17-01-2013, 03:20 PM
Date's 12 Distributed DBMS Rules
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RULE 1-LOCAL AUTONOMY
Definition: The sites in a distributed system should be autonomous or independent of each other.
Comments: A DBMS at each site in a distributed system should provide its own security, locking, logging, integrity, and recovery. Local operations use and affect only local resources and do not depend on other sites.
RULE 2-NO RELIANCE ON CENTRAL SITE
Definition: A distributed database system should not rely on a central site, because a single central site may become a single point of failure, affecting the entire system. Also, a central site may become a bottleneck affecting the distributed system's performance and throughput.
Comments: Each site of a distributed database system provides its own security, locking, logging, integrity, and recovery, and handles its own data dictionary. No central site must be involved in every distributed transaction.
RULE 3-CONTINUOUS OPERATION
Definition: A distributed database system should never require downtime.
Comments: A distributed database system should provide on-line backup and recovery, and a full and incremental archiving facility. The backup and recovery should be fast enough to be performed on- line without noticeable detrimental effect on the entire system performance.
RULE 4-LOCATION TRANSPARENCY AND LOCATION INDEPENDENCE
Definition: Users and/or applications should not know, or even be aware of, where the data is physically stored; instead, users and/or applications should behave as if all data was stored locally.
Comments: Location transparency can be supported by extended synonyms and extensive use of the data dictionary. Location independence allows applications to be ported easily from one site in a distributed database system to another without modifications.
RULE 5-FRAGMENTATION INDEPENDENCE
Definition: Relational tables in a distributed database system can be divided into fragments and stored at different sites transparent to the users and applications.
Comments: Similar to the location transparency rule, users and applications should not be aware of the fact that some data may be stored in a fragment of a table at a site different from the site where the table itself is stored.
RULE 6-REPLICATION INDEPENDENCE
Definition: Data can be transparently replicated on multiple computer systems across a network.
Comments: Similar to the data location and fragmentation independence rules, replication independence is designed to free users of the concerns of where the data is stored. In the case of replication, users and applications should not be aware that replicas of the data are maintained and synchronized automatically by the distributed database management system.
RULE 7-DISTRIBUTED QUERY PROCESSING
Definition: The performance of a given query should be independent of the site at which the query is submitted.
Comments: Since a relational database management system provides no navigational access to data (via SQL), such a system should support an optimizer that can select not only the best access path within a given node, but also can optimize a distributed query performance in regard to the data location, CPU and I/O utilization, and network traffic throughput.