---
id: 'function-background-tasks'
title: 'Background Tasks'
description: 'How to run background tasks in an Edge Function outside of the request handler'
subtitle: 'Run background tasks in an Edge Function outside of the 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 and leave the task running in the background.

This allows you to:

- Respond quickly to users while processing continues
- Handle async operations without blocking the response

---

## Overview

You can use `EdgeRuntime.waitUntil(promise)` to explicitly mark background tasks. The Function instance continues to run until the promise provided to `waitUntil` completes.

```ts
// Mark the asyncLongRunningTask's returned promise as a background task.
// ⚠️ We are NOT using `await` because we don't want it to block!
EdgeRuntime.waitUntil(asyncLongRunningTask())

Deno.serve(async (req) => {
  return new Response(...)
})
```

You can call `EdgeRuntime.waitUntil` in the request handler too. This will not block the request.

```ts
Deno.serve(async (req) => {
  // Won't block the request, runs in background.
  EdgeRuntime.waitUntil(asyncLongRunningTask())

  return new Response(...)
})
```

You can listen to the `beforeunload` event handler to be notified when the Function is about to be shut down.

```tsx
EdgeRuntime.waitUntil(asyncLongRunningTask())

// Use beforeunload event handler to be notified when function is about to shutdown
addEventListener('beforeunload', (ev) => {
  console.log('Function will be shutdown due to', ev.detail?.reason)
  // Save state or log the current progress
})

Deno.serve(async (req) => {
  return new Response(...)
})
```

## Handling errors

We recommend using `try`/`catch` blocks within your background task function to handle errors.

You can also add an event listener to [`unhandledrejection`](https://developer.mozilla.org/en-US/docs/Web/API/Window/unhandledrejection_event) to handle any promises without a rejection handler.

```tsx
addEventListener('unhandledrejection', (ev) => {
  console.log('unhandledrejection', ev.reason)
  ev.preventDefault()
})
```

<Admonition type="note">

The maximum duration is capped based on the wall-clock, CPU, and memory limits. The function will shut down when it reaches one of these [limits](/docs/guides/functions/limits).

</Admonition>

---

## 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"
```
