Distributed Databases

Question 1

What is a Distributed
Database?

Answer 1

This is a Db system that is stored on several computers, interconnected through a network.

Question 2

What are some features of a site?

Answer 2

Each site can process local transactions involving data only on that site

Each site has a local transaction manager

Each site has a transaction coordinator

Question 3

Are global interactions possible?

Answer 3

Yes, Some sites may get involved in global transactions,
which access data from several sites.

Local transaction managers communicate to achieve this.

Question 4

Examples

Answer 4

University of the West Indies
 Bursary - Financial information
 Personnel - Staff information
 Registry - Student information

Multinational
 HQ in Kingston
 Manufacturing in Trinidad
 Warehouse in Miami
 European HQ in London
 Each site keeps data on local employees, as well as data
relevant to the operation of the site.

Question 5

Why are distributed Databases Beneficial?

Answer 5

More natural for representing many real world organizations.

Local autonomy

Improved performance
- Speeds up query processing with the possibility of parallel computing
-Local Processing

Question 6

Provide 3 benefits of Distrbuted databases:

Answer 6

Improved availability/reliability
-DB can continue to function even if some subsystems are down if we replicate data or hardware.

Security
-Avoid destruction of DB by replicating vital data.

Incremental growth
-Increasing size of DB
-Increasing operations
-Example: Include R&D facility.

Question 7

What do we want from a distributed database?

Answer 7

To allow users to use the system as if it is not distributed.
No reliance on central master site as this causes bottle necks.

Continuous Operations to enforce reliability and availability.

Distributed Query Processing so that all data is available at a no. of different sites.

Distributed Transaction Management
- Same copy of a data item may be a a number of sites.

Transparency

Local Autonomy

Question 8

What is autonomy in distributed systems?

Answer 8

All operations at a site are controlled by that site.

Question 9

What are the 3 different types of autonomy?

Answer 9

Design
Communication
Execution

Question 10

Describe each type of autonomy.

Answer 10

Design
Individual DBs can use data models and transaction management techniques that they prefer.

Communication
Individual DBs can decide which information they want to make accessible to other sites

Execution
Individual DBs can decide how to execute transactions submitted to them.

Question 11

What are some issues with Distributed database design.

Answer 11

How should DB and applications be placed across sites.
How should data dictionary be placed across sites.
Replication and partitioning

Question 12

What are issues and problems associated with distributed database systems?

Answer 12

Distributed database design

Distributed query processing (How to break down a query into series of data manipulation operations.)

Distributed concurrency control

Distributed deadlock control

Reliability

Heterogeneous databases

Question 13

Describe the Problem of Reliability with DDBs(Distributed Database Systems).

Answer 13

If site becomes inaccessible, how do you ensure that DBs at other sites remain consistent and up-to-date?

Question 14

What is a heterogeneous database?

Answer 14

Sharing pre-existing DBs with different conceptual schemas.

Also called multidatabase systems

Question 15

What is Replication?

Answer 15

Maintaining multiple copies of a part of a DB.

Question 16

What is Fragmentation?

Answer 16

Storing different parts of a database on different sites.

Question 17

Advantages of Replication

Answer 17

Increased availability - If one site goes down, the data may be available from elsewhere.

Increased parallelism - More likely that the data needed is at the site that poses the query.

We may send parts of the same query to different sites.

Question 18

Disadvantages of Replication

Answer 18

Disadvantages
Increased storage space
Increased overhead on update.
Must ensure that all copies are consistent.

Question 19

Why fragment? Why not simply store different
complete tables at different sites?

Answer 19

Applications usually access only a subset of a relation.

Fragmentation gives possibility of increased parallelism.

Question 20

What are the 3 types of Fragmentation? Explain the different types.

Answer 20

Horizontal fragmentation - Assign different tuples to each fragment (e.g.,through a selection in the sense of relational algebra).

Vertical fragmentation - Assign different attributes to each fragment.
Must ensure re constructability.

Mixed Fragmentation - Mixture of the two.

Question 21

Problems with Fragmentation?

Answer 21

Increased response time

Data integrity might be harder to maintain

Issues with correctly finding where to place the various fragments.