Background Tasks

How to 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.

How it works#

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

The maximum duration is capped based on the wall-clock, CPU, and memory limits. The Function will shutdown when it reaches one of these limits.

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

Example#

Here's an example of using EdgeRuntime.waitUntil to run a background task and using beforeunload event to be notified when the instance is about to be shut down.

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async function longRunningTask() {  // do work here}// Mark the longRunningTask's returned promise as a background task.// note: we are not using await because we don't want it to block.EdgeRuntime.waitUntil(longRunningTask())// Use beforeunload event handler to be notified when function is about to shutdownaddEventListener('beforeunload', (ev) => {  console.log('Function will be shutdown due to', ev.detail?.reason)  // save state or log the current progress})// Invoke the function using a HTTP request.// This will start the background taskDeno.serve(async (req) => {  return new Response('ok')})

Starting a background task in the request handler#

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

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async function fetchAndLog(url: string) {  const response = await fetch(url)  console.log(response)}Deno.serve(async (req) => {  // this will not block the request,  // instead it will run in the background  EdgeRuntime.waitUntil(fetchAndLog('https://httpbin.org/json'))  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:

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[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.