2019 Explain the 3-tier architecture. How is it different from two-tier architecture? [5] [MAKAUT MCA]
The goal of the three-schema architecture, illustrated in above figure, is to separate the user applications from the physical database. In this architecture, schemas can be defined at the following three levels:
1. The internal level has an internal schema,which describes the physical storage structure of the database. The internal schema uses a physical data model and describes the complete details of data storage and access paths for the database.
2. The conceptual level has a conceptual schema, which describes the structure of the whole database for a community of users. The conceptual schema hides the details of physical storage structures and concentrates on describing entities, data types, relationships, user operations, and constraints.
Usually, a representational data model is used to describe the conceptual
schema when a database system is implemented.
3. The external or view level includes a number of external schemas or user
views. Each external schema describes the part of the database that a particular user group is interested in and hides the rest of the database from that user group. Each external schema is typically implemented using a representational data model.
Differences between 3-tier and 2-tier architectures
1) Two tier architecture is similar to client-server model where the application at the client end directly communicates with the database at the server side.
In three-tier architecture, the client does not directly communicate with the server. Instead, it interacts with an application server which further communicates with the database system and then the query processing and transaction management takes place. This intermediate layer acts as a medium for exchange of partially processed data between server and client
2) . Two-tier architecture is used for systems having small number of users whereas three-tier architecture is mainly used for large web applications