Distributed Databases

1. Distributed Databases Gapped handout on intranet complete as we work through.

2. Distributed databases • A single logical database that is spread physically across computers in multiple locations that are connected by a data communications link • Databases may be physically stored in two or more computer systems • 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!

3. 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 • 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

4. Distributed databases • Although geographically dispersed, a distributed database system manages and controls the entire database as a single collection of data. • 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. • An example is the Internet Domain Name System (DNS) - a general-purpose distributed data query service chiefly used on the Internet for translating hostnames into Internet addresses. (Handout on this on the intranet) • Each database server in the distributed database is controlled by its local DBMS, and each cooperates to maintain the consistency of the global database.

5. Advantages and Disadvantages of Distributed Databases • Advantages and Disadvantages vary according to the method of distribution used. • Centralised database is useful for statistical analysis (e.g. sales figures) and backup • Decentralising increases complexity but reduces network traffic

6. Advantages • Security • Limit access to different areas of the database • Local autonomy • Each local area is responsible for maintenance of its database • Errors are easier to correct at a local level than a national level • Improved availability • Give access to parts of the database as required • Improved reliability • Faster performance locally • Economics • Smaller computers required (assuming part of the database) • Less transmission cost as traffic is reduced • Modular Growth • easier to handle expansion

7. Disadvantages • Complexity • To maintain indexes, locations, updating, etc is complex • Cost • Require processing and storage at each site • permanent high speed links between sites • Security • Many locations and entry points to the system • Integrity control • Maintenance of data integrity – one record updated in two sites differently

8. Security of Distributed Databases • Security Issues of Distributed Databases • Encryption • Encryption key is spread around, increases possibility of cracking it • Protection against and checking for a virus • Virus in one of the sites can easily spread to the others • 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.

9. 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:- And transmitted as HIEMEES**RR*EP*ETXA**HAP* Using a 5 column and 6 row grid, decode the message ITOIOS*EMNTTWDPG*OA*U*HRNCTNIY 'I wanted Computing not History'