---
id: 'function-background-tasks'
title: 'Background Tasks'
description: 'How to run background tasks in an Edge Function outside of request handler'
subtitle: 'How to run background tasks in an Edge Function outside of request handler'
---

Edge Function instances can process background tasks outside of the request handler. Background tasks are useful for asynchronous operations like uploading a file to Storage, updating a database, or sending events to a logging service. You can respond to the request immediately or leave the task running in the background.

A background task can run until the Edge Function instance hits its [wall-clock limit](/docs/guides/functions/limits).

Here's an example of defining a background task using a custom event.

```ts
// define a custom event
class MyBackgroundTaskEvent extends Event {
  readonly taskPromise: Promise<string>

  constructor(taskPromise) {
    super('myBackgroundTask')
    this.taskPromise = taskPromise
  }
}

globalThis.addEventListener('myBackgroundTask', async (event: MyBackgroundTaskEvent) => {
  const res = await event.taskPromise
  console.log(await res.json())
})

Deno.serve(async (req) => {
  const fetchPromise = fetch('https://httpbin.org/json')
  const event = new MyBackgroundTaskEvent(fetchPromise)
  globalThis.dispatchEvent(event)

  return new Response('ok')
})
```

### Testing background tasks locally

When testing Edge Functions locally with Supabase CLI, the instances are terminated automatically after a request is completed. This will prevent background tasks from running to completion.

To prevent that, you can update the `supabase/config.toml` with the following settings:

```toml
[edge_runtime]
policy = "per_worker"
```

When running with `per_worker` policy, Function won't auto-reload on edits. You will need to manually restart it by running `supabase functions serve`.