Google Pub/Sub Messaging Configuration

Hono can be configured to support Google Pub/Sub as the messaging infrastructure. Google Pub/Sub can only be used to forward Telemetry and Event messages, but not Command messages. The Pub/Sub client used for this purpose can be configured by means of operating system environment variables.

Supporting Google Pub/Sub, Hono must run on Google Kubernetes Engine to authenticate to the Google Pub/Sub API. To authenticate to the Google Pub/Sub API, Workload Identity is used and has to be configured as described in the Google Cloud Documentation.

Publisher Configuration

The org.eclipse.hono.client.pubsub.CachingPubSubPublisherFactory factory can be used to create Pub/Sub publishers for Hono’s Pub/Sub based APIs.

Please refer to the Quarkus Google Cloud Services extension documentation for details regarding configuration of the PubSub client.

Configuring Tenants to use Pub/Sub based Messaging

Hono’s components by default support using Kafka based messaging infrastructure to transmit messages hence and forth between devices and applications. Hono also supports using Pub/Sub as the messaging infrastructure either as a replacement for or as an alternative in addition to the Kafka based infrastructure.

In most cases Hono’s components will be configured to use either Pub/Sub, AMQP 1.0 or Kafka based messaging infrastructure. However, in cases where more than one type of infrastructure is being used, Hono’s components need to be able to determine, which infrastructure should be used for messages of a given tenant. For this purpose, the configuration properties registered for a tenant support the ext/messaging-type property which can have a value of either pubsub, amqp or kafka.

The following example shows a tenant that is configured to use the Pub/Sub based messaging infrastructure:

{
  "tenant-id": "TEST_TENANT",
  "enabled": true,
  "ext": {
    "messaging-type": "pubsub"
  }
}

If not explicitly set, the ext/messaging-type property’s value is kafka which indicates that Kafka is to be used for the tenant.

Used Topics

For Telemetry & Event Messages

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