Advantages of DDBMS
- Data is located near the sites of greatest demand
- Faster data access
- Process data at different sites
- New sites can be added without affecting other sites
- Cheaper to add nodes to a system than updating a mainframe
- Less danger of SPOF
Disadvantages of DDBMS
- Complexity of management and control
- Technological difficulty - replication, query optimization, transaction management
- Increased storage requirements (for replication)
- Higher cost
Components of DDBMS
- TP
- DP
- Communications network
Distributed processing
database’s logical processing is shared among
two or more physically independent sites via network
Distributed database
stores logically related database over two or
more physically independent sites via a computer network
Database fragment
database composed of many parts in distributed
database system
Database level fragmentation
Table1 in Location1, Table2 in Location2
Table level fragmentation
Same tables with different data in different locations
- e.g. Payroll & Ops tables in Halifax office with Halifax data, Payroll & Ops tables in Bedford office with Bedford data
Single site processing, single site data
- TP and DP in one computer
- End user has dumb terminal
Multi-site processing, single site data
- Multiple TP run on different computers sharing a single data repository (DP)
- Accessed through LAN
- Client/server architecture
Multi-site processing, multi-site data
- Fully distributed database management system
- Support multiple DP and TP at multiple sites
Homogeneous DDBMS
- integrate multiple instances of same DBMS over a
network - e.g. MySQL v5 on 3 locations
Heterogeneous DDBMS
- integrate different types of DBMSs over a network
- e.g. MySQL in Asia, Oracle in EU, MSSQL Server in US
Fully heterogeneous DDBMS
- support different DBMSs, each supporting different data model running under different computer systems
- DB level fragmentation
- e.g. T1T2 with Oracle in L1, T3T4 with Postgres in L2
Minimum desirable DDBMS transparency
- Distribution Transparency
- Transaction Transparency
- Failure Transparency
- Performance Transparency
- Heterogeneity Transparency
What is Distribution Transparency?
Distributed DB treated as a single logical database
Levels of Distribution Transparency
fragmentation (highest), location, and local mapping (lowest)
Fragmentation Transparency
Query has no fragment name, no location
Location Transparency
Query has fragment name, no location
Local Mapping Transparency
Query has fragment name and location. Faster data retrieval.
When to use local mapping transparency?
for security researchers to track data loss
for DBA to track dupe records
What contains the entire description of Distributed DB?
distributed data dictionary (DDD) or distributed data
catalog (DDC)
What is a distributed global schema?
common database schema to translate user
requests into subqueries
Pros & cons of having large # factors in DDD (i.e. node name, IP address)
Pros: efficient data retrieval
Cons: need to update often
