G063 - Distributed Databases

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

Database storage:

Database storage:

Database storage:

Database storage:

A Distributed Database is:• a single logical database

– consisting of many entities

– possibly used by many users for different purposes

• a database is not stored in its entirety at a single physical location

• database is spread physically across a number of computers – computers could be in multiple locations

buildings or sites,

– computers connected by a data communications link LAN and/or WAN

Why distribute a database:• allows faster local queries

– faster searching

• speeds up other network operations – due to some data queries being handled locally

reduces network traffic 

• improved reliability– data may be replicated at multiple sites

• allows for modular growth of the database– can easily add new sites and/or uses

• user does not need to know where data is stored physically – looks like a single, location, centralized system to the user

Types of Distributed Database

• Replicated

• Centralised

• Partitioned

Replicated Database• complete database is duplicated at each centre

• exact copy of the database stored & accessed locally

• duplicated versions are usually read only– transaction files created of changes at each centre

• updates allowed made on a master database– a ‘new’, updated copy of database sent to each centre

at regular intervals

Replicated DatabaseAdvantages:• reliability

– data is always available locally

– not reliant on the network or central server

– work carries on even if some nodes are down

• fast response to searches– local access will be faster than WAN access

data does not have to be transmitted over the network

• reduced network traffic at prime time– faster access to network if required

Replicated DatabaseDisadvantages

• additional local requirements for storage space

• additional time for update operations

• complexity and cost of updating

• data integrity issues – if replicated data is not updated simultaneously– local copies of data may be different

Centralised Database• single database held centrally (possibly at Head Office)• each node accesses database through a network (WAN)

– access available to all branches or offices,

• an index to the central database is held locally at each node– speeds up queries/transactions

• booking systems need distributed access to a central database if they are to work effectively– sharing of up-to-date information important,

– avoids double bookings.

Centralised DatabaseAdvantages:• better security of data

– one copy rather than several (replicated copies)

– security handled centrally

• good data integrity– one copy rather than several

always sharing the same data

• data can be updated in real time– data always up-to-date

• centralised backup– can be automated

Centralised DatabaseAdvantages (from June 2011 Q13 mark scheme):

• storage is only required at the central location for the centralised database (1) the local indexes stored at each site take up far less memory (1)

• queries are processed locally(1) this speeds up searches as only the required data is retrieved from the central location (1)

• less data traffic than complete centralisation (1) as only data is sent and not the additional information /forms/reports structure (1)

• increased security (1) only central database needs increased security as that is where the data is stored (1)

• integrity of data not compromised (1) as it is stored in only one location and one database to update (1)

• centralised back-up of data (1) management backup easier as it is just one person’s responsibility (1)

Centralised DatabaseDrawbacks:

• 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

Partitioned Database• database is split into sections

• each node or site on the network stores local data– i.e the section of the database that relates to that site,

e.g. the section 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,

Horizontal partitioning• involves putting different rows into different tables.

• splitting the table into number of smaller tables – on the basis of rows (records)

i.e. specific field contents

Example:• branch offices in an organization deal mostly with a

set of local customers – Euston Road branch stores the fragment where contents of

the Branch field = 'Euston Road'

Horizontal partitioning• this table represents the database for an estate

agency with 3 branches

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

Horizontal partitioning

• this means that the data is stored like this:

Horizontal partitioning• this means that the data is stored like this:

Vertical partitioning• dividing the table based on the different columns.• involves creating tables with fewer columns

– using additional tables to store the remaining columns.

• different columns of a table located at different sites– e.g. stock descriptions (country of origin, supplier name at

one site and prices at another site)

Vertical partitioningFrom June 2011 Q13 mark scheme:

• only certain people see certain fields– e.g. financial matters not revealed to all (1)

• to conform to the law/DPA (1) – keeping personal information private (1)

• reduces amount of data being sent between locations (1) – in order to speed up data transfer (1)

– allowing faster reaction time (1)

– meaning rescue reaches emergency quicker (1)

Partitioned DatabaseAdvantages:• speed:

– faster access to local data less network access required

• local control over local data

• scalability– can add new sites as required

• not reliant on network or server for day-to-day tasks

• each partition can have its own transaction log– local reporting (access/sales)

Partitioned DatabaseDrawbacks:

• data inconsistency – possibility of different data being held centrally to that on

partition– regular batch update required to maintain consistency

• 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