AsyncIndex | Schedule Reindex from Backend | Cron Utils library

I am confused in Action and Action.async and what is the appropriate condition to use one

From the documentation:

Note: Both Action.apply and Action.async create Action objects that are handled internally in the same way. There is a single kind of Action, which is asynchronous, and not two kinds (a synchronous one and an asynchronous one). The .async builder is just a facility to simplify creating actions based on APIs that return a Future, which makes it easier to write non-blocking code.

when i simultaneously make two request to this method the second request is blocked until the first one is completed

Also from the documentation:

The web client will be blocked while waiting for the response, but nothing will be blocked on the server, and server resources can be used to serve other clients.

If your simultaneous requests are from the same synchronous client, one of the requests on the client side will be blocked until the other is completed. There is no blocking on the server side. To achieve parallel processing of requests to the same endpoint, use distinct clients to make those requests, or use a client that makes asynchronous HTTP calls. Also consider using a separate dispatcher for this endpoint, even if you're wrapping the processing inside a Future (more information on creating a custom dispatcher is in the linked documentation).