Distributed databases in general, and heterogeneous distributed databases in particular, are of increasing practical importance, as organizations attempt to integrate databases across physical and organizational boundaries. Such interconnection of databases to create a distributed or multidatabase is in fact proving crucial to competitiveness for many companies. This chapter reconsiders the issues addressed earlier in the text, such as query processing, recovery and concurrency control, from the standpoint of distributed databases. | Chapter 22: Distributed Databases Database System Concepts, 5th Ed. ©Silberschatz, Korth and Sudarshan See for conditions on reuse Chapter 22: Distributed Databases s Heterogeneous and Homogeneous Databases s Distributed Data Storage s Distributed Transactions s Commit Protocols s Concurrency Control in Distributed Databases s Availability s Distributed Query Processing s Heterogeneous Distributed Databases s Directory Systems Database System Concepts 5th Edition, Aug 22, 2005. 22. ©Silberschatz, Korth and Sudarshan Distributed Database System s A distributed database system consists of loosely coupled sites that share no physical component s Database systems that run on each site are independent of each other s Transactions may access data at one or more sites Database System Concepts 5th Edition, Aug 22, 2005. 22. ©Silberschatz, Korth and Sudarshan Homogeneous Distributed Databases s In a homogeneous distributed database q q q q All sites have identical software Are aware of each other and agree to cooperate in processing user requests. Each site surrenders part of its autonomy in terms of right to change schemas or software Appears to user as a single system s In a heterogeneous distributed database q Different sites may use different schemas and software q Difference in schema is a major problem for query processing Difference in software is a major problem for transaction processing Sites may not be aware of each other and may provide only limited facilities for cooperation in transaction processing Database System Concepts 5th Edition, Aug 22, 2005. 22. ©Silberschatz, Korth and Sudarshan Distributed Data Storage s Assume relational data model s Replication q System maintains multiple copies of data, stored in different sites, for faster retrieval and fault tolerance. s Fragmentation q Relation is partitioned into several fragments stored in distinct sites s .
