Client-Server Architectures

A Client-Server Architecture consists of two types of components: clients and servers. A server component perpetually listens for requests from client components. When a request is received, the server processes the request, and then sends a response back to the client. Servers may be further classified as stateless or stateful. Clients of a stateful server may make composite requests that consist of multiple atomic requests. This enables a more conversational or transactional interactions between client and server. To accomplish this, a stateful server keeps a record of the requests from each current client. This record is called a session.

In order to simultaneously process requests from multiple clients, a server often uses the Master-Slave Pattern. In this case the Master perpetually listens for client requests. When a request is received, the master creates a slave to processes the request, and then resumes listening. Meanwhile, the slave performs all subsequent communication with the client.

Here is a simple component diagram showing a server component that implements operations specified in a Services interface, and a client component that depends on these services.

Internally, the client component may consist of a ClientUI that forwards user requests to a controller component. The controller component forwards the request across a process or machine boundary to a RequestListener inside the server. The listener, which acts like a master, creates a RequestHandler slave and forwards the request to it:

The following sequence diagram shows a typical client-server interaction: