Distributed Client Server Architectures

•A single logical database spread physically across computers in multiple locations that are connected & communicated by a data communications link (high-speed networks or telephone lines).

•They do not share main memory or disks.

•The computers in a distributed system may vary in size and function, ranging from workstations up to mainframe systems.

Major Objectives
•Location Transparency
–User does not have to know the location of the data
–Data requests automatically forwarded to appropriate sites

•Local Autonomy
–Local site can operate with its database when network connections fail
–Each site controls its own data, security, logging, recovery

The computers in a distributed system are referred to by a number of different names, such as sites or nodes, depending on the context in which they are mentioned.

Advantages and Disadvantagesof DDB

•Advantages 
i) Sharing data
ii) Autonomy
iii) Availability

•Disadvantages
i) Software cost and complexity
ii) Processing overhead
iii) Data integrity exposure
iv) Slower response for certain queries

Need to Distributing a Database
•Data replication 
–Copies of data distributed to different sites 

•Horizontal partitioning
–Different rows of a table distributed to different sites 

•Vertical partitioning 
–Different columns of a table distributed to different sites 


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