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  Centralized and Client-Server DBMS Architectures

Centralized and Client-Server DBMS Architectures:

Centralized DBMS:

a) Merge everything into single system including- Hardware, DBMS software, application programs, and user interface processing software.

b) User can still connect by a remote terminal – but all processing is done at centralized site.

Physical Centralized Architecture:

1695_physical centralized architecture.jpg

Architectures for DBMS have pursued trends similar to those generating computer system architectures. Earlier architectures utilized mainframes computers to provide the main processing for all system functions including user application programs as well as user interface programs as well all DBMS functionality. The reason was that the majority of users accessed such systems via computer terminals that did not have processing power and only provided display capabilities. Thus all processing was performed remotely on the computer system and only display information and controls were sent from the computer to the display terminals which were connected to central computer via a variety of types of communication networks.

As prices of hardware refused most users replaced their terminals with PCs and workstations. At first database systems utilized these computers similarly to how they have used is play terminals so that DBMS itself was still a Centralized DBMS in which all the DBMS functionality application program execution and user interface processing were carried out on one Machine.

Basic 2-tier Client-Server Architectures:

• Specialized Servers with Specialized functions
• Print server
• File server
• DBMS server
• Web server
• Email server
• Clients are able to access the specialized servers as needed

Logical two-tier client server architecture:

1935_two tier client server architecture.jpg

Clients:

• Offer appropriate interfaces through a client software module to access as well as utilize the various server resources.
• Clients perhaps diskless machines or PCs or Workstations with disks with only the client software installed.
• Connected to the servers by means of some form of a network.
• (LAN- local area network, wireless network and so on.)

DBMS Server:

• Provides database query as well as transaction services to the clients

• Relational DBMS servers are habitually called query servers, SQL servers, or transaction servers

• Applications running on clients use an Application Program Interface (API) to access server databases via standard interface such as:

ODBC- Open Database Connectivity standard
JDBC- for Java programming access
Client and server should install appropriate client module and server module software for ODBC or JDBC

Two Tier Client-Server Architecture:

a) A client program may perhaps connect to several DBMSs sometimes called the data sources.

b) In general data sources are able to be files or other non-DBMS software that manages data. Other variations of clients are likely- example in some object DBMSs more functionality is transferred to clients including data dictionary functions, optimization as well as recovery across multiple servers etc.

Three Tier Client-Server Architecture:

a) Common for Web applications.

b) Intermediate Layer entitled Application Server or Web Server.

c) Stores the web connectivity software as well as the business logic part of the application used to access the corresponding data from the database server.

d) Acts like a conduit for sending moderately processed data between the database server and the client.

e) Three-tier Architecture is able to Enhance Security:

• Database server merely accessible via middle tier.
• Clients can’t directly access database server.

1251_three tier client server architecture.jpg

Classification of DBMS's:

• Based on the data model used
• Traditional- Network, Relational, Hierarchical.
• Emerging- Object-oriented and Object-relational.
• Other classifications
• Single-user (typically utilized with personal computers) v/s multi-user (most DBMSs).
• Centralized (utilizes a single computer with one database) v/s distributed (uses multiple computers and multiple databases)

Variations of Distributed DBMSs (DDBMSs):

• Homogeneous DDBMS
• Heterogeneous DDBMS
• Federated or Multi-database Systems
• Distributed Database Systems have at the present come to be known as client-server based database systems because
• They don’t support a totally distributed environment however rather a set of database servers supporting a set of clients.

Cost considerations for DBMSs:

• Cost Range- from free open-source systems to configurations costing millions of dollars
• Instances of free relational DBMSs- MySQL, PostgreSQL and others.

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