Wednesday, April 27, 2011
Centralised & Distributed Databases @ 5:14 PM
~Centralised & Distributed Databases~
There are THREE strategies involved with centralised and distributed databases. These are as follows:
1. Fragmentation- Parts of a database are stored in different location with each item only stored once. During queries data may be transferred from a remote location. This requires a fast and reliable connection.
E.g. Each example only occurs at one location
ABC DEF GHI
2. Downloading- Each server downloads copies as required, hence with frequently used data, copies will be located on the local server. This is suited to data that rarely changes and has a fast access time.
E.g. All location have copies of data D and E
ABCDE DEF GHIDE
3. Replication- Uses master and replicant databases. Local databases hold copies of all data of the time (goal). Local replicant databases are constantly updating with the master database, which usually takes place at night and does not require a fast connection.
E.g. Ideally complete copies occur at each location (the same)
ABCDEFGHI ACDEFGHI ACDEFGHI