
  routes.draw do
    topic :a do
      config(active: false)
    end
  end
end

This will effectively ignore this topic from being altered in any way by Karafka. Karafka will ignore this topic together in all the CLI topics related operations.

Keep in mind that setting active to false inside the #config is not equivalent to disabling the topic consumption using the active method.

You can use Karafka to manage topics that you do not consume from as well by defining their config and making them inactive at the same time:

class KarafkaApp < Karafka::App
  routes.draw do
    topic :a do
      config(
        partitions: 2,
        replication_factor: 3
      )

      active false
    end
  end
end

Setting such as above will allow Karafka to manage the topic while instructing Karafka not to try to consume it. A configuration like this is helpful in a multi-app environment where you want Karafka to manage topics, but their consumption belongs to other applications.

Production usage

The topics management CLI never performs any destructive actions except the delete and reset commands. This means you can safely include the karafka topics migrate in your deployment pipelines if you wish to delegate topics management to Karafka.

Please keep in mind two things, though:

Karafka currently does not update settings different than the partition count.
Topics management API does not provide any means of concurrency locking when CLI commands are being executed.

Limitations and other info

Topics management is enabled by default but will not be used unless any CLI commands are invoked.
migrate does not wait for a confirmation. Use plan command to check the changes that would be applied.
If a topic is used by several consumer groups defined in one application, only the first config defined will be used.
Topics management API does not support the management of multiple independent Kafka clusters. Only the primary one will be managed.
Topics management API does not provide any means of concurrency locking when CLI commands are being executed. This means it is up to you to ensure that two topic CLI commands are not running in parallel during the deployments.
Topics commands are not transactional. It means that the state application may be partial in case of errors.
Topics commands are idempotent. Broken set of operations can be retried after fixes without worry.
Karafka will never alter any topics that are not defined in the routing.