What are the main differences between single-tier and multi-tier site architectures?

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Single-tier site architecture simply consists of a server machine running the basic Web server software. Multi-tier site architecture, on the other hand, provides much more functionality by linking a Web server layer that can include multiple Web servers to a middle tier that includes many Web application servers, which provide a wide variety of transaction processing tasks. This middle layer is also linked to a backend layer that includes existing databases, human resources systems, corporate applications, financial data, and enterprise systems. A multi-tiered site typically employs several or more physical computers each running some of the software applications and sharing the workload across many computers.

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