> ## Documentation Index
> Fetch the complete documentation index at: https://wundergraphinc-brendan-add-sof-link.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom Modules

> Customize Streams behavior with powerful handlers for subscription lifecycle, event processing, and data transformation.

Cosmo Router provides powerful handlers to hook into the event processing of Cosmo Streams.
These handlers allow you to implement custom logic for subscription lifecycle management, event processing and data transformation.

<Info>
  Custom Modules in Cosmo Streams are available since Router version 0.266.0
</Info>

## Available Hooks

The Cosmo Streams system provides three main hook interfaces that you can implement with [Custom Modules](/router/custom-modules):

* [`SubscriptionOnStartHandler`](/router/cosmo-streams/custom-modules/subscription-on-start): Called when a client subscribes
* [`OnReceiveEventHandler`](/router/cosmo-streams/custom-modules/on-receive-event): Called when events are received from a message broker
* [`OnPublishEventHandler`](/router/cosmo-streams/custom-modules/on-publish-event): Called when events are going to be sent to a message broker