Asynchronous Request-Reply Pattern

Problem

• Design APIs for a client app to respond quickly (100 ms or less), code running in a web-client (browser).

• Manage the response latency : factors such as current load, network infra, size of a request payload, etc.

• We prefer the owrk done by the frontend and not by the backend (which could be lon-running and done in seconds).

Solution

• Use HTTP polling which is useful to client-side code :

  1. Make asynchronous call to the API;

  2. API responds synchronously as quickly as possible (http 202 - OK);

  3. Client can poll to check for the result : reponse holds a location reference pointing to an endpoint;

  4. API offloads processing to another component (message queue, …);

  5. API returns HTTP 200. Once the work is complete, endpoint can redirect to another resource URL.

Asynchornous Request-Reply In Practice

API call with Azure Function which can listen to changes in Azure resources : decouple backend processing from a frontend host where backend processing needs to be asynchronous, but the frontend still needs a clear response.