Background Tasks

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.

// 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.

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.

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 to handle any promises without a rejection handler.

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

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.

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:

[edge_runtime]
policy = "per_worker"