Distributed DBMS Concepts of Distributed DBMS Advantages and Disadvantages Functions and Architechure Objective and strategy of Allocation Date's 12 Rules Distributed Concurrency Control Replication Servers Distributed Query Optimization Mobile Database Chapter 24 25 26( 22 23 24)

Concepts of Distributed DBMS DDBMS consists of a single logical DB that is split into a number of fragments Each fragments is stored on one or more computers connected by network characteristics pp.689 DDBMS make the distribution transparent to user. Parallel DBMSs:share memory(SMP), share disk(clusters),share nothing Fig. 24(22).3

Advantages and Disadvantages Reflect ORG. structure Improve shareability and local autonomy Improved availability and reliability Improved performance Economics and Modular growth Integration, Remaining competitive Disadvantages: Complexity,Cost Security,Integrity control more difficult Lack of standards and experience

Homogenous and Heterogeneous different hardware different DBMS products different hardware and different DBMS products gateway solution not support transaction management can not translate differnet schemas in general

Functions and Architechure Functions: pp.747(703) 24(22).3.1 Architecture: pp.748(704) Fig. 24.5(22.4) Fragmentation: a sub-table (horizontal or vertical) Allocation: a fragment is stored at a site Replication: maintain a copy of a fragmentat several different sites.

Objective and strategy of Allocation objectivies locality of reference improved reliability and availability acceptable performance balanced storage capacities and costs minimal communication costs strategies: table 24(22).3 pp.754(710) centralized, Fragmented, Complete replication(snapshots), selective replication

Date's 12 Rules local autonomy no reliance on a central site,continuous operation location, fragmentation,replication independence distributed query,transaction processing hardware,OS,network,DBMS independence pp.771(729)

Distributed Concurrency Control additional problem: multiple-copy consistency problem Locking protocols Centralized 2PL:easy, but central site becomes a bottleneck. Primary copy 2PL:distributing lock manager to a number of sites. a degree ofcentrali-zation, primary copy handle by one site. Distributed 2PL: lock manager control data at that site,ROWA protocol Majority locking:obtain locks on a majority of copies

Distributed Query Optimization Consideration: memory is fastest disk access is slow LAN is slower than disk access (Not always true, 160/320MB of SCSI 1/10 Gbit of Ethernet) WAN is much slower than LAN(Not always true)

Replication Servers(1) Replication: generating multiple copies of data on one or more sites synchronous and asynchronous replication Functionality scalability mapping and transformation object replication specification of replication schema subscription mechanism Initialization mechanism Chap. 26.2

Replication Servers(2) Data ownership master/slave ownership workflow ownership(update ability can move from one site to another) update-anywhere ownership Implementation pp.829(789) transactional updates snapshots versus Database triggers conflict detection and resolution (when multiple sites can update data) time zone (time server)

Mobile Database Definiation: A database that is portable and physically separate from a centralized database server but is capable of communicating with that server from remote sites components of a mobile database:pp.833 (793) Figure 26.6(24.5) additional functionality: pp.834(794)

Oracle's Implementation do not support fragmantation transparency based on database link(communication from one database to another) Heterogeneous DB solutions 25(23).7 Transparent gateways to some DBs Procedural gateways to some Aps features: pp.818(775) Types of replication pp.843(795) Materialized view replication Single,Multimaster replication Hybrid replication