> ## Documentation Index
> Fetch the complete documentation index at: https://wundergraphinc-ahmet-router-592-mcp-add-structured-tool-out.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 the following hook interfaces that you can implement with [Custom Modules](/router/custom-modules):

* [`SubscriptionOnCreateHandler`](/router/cosmo-streams/custom-modules/subscription-on-create): Called before a subscription is registered, allows modifying the event configuration (experimental)
* [`SubscriptionOnStartHandler`](/router/cosmo-streams/custom-modules/subscription-on-start): Called when a client subscribes
* [`StreamBeforeEventsDispatchHandler`](/router/cosmo-streams/custom-modules/before-events-dispatch): Called once per batch of events received from a message broker, before it is dispatched to any subscriber
* [`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