2011-11-21-DistributedDBs - Distributed Databases Chapter...

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1 Distributed Databases Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 1 Chapter 22, Part B Introduction Data is stored at several sites, each managed by a DBMS that can run independently. Distributed Data Independence: Users should not have to know where data is located (extends Physical Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 2 and Logical Data Independence principles). Distributed Transaction Atomicity: Users should be able to write Xacts accessing multiple sites just like local Xacts. Note: We focus on concepts today, systems next lecture Recent Trends Users have to be aware of where data is located, i.e., Distributed Data Independence and Distributed Transaction Atomicity are not supported. Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 3 These properties are hard to support efficiently. For globally distributed sites, these properties may not even be desirable due to administrative overheads of making location of data transparent.
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2 Types of Distributed Databases Homogeneous: Every site runs same type of DBMS. Heterogeneous: Different sites run different Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 4 DBMSs (different RDBMSs or even non- relational DBMSs). DBMS1 DBMS2 DBMS3 Gateway Distributed DBMS Architectures Client-Server CLIENT CLIENT QUERY Client ships query to single site. All query processing at server. Thin vs fat clients Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 5 Collaborating-Server SERVER SERVER SERVER SERVER SERVER SERVER QUERY - vs. fat clients. - Set-oriented communication, client side caching. Query can span multiple sites. Storing Data Fragmentation Horizontal: Usually disjoint. Vertical: Lossless-join; tids. TID t1 t2 t3 t4 Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 6 Replication – Gives increased availability. – Faster query evaluation. Synchronous vs. Asynchronous . Vary in how current copies are. R1 R1 R2 R3 SITE A SITE B
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3 Distributed Catalog Management Must keep track of how data is distributed across sites. Must be able to name each replica of each fragment To preserve local autonomy Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 7 fragment. To preserve local autonomy: –< local-name , birth-site > Site Catalog: Describes all objects (fragments, replicas) at a site + Keeps track of replicas of relations created at this site. – To find a relation, look up its birth-site catalog. – Birth-site never changes, even if relation is moved. Distributed Queries
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2011-11-21-DistributedDBs - Distributed Databases Chapter...

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