Ch22b_DistributedDBs-95

Ch22b_DistributedDBs-95 - Distributed Databases Chapter 22,...

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1 Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 1 Distributed Databases Chapter 22, Part B Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 2 Introduction Y Data is stored at several sites, each managed by a DBMS that can run independently. Y Distributed Data Independence: Users should not have to know where data is located (extends Physical and Logical Data Independence principles). Y Distributed Transaction Atomicity: Users should be able to write Xacts accessing multiple sites just like local Xacts. Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 3 Recent Trends Y Users have to be aware of where data is located, i.e., Distributed Data Independence and Distributed Transaction Atomicity are not supported. Y These properties are hard to support efficiently. Y 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 Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 4 Types of Distributed Databases Y Homogeneous: Every site runs same type of DBMS. Y Heterogeneous: Different sites run different DBMSs (different RDBMSs or even non- relational DBMSs). DBMS1 DBMS2 DBMS3 Gateway Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 5 Distributed DBMS Architectures Y Client-Server Y Collaborating-Server CLIENT CLIENT SERVER SERVER SERVER QUERY SERVER SERVER SERVER QUERY Client ships query to single site. All query processing at server. - Thin vs. fat clients. - Set-oriented communication, client side caching. Query can span multiple sites. Database Management Systems, 2 nd Storing Data Y Fragmentation Horizontal: Usually disjoint. Vertical: Lossless-join; tids. Y Replication – Gives increased availability. – Faster query evaluation. Synchronous vs. Asynchronous . X Vary in how current copies are. TID t1 t2 t3 t4 R1 R1 R2 R3 SITE A SITE B
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3 Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 7 Distributed Catalog Management Y Must keep track of how data is distributed across sites. Y Must be able to name each replica of each fragment. To preserve local autonomy: –< local-name , birth-site > Y 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. Database Management Systems, 2 nd Edition. R. Ramakrishnan and Johannes Gehrke 8 Distributed Queries Y Horizontally Fragmented: Tuples with rating < 5 at Shanghai, >= 5 at Tokyo. – Must compute SUM(age), COUNT(age) at both sites. –I f WHERE contained just S.rating>6, just one site.
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This note was uploaded on 02/06/2010 for the course CSE 302 taught by Professor Joel during the Summer '05 term at Punjab Engineering College.

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Ch22b_DistributedDBs-95 - Distributed Databases Chapter 22,...

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