Evolution of information systems

¨             Centralised processing systems
Until 70s, usually a mainframe with ‘dumb’ terminals

¨             Dispersed systems
With cheaper hardware, processing power moved to users’ desks – many standalone computers

¨             Distributed processing
linked computers at remote sites

¨             Local area networks (LANs)
group of linked computers on the same site, connected by cables

¨             Wide area networks (WANs)
link whole organisations for communication and data exchange, countrywide or worldwide

¨             Distributed databases
need to store databases locally

¨             Public networks
e.g. The Internet, BT’s Home Campus, or MSN
Ways of distributing a database.

1.       Central database can be partitioned
each remote processor has the data on its own customers, stock, etc.

2.       Entire database is duplicated at each remote site.

In both cases, the two databases have to be reconciled - usually by updating the central database each night

3.       Central database contains only an index
to entries which are held on local databases.  This system is used for very large databases

A variation of this system is not to hold an index, and to simply poll all remote databases until the required record is found. The complete record is then transferred to the local computer that requests it.

Distributed databases

Advantages

¨             reduce the dependence on a single, central database.

¨             increase responsiveness to local users’ and customers’ needs

Limitations

¨             dependent on powerful and reliable telecoms systems

¨             local databases can sometimes depart from central data definitions and standards,

¨             security can be compromised when distribution widens access to sensitive data.

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