1. Database Design – Lecture 16
Distributed Databases

2. Lecture Objectives Distributed Processing and Distributed Databases
Distributed Database Management System (DDBMS)
Distributed Database Design

3. Distributed Processing
Shares the
database’s logical
processing among
two or more
physically
independent sites
that are connected
through a network.
Note: data resides at only one site and is shared by other sites (“centralized”)

4. Distributed Databases
Stores a logically
related database
over two or more
physically
independent sites.
The sites are
connected by a
computer network.
Note: database is composed of several parts know as database fragments. These fragments are located at several different sites.

5. Distributed Processing and Distributed Databases
In a distributed database environment, the users do not need to know the name or location of each database fragment in order to access the database – transparent to the user
Distributed processing does not require a distributed database but a distributed database requires distributed processing
Both distributed processing and distributed databases require a network to connect all components

6. Lecture Objectives Distributed Processing and Distributed Databases
Distributed Database Management System (DDBMS)
Distributed Database Design

7. DDBMS Advantages
Data are located near/at “greatest demand” site – improved performance
Improved reliability – data replication
Growth facilitation
Reduced operating costs

8. DDBMS Disadvantages
Complexity
Cost
Database design more complex

9. Distributed Database Management System(DDBMS)
Governs the storage and processing of a single logically related database over interconnected computer systems in which both data and processing functions are distributed among several sites.

10. Distributed Database Management System(DDBMS)
A DDBMS must have at least the following functions to be classified as distributed:
- Application Interface - Validation
- Transformation - Query Optimization
- Mapping - I/O Interface
- Formatting - Security
- Backup & Recovery - DB Administration
- Concurrency Control - Transaction Management
- Computer Workstations (sites or nodes)
- Network Hardware & Software
- Communications Media

11. Distributed Database Management System(DDBMS)
A DDBMS must have at least the following functions to be classified as distributed:
Application Interface
Allows the interaction with the end user or application programs and with other DBMSs within the distributed database
Validation
Able to analyze data requests
Transformation
To determine which data request components are distributed and which ones are local

12. Distributed Database Management System(DDBMS)
A DDBMS must have at least the following functions to be classified as distributed:
Query Optimization
To find the best access strategy
Mapping
To determine the data location of local and remote fragments
I/O Interface
To read or write data from or to permanent local storage

13. Distributed Database Management System(DDBMS)
A DDBMS must have at least the following functions to be classified as distributed:
Formatting
To prepare the data for presentation to the end user or an application program
Security
To provide data privacy at both local and remote databases
Backup and Recovery
To ensure the availability and recoverability of the database in case of a failure

14. Distributed Database Management System(DDBMS)
A DDBMS must have at least the following functions to be classified as distributed:
DB Administration
To allow the Database Administrator to maintain the databases
Concurrency Control
To manage simultaneous data access and ensure data consistency across database fragments in the DDBMS
Transaction Management
To ensure that the data move from on consistent state to another – synchronizing transactions

15. Distributed Database Management System(DDBMS)
A DDBMS must have at least the following components:
Computer Workstations (sites or nodes)
Form the network system
Network Hardware and Software
Components that reside in each workstation
Allows all sites to interact and exchange data
Communications media
Carries data from one workstation to another

16. Distributed Database Management System(DDBMS)
A DDBMS must have at least the following components:
Transaction Processor (TP)
Software component found in each computer that requests data
Receives and processes the application’s data requests (remote and local)
Data Processor (DP)
Software component residing on each computer that stores and retrieves data located at the site

17. Distributed Database Environment

18. Lecture Objectives Distributed Processing and Distributed Databases
Distributed Database Management System (DDBMS)
Distributed Database Design

19. Distributed Database Design
Designing for a relational data base structure does not change – start with a top down approach
HOWEVER, need to consider the following as well:
How to partition the database into fragments
Which fragments to replicate
Where to locate those fragments and replicas
More frequently used fragments should be stored locally
Fragments used by all users should be stored centrally

