1. 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 ( )

2. 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

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

4. 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

5. 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.

6. 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

7. 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)

8. 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

9. 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)

10. 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

11. 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)

12. 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)

13. 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