T1 Week 9: Centralised and Distributed Databases

Centralised: a single database under the control of a single DBMS. All users and client applications connect directly to the DBMS

Distributed: A set of connected databases stored on multiple computers that appears to users as a single database.

Types of Distributed Database Systems:

• Fragmentation: individual data items are physically stored once only at one single location, however parts of teh databse are stored at different locations. During queries data may be transfered from a remote location. It requires fast reliable connection.
• Downloading: server downloads copies of data as it is required from remote databases and stores the data within its local database. With frequently used data, copies are located on the local server. This is suited to data that rarely change. This also required fast data access time.
  e.g. if one data is changed than data that is stored in another location must also be changed
• Replication: uses master and replicant databases. Local databases hold copies of all data all of the time (goal) . Local replicant DB are constantly updating with the master DB. Usually replication takes place at night - does not require fast connection
  e.g. ideally, complete copies occur at each location