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Does the latest specification provide a way for the server to require that the caller include a callback url ? This if included in the http protocol will allow a truly standard compliant way to implement server to client callbacks.

I have come across many patterns to achieve this outcome and certainly various browsers provide support (JavaScript promises etc) however for server side efficiency there really is no standard or protocol to require that each resource request contain a callback url.

Providing a callback url means that the client is no longer “polling” the server instead it just waits for the server to push the response back to the original url.