Distributed Databases
Initially distributed systems still maintained a single central database. However the use of distributed data soon developed since otherwise a large amount of data needed to be moved through the network and a mainframe or telecommunications failure isolated the local processor from the data that it needed.
In a distributed database system, elements of the database are distributed throughout the network and are stored at the location where they are needed. Various approaches are used to implement this.
1. Partition. The database is partitioned with each node on the network containing that section of the database that relates to it. For example the section of the database that relates to customers served at that node. Other data is held centrally and any changes to central data can be dealt with overnight by a batch update.
An extreme case of this approach is when the entire database is replicated to local processors. Here again the central copy will be updated by batch processing, probably overnight.
Both of these approaches lead to data inconsistency and will therefore be unsuitable for applications such as holiday bookings where data changes at one node need to be available to other nodes. They will however work well in situations where local data processing is compartmentalised and has no immediate effect on other nodes. An example of this is supermarket stock control.
2. Another approach is to hold only one working copy of the data at the local node with each node storing the data that is most closely associated with it. The database is in fact distributed throughout the network. If a node needs access to records that are not held locally then this is obtained through the network - possibly by initially accessing a central index to find the location of the data. Software handles access to the database so that the fact that it is spread over a number of sites is not apparent. Another approach is to hold only one w
This approach requires more constant and heavier use of the network but it eliminates the problems of data redundancy and it removes the need for overnight reconciliation.
Advantages of distributed databases include:
- Faster response to local queries
- Reduction in amount of network traffic
- Effect of breakdowns is minimised
- Better local control over the system
- Less powerful cheaper processors needed
There are however some disadvantages in the use distributed database
- Increased tendency to data redundancy and data inconsistency.
- System is dependent on high quality telecommunication lines, which may be vulnerable.
- Need to maintain and enforce consistent standards and data definitions over a wide area.
- Increased security problems - need to enforce security procedures over wider area plus increased problems over data transmission.
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