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# Asynchronous Request and Reply

* synchronously processing the client request before all the back end work is completed may not always be feasible
  * Becuase of
    * Takes too long, lots of processing
    * microservice architecture, lots of calls between services
    * Lots of wait time between I/O
  * Cannot have as client may need low latency (set SLAs) for a response to come back
* Solution
  * Asynchronous Processing (HTTP Polling or Event Notification)
    * Benefit
      * client does not need to wait
      * If passed to a message queue, can be sure that the requests reaches the server, which is not certain with direct sync requests.
    * How it works
      * The client makes an API request
      * The application now offloads the work to another back end service or message queue
      * The client can poll for the resource/process status using an HTTP GET (HTTP Polling) at regular intervals as appropriate for the client application
      * The status API returns “In-Progress” indicating the back end process is still running
      * Once the process completes, the status will return the required output or another reference to the resource
      * The application can also push event notifications once the process is complete without the need for the client to poll repeatedly
    * Example
      * Imagine you’re too tired to make dinner and decide to place a takeaway order over the phone at your favorite restaurant (API request)
      * The restaurant provides you with an order number (Acknowledgement and reference identifier)
      * You arrive at the restaurant to pick up your order, provide them the order number and the restaurant staff informs you that your order is still not complete
      * If you’re particularly hungry, you might return to the reception to inquire about your order every 5–10 mins (HTTP Polling)
      * Or, the restaurant can provide you with a pager that buzzes once your order is complete (Event Notification)