CHAPTER 25 - Distributed Databases and Client–Server Architectures GROUP MEMBERS Edwin Acquah Andria Akua Abraham Henry Cleland Francis Asante

Presentation Outline Introduction and Concepts Types of DDBMS Techniques for DDBMS Implementation Fragmentation Replication Query Processing Concurrency Architecture

INTRODUCTION & CONCEPTS DDB Technology is a merger between Database Technology Networks and Data Communication Technology However, developments have been shifted from Pure DDBMS products to Systems based on Client-Server Architectures or Developing Technologies for accessing Distributed Heterogeneous Data sources

THE MAIN AIM Decentralizing Of Processing (at the system level) while achieving an Integration of Information sources (at the logical level) within a geographically distributed systems of databases, applications and users

ADVANTAGES OF DISTRIBUTED DATABASES Management of distributed data with different levels of transparency Increased reliability and availability Improved performance Easier expansion

ADDITIONAL FUNCTIONS OF DISTRIBUTED DATABASES Ability to keep track of the data distribution, fragmentation, and replication by expanding the DDBMS catalog. Ability to access remote sites and transmit queries and data among the various sites via a communication network. Executed with the proper management of the security of the data and the authorization /access privileges of users.

TYPES OF DISTRIBUTED DATABASES (FACTORS TO CONSIDER) Degree of homogeneity Degree of local autonomy

FEDERATED DATABASE SYSTEM Each server is an independent and autonomous centralized DBMS. Used when there is a global view or schema of the federation of databases that is shared by the applications. In heterogeneous FDBS one server may be a relational DBMS, another a network DBMS and the third an object or hierarchical DBMS.

FEDERATED DATABASE SYSTEM ISSUES Differences in data models Differences in constraints Differences in query languages

SEMANTIC HETEROGENEITY The universe of discourse from which the data is drawn Representation and meaning The understanding, meaning and subjective interpretation of data Transaction and policy constraints Derivation of summaries

DESIGN AUTONOMY OF COMPONENT DBSs Communication autonomy Execution autonomy Association autonomy.

Techniques for DDBMS Implementation Fragmentation Vertical Fragmentation (PROJECTION) Horizontal Fragmentation (SELECT) Mixed(Hybrid) Fragmentation Replication Global catalog schema Degree of replication Query Processing Query Decomposition Semijoins

CONCURRENCY CONTROL AND RECOVERY IN DISTRIBUTED DATABASES Issues that arise include: Dealing with multiple copies of data items Failure of individual sites Failure of communications links Distributed commit Distributed DEADLOCK

CONCURRENCY CONTROL BASED ON DISTINGUISHED COPY OF DATA ITEM Based on an extension of centralized locking. Idea is to designate a particular copy of each data item as a distinguished copy. Locks for that data item deal with the distinguished copy as such all locking and unlocking requests are sent to the site that contains that copy.

IMPLEMENTATION OF DISTINGUISHED COPY Primary site technique (Coordinator site) All distinguished copies are kept on one site. Risk and impact of failure is high Primary Site with Backup -Adds a backup site to curb the high risk of failure. Primary copy technique -distribute locks over several sites

3 tier architecture Presentation layer Application layer Database sever Distribution transparency

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