14-01-2013, 04:44 PM
Distributed Databases
1Distributed Databases.ppt (Size: 136 KB / Downloads: 123)
A distributed database system consists of loosely coupled sites that share no physical component
Appears to user as a single system
Database systems that run on each site are independent of each other
Processing maybe done at a site other than the initiator of request
Homogenous Distributed Database Systems
All sites have identical software
They are aware of each other and agree to cooperate in processing user requests
It appears to user as a single system
What can we do?
A single query from a Manufacturing client on local database mfg can retrieve joined data from the products table on the local database and the dept table on the remote hq database.
For a client application, the location and platform of the databases are transparent.
Heterogeneous Distributed Database System
In a heterogeneous distributed database system, at least one of the databases uses different schemas and software.
A database system having different schema may cause a major problem for query processing.
A database system having different software may cause a major problem for transaction processing.
Distributed Data Storage
Replication
System maintains multiple copies of data, stored in different sites, for faster retrieval and fault tolerance.
Fragmentation
Relation is partitioned into several fragments stored in distinct sites
Replication and fragmentation can be combined
Relation is partitioned into several fragments: system maintains several identical replicas of each such fragment.
Fragmentation
Data can be distributed by storing individual tables at different sites
Data can also be distributed by decomposing a table and storing portions at different sites – called Fragmentation
Fragmentation can be horizontal or vertical