What is a Two-Tier Architecture?

A two-tier architecture is one where a client talks directly to a server, with no intervening server. It is typically used in small environments (fewer than 50 simultaneous clients).

A common error in client/server development is to prototype an application in a small, two-tier environment, and then attempt to scale up by simply adding more client connections to the server.

This approach will usually result in an ineffective system, as the server becomes overwhelmed. To properly scale to hundreds or thousands of users, it is usually necessary to move to a three-tier architecture.

What is a Three-Tier Architecture?

A three-tier architecture introduces (another) server (or an agent) between the client(s) and the traditional server.

The role of the agent is manyfold. It can provide translation services (as in adapting a legacy application on a mainframe to a client/server environment), metering services (as in acting as a transaction monitor to limit the number of simultaneous requests to a given server), or intelligent agent services (as in mapping a request to a number of different servers, collating the results, and returning a single response to the client.