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Massively Distributed Database Systems- Distributed DBS
Spring 2014Ki-Joune Li
http://isel.cs.pusan.ac.kr/~likPusan National University
Pros and ConsReliability and Availability
Local Control
Incremental Growth
Communication Costs
Fast Response
Advantages
Software Cost and Complexity
Processing Overhead
Data Integrity
Slow Response
Disadvantages
3-layer model of databases
External Layer
Conceptual Layer
Physical LayerData Storage Format
Conceptual Schema
View Definitions
Implementation- Systems
Modeling
Local DatabaseLocal Database
Distributed Databases as a Logical Layers
External Layer
Conceptual Layer
Physical Layer
External Layer
Conceptual Layer
Physical Layer
External Layer
Conceptual Layer
Physical Layer
Global Conceptual Layer
Local Database
Global Physical Layer
GlobalDatabase
View from client View from client View from client
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Issues
• Replication vs. Partitioning• Distributed DBMS • Transparency• Query Optimization• Transaction Management
Replication vs. Partitioning
• Replication• Partitioning• Vertical vs. Horizontal• Hybrid
• Replicate all or parts of databases to local DB
Site 1
Replication
DB-1
DB-2
DB-3Site 2
DB-1
DB-2
Site 2
DB-2
DB-3
How to manage replicated DBs?
• Issue 1 – Consistency • If updates at a site, how to manage the integrity of
global databases
• Issue 2 – How to duplicate• All or only some parts• Factors to consider
Replication – Management of Consistency
• Snapshot replication• Store all update logs at a central site from a given time• Periodically send proper logs to local sites• Each local site reflects the update logs for its local DB
• Near Real-Time replication• When an update occurs, it triggers updates at other sites
• Pull Replication• Instead of push protocol, each local site asks update logs
when it is necessary
Replication – Management of Consistency
• Exclusive ownership vs. Shared ownership• Single update vs. Multiple update
• Synchronous updates vs. Asynchronous update• Simple snapshot vs. Multiple snapshot
Replication – How to replicate
• Fast Response• Communication Overhead• Security• Query Optimization
Partitioning – Horizontal Partitioning
Split a table into sev-eral subtables
Partitioning – Horizontal Partitioning
• How to split a table?• Efficiency• Local Optimization• Communication Overhead• Security
• Dynamic reconfiguration of Partitioning
Partitioning – Vertical Partitioning
Split a DB into severaldisjoint tables
Shared Primary Keys – Join operations are inevitable
Comparison btw Replication and Partitions
Distributed DBMS
• What a distributed DBMS should do⁻ Management of Data Dictionary⁻ Resolving Heterogeneity: Schema, QL, DBMS⁻ Keeping distributed DBs secure and consistent: TM⁻ Transparency: single logical view to user⁻ Dynamic load balancing⁻ Query processing (Optimization)
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