Generally, server applications respond to clients requests. That is, the request and reply method is the main one for interactions between clients and servers as per the famous client-server architectural style. This is kind of pulling information from servers. The communication is also synchronous. In this case, both clients and servers have to be available online in order to initiate and accomplish the tasks. Further on, when service requests are being processed and performed by server machines, the requesting services/clients have to wait to receive the intended response from servers. That means clients cannot do any other work while waiting to receive servers' responses.
The world is eventually becoming ...
