# NATS triggers

> How do I trigger scripts and flows from NATS messages in Windmill?

Windmill can connect to [NATS](https://nats.io/) servers and trigger runnables (scripts, flows) when a message is received.
Listening is done from the servers, so it doesn't take up any workers.
NATS triggers is a [self-hosted Enterprise](/pricing) feature.

![NATS triggers](./nats_triggers.png 'NATS triggers')

## How to use

### Core NATS

Create a new trigger on the NATS triggers page.
Add a NATS resource with the server hostnames (`hostname[:port]`, without the `nats://` prefix) and the authentication configuration.
Specify the subjects the trigger should listen to (wildcards are supported).
Only one subject is supported in core NATS.

Once the NATS resource and settings are set, select the runnable that should be triggered by this trigger.
The received webhook base64 encoded payload will be passed to the runnable as a string argument called `payload`.

Here's an example script:

```TypeScript
export async function main(payload: string) {
  // do something with the message
}
```

And if you use a [preprocessor](../../core_concepts/43_preprocessors/index.mdx), the script could look like this:

```TypeScript

    subject: string; // the specific subject the message was received from
    length: number;
    headers?: Record<string, string[]>;
    status?: number;
    description?: string;
  }
) {
  if (event.kind !== "nats") {
    throw new Error(`Expected a nats event`);
  }

  // assuming the message is a JSON object
  const msg = JSON.parse(atob(event.payload));

  // define args for the main function
  // let's assume we want to use the message content and the subject
  return {
    message_content: msg.content,
    subject: event.subject
  };
}

export async function main(message_content: string, subject: string) {
  // do something with the message content and subject
}
```

### JetStream
[JetStream](https://docs.nats.io/nats-concepts/jetstream) is also supported and enables persistence as well as listening to multiple subjects.
Persistence makes sure that even if the trigger stops listening for a while, it will receive the messages it missed when it starts listening again.

A stream will be created with the specified name and subjects, if no stream exists with this name.
If one already exists, whether created outside or inside of Windmill, its config will be adapted to include the specified subjects but only if they are missing (considering wildcards).
The rest of the config will be left untouched.
**Stream subjects are never deleted automatically. If you need to delete old subjects, you have to do it manually.**

A [durable push-consumer](https://docs.nats.io/nats-concepts/jetstream/consumers) will be created with the specified name and subjects.
If one already exists, it will be overwritten.
The consumer name is also used as the `DeliverSubject`, so make sure it's unique.

## Error handling

NATS triggers support local error handlers that override workspace error handlers for specific triggers. See the [error handling documentation](../../core_concepts/10_error_handling/index.mdx#trigger-error-handlers) for configuration details and examples.