20. Distributed Database Design
Data Fragmentation:
Allows a single object to be broken into two or more segments or fragments
Each fragment can be stored at any site on the network
Data fragmentation information is stored in the distributed data catalog (DDC), from which it is accessed by the TP to process user requests

21. Distributed Database Design
Types of Data Fragmentation:
Horizontal
Vertical
Mixed

22. Distributed Database Design
Types of Data Fragmentation:
Horizontal
The division of a relation into tuples (rows)
Each fragment is stored at a different node and each fragment has unique rows
Each tuple has the same attributes (columns) but the rows are fragmented

23. Distributed Database Design
Example of horizontal fragmentation
Original structure:
5th Edition
Fragmented
structure: Split
by state
6th Edition

24. Distributed Database Design
Example of horizontal fragmentation
Resulting structure:
Fragmented
structure: Split
by state
5th Edition

25. Distributed Database Design
Types of Data Fragmentation:
Vertical
The division of a relation into subsets by attributes (column)
Each subset is stored at a different node, and each fragment has unique columns – with the exception of the key column, which is common to all fragments
Transaction issues here because same record may need to be inserted into two tables (part of record into 1 table and other part into another table). If only 1 insert is successful; end up with inconsistent data.

26. Distributed Database Design
Original structure:
5th Edition
Fragmented
structure: Split
by location
6th Edition

27. Distributed Database Design
Original structure:
5th Edition
Example of Vertical Fragmentation
Fragmented
structure: Split
by location
5th Edition

28. Distributed Database Design
Types of Data Fragmentation:
Mixed
A combination of horizontal and vertical strategies

29. Distributed Database Design
Example of Mixed Fragmentation:

30. Distributed Database Design
Example of Mixed Fragmentation:

31. Data Replication
Storage of data copies at multiple sites served by a computer network
Fragment copies can be stored at several sites to serve specific information requirements
Can enhance data availability and response time
Can help to reduce communication and total query costs

32. Replication Scenarios
Fully replicated database:
Stores multiple copies of each database fragment at multiple sites
Can be impractical due to amount of overhead
Partially replicated database:
Stores multiple copies of some database fragments at multiple sites
Most DDBMSs are able to handle the partially replicated database well
Unreplicated database:
Stores each database fragment at a single site
No duplicate database fragments

33. Data Allocation Deciding where to locate data Allocation strategies:
Centralized data allocation
Entire database is stored at one site
Partitioned data allocation
Database is divided into several disjointed parts (fragments) and stored at several sites
Replicated data allocation
Copies of one or more database fragments are stored at several sites
Data distribution over a computer network is achieved through data partition, data replication, or a combination of both

34. Distributed Database Design
How is a distributed database managed?
Distributed Data Catalog (DDC)
Contains the description of the entire database as seen by the DBA
Translates user requests into sub-queries (remote requests) that will be processed by different DPs
DDC is distributed and replicated at network nodes (the location of a database fragment)

35. Examples of Distributed Databases
Banking
Account data distributed at each local branch
Loan data distributed at each local branch
Corporate data at head office (summarized branch information)
Insurance
Policy data with each branch
Corporate data at head office

36. Examples of Distributed Databases
Retail
Inventory data distributed at each local store
Employee Scheduling data at each store
Corporate data at head office (summarized store information)
Payroll data at head office
Utilities
Utility monitoring data at each location (I.e. nuclear station monitoring – air, water etc at each location)
Corporate data at head office

37. Distributed Database vs Client Server
Client/Server is really an architecture which models a computerized solution based on the distribution of functions between servers and clients. A client requests specific services from a server and a server provides requested services to clients
Distributed processing could be one aspect of client/server architecture – data ‘centralized’
The DDBMS distributes data to different locations – could be used in a Client/Server architecture

38. Distributed Database Design
Steps:
Always start with a centralized view design
Consider horizontal fragmentation of a centralized database
Consider vertical fragmentation of a horizontally fragmented database
Re-consider PK for all fragments of the database
Define data replication rules (scenarios)
Complete Design