Announcing QStash Callbacks

Earlier this year, we released QStash, a message queue and scheduler designed for serverless and edge platforms. Today we're excited to announce: QStash Callbacks.

QStash solved the problem of using queues and scheduling messages in serverless functions. But until now this was a one way street, where you could only send messages but had no way of accessing the response of the destination API. With QStash Callbacks, you can now send messages and get the response pushed back to you.

How it works

Currently, when you publish a message, we send it to the destination API and wait for a response. If the response is a 2xx status code, we consider the message delivered. If the response is a 4xx or 5xx status code, we consider the message undelivered and retry it.

With QStash Callbacks, you can now send a message and get the response pushed back to you. This is how it works:

1. You publish a message to QStash with a callback URL.
2. QStash sends the message to the destination API and waits for a response.
3. QStash waits for the response
4. The response status, body and headers are sent to the callback URL.

How to use it

Callback URLs are added when publishing a message or scheduling a message. To add a callback URL, you can use the Upstash-Callback header:

curl -X POST \
  https://qstash.upstash.io/v2/publish/<DESTINATION_URL_OR_TOPIC> \
  -H 'Content-Type: application/json' \
  -H 'Authorization: Bearer <QSTASH_TOKEN>' \
  -H 'Upstash-Callback: https://my-project.vercel.app/api/callback' \
  -d '{
    "message": "Hello World"
}'

After receiving a response from the destination api, we immediately publish a new message to the callback URL. The message body is a JSON object with the following fields:

{
  "status": 200,
  "sourceMessasgeId": "msg_xxx" // The ID of the message that triggered the callback
  "headers": {
    "key": ["value"]
  },
  "body": "base64 encoded response body",
  "retried": 1 // How many times we retried to deliver the original message
}

You should then verify the signature just like you would for a regular message.

Example

Using callbacks opens up a lot of new use cases. For example, you can now use QStash to call 3rd party APIs and use the response to update your database.

Dom from our friends at Vercel has already built an example project that uses QStash callbacks to call the DALL-E API and store the generated images in redis to show them in the UI. Check it out here:

• Source Code
• App

Future

Please let us know what you would like to build with callbacks or if you have feedback on Twitter or Discord, we're excited to see what you come up with!

For a more in-depth guide on how to use callbacks, please visit our documentation.