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A two-tier architecture is a software architecture in which a presentation layer or interface runs on a client, and a data layer or data structure gets stored on a server. Separating these two components into different locations represents a two-tier architecture, as opposed to a single-tier architecture. Other kinds of multi-tier architectures add additional layers in distributed software design.
Experts often contrast a two-tier architecture to a three-tier architecture, where a third application or business layer is added that acts as an intermediary between the client or presentation layer and the data layer. This can increase the performance of the system and help with scalability. It can also eliminate many kinds of problems with confusion, which can be caused by multi-user access in two-tier architectures. However, the advanced complexity of three-tier architecture may mean more cost and effort.
An additional note on two-tier architecture is that the word "tier" commonly refers to splitting the two software layers onto two different physical pieces of hardware. Multi-layer programs can be built on one tier, but because of operational preferences, many two-tier architectures use a computer for the first tier and a server for the second tier.