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Distributed Database Management Systems Reading Farkas Textbook: Ch. 1, Ch. 3 For next class: Ch. 4 CSCE 824 2 Database Management System (DBMS) Collection of – interrelated data and – set of programs to access the data Convenient and efficient processing of data Database Application Software Farkas CSCE 824 3 Abstraction View level: different perspectives – application programs hide irrelevant data Logical level: data models – Logical representation of data – Different approaches: relational, hierarchical, network, object oriented, semi-structured, etc. – Data independence principle Farkas Physical level: how data is stored CSCE 824 4 Motivation for DBMS Farkas Integrate related data Provide centralized and controlled access to data CSCE 824 5 Computer Network Distributed processing: – Number of autonomous processing elements that are interconnected by computer network – Cooperate to perform their assigned tasks Farkas CSCE 824 6 What to distribute? Farkas Processing logic/element Functions Data Control of execution CSCE 824 7 Why to distribute? Farkas Intuition Reliability Performance CSCE 824 8 Distributed Database Systems Distributed database: – Collection of multiple, logically interrelated databases that are distributed over a computer network Distributed DBMS: software system that – Permits the management of the distributed database and – Makes the distribution transparent to the user Farkas CSCE 824 9 Data Delivery Data storage and query processing Data delivery: – Delivery mode: push, pull, hybrid – Frequency: periodic, conditional, ad-hoc, irregular – Communication method: unicast, one-to-many Farkas CSCE 824 10 DDBMS Services Transparent data management – Distributed, replicated data – Transparency: network, replica, fragmentation Reliable access to data – Distributed transactions – Failure atomicity Improved performance Flexible expansion Farkas CSCE 824 11 Difficulties Everything that is present in traditional DBs Fragmentation and replica control – Data retrieval – Data update Farkas Dealing with failures Synchronization CSCE 824 12 DDBMS Issues Farkas Database design Directory management Query processing Concurrency control Deadlock management Reliability Replication CSCE 824 13 DDBMS Architecture Chapter 1.7 (read only) – Client/server – P2P – Multi-database Farkas CSCE 824 14 DISTRIBUTED DATABASE DESIGN Farkas CSCE 824 15 Design Issues Farkas Placing of data and programs (DBMS and application) Network issues CSCE 824 16 Level of Sharing No sharing Data sharing Data and program sharing Heterogeneous environment! Farkas CSCE 824 17 Access Pattern Farkas Static Dynamic CSCE 824 18 Level of Knowledge on Access Behavior Farkas Complete information Partial information CSCE 824 19 Top-Down Design Farkas Figure 3.2 CSCE 824 20 Fragmentation Why to fragment the data? – Application views – Limit replication while increase availability – Increased concurrency Farkas CSCE 824 21 Fragmentation Types: – Horizontal – Vertical – Hybrid Degree: – From no fragmentation to individual tuples/attributes Farkas CSCE 824 22 Correctness of Fragmentation 1. 2. Completeness: FR={R1, …, Rn} Reconstruction: R=Ri, RiR 1&2: Lossless-join (normalization) 3. Disjointness: – – Farkas Horizontal: does not djRi such that djRk where ki Vertical: same as horizontal for nonprimary key attributes CSCE 824 23 Allocation Replication or single copy? – Read-only transactions Issues: (Figure 3.6) – – – – Query processing Directory management Concurrency control Reliability Real world applications Farkas CSCE 824 24 Fragmentation Design Information need: – Database information – Application information – Communication network information – Computer system information Farkas CSCE 824 25 Horizontal Fragmentation Primary horizontal fragmentation: defined by selection operation on the relations of a database schema, Ri= Fi(R) Correctness: – Completeness – Reconstruction (union) – Disjointness Farkas CSCE 824 26 Vertical Fragmentation R={R1, …, Rn}, where each Ri (i=1, …, n) contains a primary key and some of the attributes in R More difficult than horizontal fragmentation – heuristics – Grouping – Splitting Farkas CSCE 824 27 Vertical Fragmentation Correctness: – Completeness – Reconstruction (join) – Disjointness Farkas CSCE 824 28 Hybrid Fragmentation Farkas Horizontal or vertical fragmentations are not sufficient the user application requirements Nested or mixed fragmentation CSCE 824 29 Data Directory Global vs. local conceptual schemas – How to search? – Where to store? – Single vs. multiple copies? Farkas CSCE 824 30 Current Research Allocation: new requirements, technology, etc. Where to store the fragments? Dynamic environment – – – – Farkas Usage pattern Application characteristics Network changes Security CSCE 824 31 Next Class Commit Protocols Farkas CSCE 824 32