A server includes a first server configured to receive requests and a plurality of second servers configured to service requests. A method of client-server communication includes receiving, at the first server, a request from the client and allocating the request to one of the second servers. The server determines if the allocated second server can provide a response to the request within a predetermined time period. If the allocated second server cannot provide a response to the request within the predetermined time period it sends a reply to the client indicating when the server system can provide a response to the request and including an identifier of the allocated second server. Server generates a response to the request. Server receives a callback message from the client. Server sends the response to the client.