1. Distributed databases
A brief introduction
(Figure numbers may not be the same as in the book)
Distributed databases

2. Distributed database concepts
Distributed database (DDB)
Collection of multiple logically interrelated databases distributed over a computer network
Distributed database management systems (DDBMS)
Software systems managing a distributed database, making distribution transparent to the users
Distributed databases

3. Distributed databases
Transparency
Hiding implementation details from the users of the database
Data organization transparency
Location transparency
Use does not depend on location
Naming transparency
Naming is independent from location
Replication transparency
Copies can be kept for availability, performance, and availability
User are unaware of the existence of these copies
Fragmentation transparency
One table is divided into more locations
Horizontal fragmentation
Table divided by rows
Vertical fragmentation
Table divided by columns
Distributed databases

4. Example: Replication and horizontal fragmentation
Distributed databases

5. Reliability and Availability
Two common advantages of distributed databases
Reliability
The probability that a system is running at a certain time point
Availability
The probability that a system is continuously available during a time interval
Distributed databases

6. Advantages of distributed databases
Improved ease and flexibility of application development
Transparency: Developers do not have to know …
Increased reliability and availability
Faults are isolated to a single site
Improved performance
Data localization, means less network traffic
Parallelism
Easier expansion
Easy to add more data, processors, etc.
Distributed databases

7. Types of distributed database systems
Degree of homogeneity
Homogeneous: All local DBMSs run identical software
Heterogeneous: Local DMBSs run different software
Autonomy
Local autonomy: Local site can function as a standalone DBMS
No autonomy: Local site can not function as a standalone DBMS
Distributed databases

8. Classification of distributed databases

9. Database system architectures
Distributed databases

10. Distributed databases
General architecture
Distributed databases

11. Component architecture of distributed databases

12. Distributed databases
Data fragmentation
Which site should store which portion of the database?
Simple fragmentation
Each site has a whole relation
Horizontal fragmentation
Subset of rows in each site
Sometimes based on location
Vertical fragmentation
Subset of columns in each site
Primary key must be in all sites
Mixed / hybrid fragmentation
Horizontal + vertical fragmentation
Described by fragmentation schema
Distributed databases

13. Example fragmentation
Distributed databases

14. Example fragmentation, continued
Distributed databases

15. Distributed databases
Data replication
Replication to improve availability
Fully replicated database
All data is replicated to each site
Non replication
All data is stored at exactly one site
Partial replication
Some data is replicated to some sites
Described by replication schema
Distributed databases

16. Distributed query processing
Query mapping
Query mapped from SQL to relational algebra using the global conceptual schema
Localization
Map query on the global schema to separate queries on the local schemas
Using fragmentation and replication information
Global query optimization
Cost = CPU time + I/O time + communication time
Local query optimization
Same as in centralized databases
Distributed databases

17. Distributed transaction management, Two-phase commit protocol (2PC)
Global transaction manager / coordinator
Coordinates the results of local transaction managers.
All local transaction managers must be able to ”commit”, before actually doing the ”commit”
Two-Phase commit protocol (2PC)
Phase 1
Individual databases tell the coordinator that they have finished transaction
All individual databases have finished: Coordinator sends ”prepare for commit” to all databases
Individual databases answer ”read to commit” or ”cannot commit”
Phase 2
If all databases answered ”ready to commit”, coordinator sends ”commit” to all databases
If one (or more) databases answered ”cannot commit”, coordinator sends ”abort” to all databases.
Timeout: if one (or more) databases does not answer within a given amount of time, coordinator sends ”abort”.
Distributed databases

18. Two-phase commit protocol (2PC)
Problems with 2PC
Coordinator crashes: All participating sites are waiting
No way of knowing whether participating sites really got the ”commit” / ”abort”
Distributed databases

19. Three-phase commit (3PC)
Distributed databases