1. G063 - Distributed Databases
“Sharing is good!”

2. Learning Objectives: By the end of this topic you should be able to:
explain how databases may be stored in more than one physical location
explain the methods by which this distribution may be carried out
explain reasons why distribution would be carried out
explain the security issues of distributed databases

3. A Distributed Database is:
a database not stored in its entirety at a single location
database spread across a number of computers
in multiple locations
computers connected by a data communications link
LAN and/or WAN
e.g.
Tesco - stock & sales databases
Thomas Cook travel agent – holiday database

4. 3 Types of Distributed Database
Replicated
Centralised
Partitioned

5. Replicated database: = a copy of the complete database at each site

6. Replicated Database a copy of the complete database at each site
exact copy of the database stored & accessed locally
transaction files created of changes at each centre
updates made on a master database
a ‘new’, updated copy of database sent to each centre

7. Replicated Database Advantages: reliability fast response to searches
data is always available locally
not reliant on the network or central server
fast response to searches
local access will be faster than WAN access

8. Replicated Database Disadvantages: data integrity issues
local copies of data may be different to each other
need additional local storage space requirements
additional time required for update operations

9. Centralised database:

10. Centralised Database
single database held centrally (possibly at Head Office)
an index to the central database is held locally
speeds up queries
each site accesses database through a WAN
Example: Booking systems
need distributed access to a central database
sharing of up-to-date information important,
avoids double bookings.

11. Centralised Database Advantages: better security of data
one copy rather than several (replicated copies)
good data integrity
one copy rather than several
always sharing the same data
data always up-to-date
data is updated in real time
centralised backup
can be automated

12. Centralised Database Drawbacks:
a virus in the central system could spread
throughout all sites
possibility of update clashes
two sites trying to modify the same record at the same time

13. Partitioned database:

14. Partitioned Database not every site needs to have all the data
→ give each site just the data that is relevant to that site
database is split into sections (partitions)
each site on the network stores its own data
i.e. the part of the database that relates to that site,
e.g. the part of the database that relates to a single supermarket’s stock is stored at that site,
other (global) data is held centrally
changes to central data can be dealt with overnight
by a batch update from the sites,

15. Partitioned Database Advantages: speed: local control over local data
faster access to local data
local control over local data
scalability
can add new sites as required
not reliant on network or server for day-to-day tasks

16. Partitioned Database Drawbacks: data inconsistency
possibility of different data being held centrally to that on partition
unsuitable for certain applications
if data changes at one node must be instantly seen by all nodes
e.g. holiday bookings
high network usage during update process
will slow down other network processes

17. Partitioned Database Two types: Horizontal partitioning
Vertical partitioning

18. Horizontal partitioning
Example:
branch offices deal mostly with a set of local customers
e.g. Northampton branch stores the fragment where contents of the Branch field = ‘Northampton'
So:
split the table into number of smaller tables
on the basis of rows (records)
i.e. specific field contents
each site (branch) stores just the table relevant to them

19. Horizontal partitioning
this table represents the database for an estate agency with 3 branches

20. Horizontal partitioning
the database is horizontally partitioned
so that the data for each branch is stored on the server in that branch:
this will speed up local queries
Boldmere staff searching for properties in Boldmere

21. Horizontal partitioning
this means that the data is stored like this:

22. Horizontal partitioning
this means that the data is stored like this:

23. Vertical partitioning
data is separated across sites based upon fields
dividing the table based on the different columns
different columns of a table located at different sites
e.g. stock descriptions:
item descriptions & prices at sales outlet
item’s country of origin, supplier name at Head Office
e.g. employee wages:
staff name & hours worked at individual shop
personal data stored at Head Office

24. Vertical partitioning
Advantages:
faster searching
reduced amount of data being sent between sites
access rights
ensures that only certain people see certain fields
help to conform to the DPA
personal information kept separate from sales records

25. Vertical partitioning
Disadvantages:
potential exists for inconsistency in the data stored
complex & time consuming to set up and modify
regular backups essential since there is no data replication