What is a callback URL in relation to an API?

I've been scouring the net, and can't seem to wrap my head around the idea of a callback URL. In my case I have a few callback URLs that I have to define myself. A popular one is a "default callback URL". What is this exactly? Can you give an example in plain english?

A callback URL will be invoked by the API method you're calling after it's done. So if you call

POST /api.example.com/foo?callbackURL=http://my.server.com/bar

Then when /foo is finished, it sends a request to http://my.server.com/bar. The contents and method of that request are going to vary - check the documentation for the API you're accessing.

Think of it as a letter. Sometimes you get a letter, say asking you to fill in a form then return the form in a pre-addressed envelope which is in the original envelope that was housing the form.

Once you have finished filling the form in, you put it in the provided return envelope and send it back.

The callback URL is like that return envelope. You are basically saying, "I am sending you this data; once you are done with it, I am listening on this callback URL waiting your response." So the API will process the data you have sent then look at the callback to send you the response.

This is useful because sometimes you may take ages to process some data and it makes no sense to have the caller wait for a response. For example, say your API allows users to send documents to it and virus scan them. Then you send a report after. The scan could take maybe 3 minutes. The user cannot be waiting for 3 minutes. So you acknowledge that you got the document and let the caller get on with other business while you do the scan, then use the callback URL when done to tell them the result of the scan.

If you use the callback URL, then the API can connect to the callback URL and send or receive some data. That means API can connect to you later (after API call).

Example

1. YOU send data using request to API
2. API sends data using second request to YOU

Exact definition should be in API documentation.

It's a mechanism to invoke an API in an asynchrounous way. The sequence is the following

1. your app invokes the url, passing as parameter the callback url
2. the api respond with a 20x http code (201 I guess, but refer to the api docs)
3. the api works on your request for a certain amount of time
4. the api invokes your app to give you the results, at the callback url address.

So you can invoke the api and tell your user the request is "processing" or "acquired" for example, and then update the status when you receive the response from the api.

Hope it makes sense. -G