Threetier

Threetier, also written as three-tier architecture, is a software design pattern that divides an application into three distinct logical layers: presentation, application (business logic), and data. The pattern promotes separation of concerns by assigning specific responsibilities to each tier, and it is commonly deployed across multiple processes or machines. In typical web deployments, the layers may reside on separate servers: a presentation tier handles the user interface, an application tier runs business logic, and a data tier manages database storage.

The presentation layer focuses on user interaction and input handling, often implemented as a web or desktop client. The application layer contains the core business rules, data processing, and workflow orchestration, acting as an intermediary between the UI and data layers. The data layer is responsible for data persistence, retrieval, and integrity, usually interfacing with a database management system. Communication between tiers occurs through defined interfaces and protocols, such as APIs and query languages, enabling independent evolution of each tier.

Benefits of threetier architecture include improved maintainability, scalability, and reusability, since changes in one tier have limited impact on others. It also supports distributed deployment and easier testing. Potential drawbacks include increased system complexity, higher latency, and deployment overhead. Variants exist, including two-tier (client–server) and multi-tier or n-tier architectures that extend the concept with additional layers or services. Threetier patterns are common in enterprise and web applications, often implemented with web servers, application servers, and database servers, sometimes leveraging service-oriented or microservices approaches.