Kafka Configuration

Hono may use an Apache Kafka cluster as messaging infrastructure.

In order to configure the Hono components for using a Kafka cluster, see the protocol adapter, command router and device registry admin guides.

Kafka Broker Configuration

Hono requires the following broker configuration properties to be set:

Broker config property	Mandatory	Description
`delete.topic.enable`	`true`	Enables deletion of topics. This is needed for the automatic deletion of `hono.command_internal.[adapterInstanceId]` topics after the corresponding protocol adapter instance was stopped.

Used Topics

For Telemetry, Event and Command & Control Messages

The Hono components publish messages to the following, tenant-specific Kafka topics, from which downstream applications will consume the messages:

Topic name	Description
`hono.telemetry.[tenantId]`	Topic for telemetry messages.
`hono.event.[tenantId]`	Topic for event messages.
`hono.command_response.[tenantId]`	Topic for command response messages.

For command & control messages published by downstream applications and consumed by the Hono command router, the following topic is used:

Topic name	Description
`hono.command.[tenantId]`	Topic for command & control messages.

The above topics may be created in advance with appropriate replication factor and partition count settings. Otherwise, the auto.create.topics.enable broker configuration property needs to be set to true to enable auto-creation of these topics.

For Hono internal messages

For messages published and consumed only by Hono components, the following topics are used:

Topic name	Description
`hono.command_internal.[adapterInstanceId]`	Topic used for routing of command & control messages between Hono components.
`hono.notification.registry-tenant`	Topic used for notification messages between Hono components about changes to tenant registration data.
`hono.notification.registry-device`	Topic used for notification messages between Hono components about changes to device registration data.

The hono.command_internal.[adapterInstanceId] topic name contains a unique identifier as suffix, created dynamically on protocol adapter start. Therefore this topic cannot be created in advance. It has to be made sure that the Kafka admin clients in the protocol adapters are able to create this kind of topic.

The hono.notification.[suffix] topics either need to be created in advance, or the auto.create.topics.enable broker configuration property needs to be set to true to enable auto-creation of these topics.