Transcript Decentralized Database

Distributed Databases
Gapped handout on intranet
complete as we work through.
Distributed databases
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A single logical database that is spread physically
across computers in multiple locations that are
connected by a data communications link
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Databases may be physically stored in two or more computer
systems
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or it can be split into segments and each segment stored in a
different location.
Decentralized Database: A collection of independent
databases on non-networked computers
They are NOT the same thing!
Distributed databases
Distribution may be carried out using different
approaches:
• Partitioned between sites
» Each remote processor has the data that is relevant to itself
only
• Entire databases duplicated at each site
» Every site contains a copy of the entire database
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In these two above methods, the changes to the database at each site need to
be integrated with the each other. This is usually done at night and new copies
distributed to the sites.
• Central database with remote local databases
» The index to the data is held locally with the database
retained at a central location
Distributed databases
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Although geographically dispersed, a distributed database system
manages and controls the entire database as a single collection of
data.
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If redundant data are stored in separate databases due to
performance requirements, updates to one set of data will
automatically update the additional sets in a timely manner.
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An example is the Internet Domain Name System (DNS) - a generalpurpose distributed data query service chiefly used on the Internet for
translating hostnames into Internet addresses. (Handout on this on
the intranet)
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Each database server in the distributed database is controlled by its
local DBMS, and each cooperates to maintain the consistency of the
global database.
Advantages and Disadvantages of Distributed Databases
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Advantages and Disadvantages vary
according to the method of distribution
used.
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Centralised database is useful for statistical analysis
(e.g. sales figures) and backup
Decentralising increases complexity but reduces network
traffic
Advantages
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Security
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Limit access to different areas of the database
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Each local area is responsible for maintenance of its database
Errors are easier to correct at a local level than a national level
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Give access to parts of the database as required
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Faster performance locally
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Smaller computers required (assuming part of the database)
Less transmission cost as traffic is reduced
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easier to handle expansion
Local autonomy
Improved availability
Improved reliability
Economics
Modular Growth
Disadvantages
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Complexity
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Cost
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Require processing and storage at each site
permanent high speed links between sites
Security
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To maintain indexes, locations, updating, etc is complex
Many locations and entry points to the system
Integrity control
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Maintenance of data integrity – one record updated in two
sites differently
Security of Distributed Databases
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Security Issues of Distributed Databases
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Encryption
» Encryption key is spread around, increases possibility of
cracking it
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Protection against and checking for a virus
» Virus in one of the sites can easily spread to the others
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Data access points
» Multiple points of access into the database (each site)
The Data Protection Act states that companies must keep their
data secure, but this is more difficult on a network. It is the
responsibility of the company to put security measures in
place which keep their data secure.
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The following encryption could be used by writing the message in a
grid row by row and transmitting it column by column. “Hers is the
exam paper” could be written thus:H
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A
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And transmitted as HIEMEES**RR*EP*ETXA**HAP*
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Using a 5 column and 6 row grid, decode the message
ITOIOS*EMNTTWDPG*OA*U*HRNCTNIY
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'I wanted Computing not History'