Clients and Servers

The terms client and server are not firm designations for particular parts of an application; rather, they denote roles that are taken by parts of an application for the duration of a request:

• Clients are active entities. They issue requests for service to servers.
• Servers are passive entities. They provide services in response to client requests.

Frequently, servers are not "pure" servers, in the sense that they never issue requests and only respond to requests. Instead, servers often act as a server on behalf of some client but, in turn, act as a client to another server in order to satisfy their client's request.

Similarly, clients often are not "pure" clients, in the sense that they only request service from an object. Instead, clients are frequently client server hybrids. For example, a client might start a long-running operation on a server; as part of starting the operation, the client can provide a callback object to the server that is used by the server to notify the client when the operation is complete. In that case, the client acts as a client when it starts the operation, and as a server when it is notified that the operation is complete.

Such role reversal is common in many systems, so, frequently, client server systems could be more accurately described as peer-to-peer systems.