We require a framework on which we can discussed architectural issues for databases. The ANSII-SPARC study group define three level architecture for database systems.
The internal level is the one closest to physical storage structure of the database. 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.
The external level is the one closest to the users - i.e. it is the one concerned with the way the data is viewed by individual users. The external level includes a number of external schemas or user views. Each external schema describes the database view of one group of the database users. Each view typically 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.
The conceptual level is a level of indirection between the two. The conceptual level has a conceptual schema, which describes the structure of the whole database for community of users. The conceptual schema is a global description of the database that hides the details of physical storage structure and concentrates on describing entities, data types, relationships, constraints.
In DBMS based on the three schema architecture, each user group refers only to its own external schema which is converted into a request on the conceptual schema, then into a request on the internal schema for processing on the stored database. lf the request is a database retrieval, the data extracted from the stored database must be reformatted to match the user's external view before it is presented to the user.
The process of transforming request and results between levels are called mappings. There are two levels of mapping in the architecture, one between the external and conceptual levels of the system and one between the conceptual and internal levels.
The conceptual/internal mapping defines the correspondence between the conceptual view and the stored database; it specifies how conceptual records and fields are represented at the internal level. If the structure of the stored database is changed - i.e. if a change is made to the storage structure definition- then the conceptual internal mapping must also be changed accordingly, so that the conceptual schema may remain invariant.
An external/conceptual mapping defines the correspondence between particular external view and the conceptual view. The differences that may exist between these two levels are similar to those that may exist between the conceptual view and the stored database. For example fields can have different data types, field and record names can be changed, multiple conceptual fields can be combined into a single(virtual) external field, and so on any number of external views can exist at the same time; any number of users can share a given external view; different external view can overlap.
Client server architecture allows the end users to work in a client computer that has connected to the server in the network environment. The basic functionality of the client server database; is that the data base is kept in the centralized server and clients are requested to maintain their data in the server computer. For the retrieval of data clients have to request data from the server computer and for the data storage clients have to store the data to the server computer over the network. Many clients may connect to the server and the server will control and maintain all the clients data; by using accessibility concepts like data integrity and the security concepts.
locations. Client, Server, Database, Clients are fully separated from the server as well as database; and will function independently. Clients Servers and Databases are connected by network. When ever the client wants to communicate to the Database; that should be done via server computer. Server computer will control the client server environment, for data insertion and retrieval clients has to communicate the server and server will communicate to database.
Internal level
The internal level is the one closest to physical storage structure of the database. 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.
External level
The external level is the one closest to the users - i.e. it is the one concerned with the way the data is viewed by individual users. The external level includes a number of external schemas or user views. Each external schema describes the database view of one group of the database users. Each view typically 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.
Conceptual Level
The conceptual level is a level of indirection between the two. The conceptual level has a conceptual schema, which describes the structure of the whole database for community of users. The conceptual schema is a global description of the database that hides the details of physical storage structure and concentrates on describing entities, data types, relationships, constraints.
Three schema architecture
In DBMS based on the three schema architecture, each user group refers only to its own external schema which is converted into a request on the conceptual schema, then into a request on the internal schema for processing on the stored database. lf the request is a database retrieval, the data extracted from the stored database must be reformatted to match the user's external view before it is presented to the user.
The process of transforming request and results between levels are called mappings. There are two levels of mapping in the architecture, one between the external and conceptual levels of the system and one between the conceptual and internal levels.
The conceptual/internal mapping defines the correspondence between the conceptual view and the stored database; it specifies how conceptual records and fields are represented at the internal level. If the structure of the stored database is changed - i.e. if a change is made to the storage structure definition- then the conceptual internal mapping must also be changed accordingly, so that the conceptual schema may remain invariant.
An external/conceptual mapping defines the correspondence between particular external view and the conceptual view. The differences that may exist between these two levels are similar to those that may exist between the conceptual view and the stored database. For example fields can have different data types, field and record names can be changed, multiple conceptual fields can be combined into a single(virtual) external field, and so on any number of external views can exist at the same time; any number of users can share a given external view; different external view can overlap.
Client Server Database Architecture
Client server architecture allows the end users to work in a client computer that has connected to the server in the network environment. The basic functionality of the client server database; is that the data base is kept in the centralized server and clients are requested to maintain their data in the server computer. For the retrieval of data clients have to request data from the server computer and for the data storage clients have to store the data to the server computer over the network. Many clients may connect to the server and the server will control and maintain all the clients data; by using accessibility concepts like data integrity and the security concepts.
One tier architecture:
Monolithic, "all-in-one" model - e.g. clients are "dumb" terminals connected directly to the mainframe.- Any change affects the entire system
- Single point of failure
Two tier architecture:
Typically composed of multiple clients and a single server, the clients connect to the server over a network.Client:
The client is fat - e.g. implements the GUl, from the server(s), performs business logic based on the data.Server:
Typically a database. Provides a shared data-store for the client. The server doesn't typically implement business logic. Later revisions to the model implement some of the business logic on the server as stored procedures (a.k.a. fat servers).Three tire database architecture:
ln a three tire database architecture three levels of layers are located separately in three differentlocations. Client, Server, Database, Clients are fully separated from the server as well as database; and will function independently. Clients Servers and Databases are connected by network. When ever the client wants to communicate to the Database; that should be done via server computer. Server computer will control the client server environment, for data insertion and retrieval clients has to communicate the server and server will communicate to database.
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