A technique for reducing the number of events that are transmitted by a server to a client in a client/server application programming interface (API). At the server host, each of different event messages is associated with at least one event category in a hierarchy of event categories. Each client host sends a registration request to the server host to inform the server host that the client host wants to receive only event messages that are associated with a specified event category. The server host associates the specified event category with each client host, and communicates only the event messages that are descendants of the specified event category.