Purposes of Replication

All user processes are local. Any data from other locations is replicated into the local database. There is no DDMS or global schemas.

• Minimizes use of connections
• Reduces risk of site or connection failures.

Possibilities exist for disagreement between copies but there are advantages in that, availabilty is higher and access is faster.

'Snapshots' of the data at specific times can be stored. This can be used to provide data history for auditing purposes.

Although there may be no DDBMS there is a replication server. The replication server is connected to the local DBMS and other replication servers on the network. Primary functions are:

• to collect and receive updates from the local database for transmission to other replication servers
• to collect and receive from remote sites to pass on to the local database

Update methods:

• DBA for SysA declares he will update SysB. All updates are pushed over to SysB.
• DBA for SysB requires updates from SysA. All updates are pulled over from SysA.

Users can continue to use their own local database and the replication server will hold the descriptions of replicated data.

Consolidation

Data fragments are collected from multiple locations in to one location in order to construct a global view. An example is an administration department from a hospital consolidating tables for the patients on all wards into one central location. A central location can be kept up to date by a DBA pushing over changes as they happen. If a central location has to pull any changes over from outlying tables, the central location may not be up to date until the pull transaction has been completed.

Dissemination

Dissemination involves a system where one database copies fragmenst of data that overlaps out to other locations. An example being where one student may be doing two courses. Data about the student is held by one central location. This data may be accessed by two tutors. Multiple copies of subsets of the central copy are passed to the tutors. The tutors can make use of stored procedures to update the central location via the replication servers. In this way the tutors are pushing their changes through the system.

Other remote users can access the central location testing for changes to their copies. In doing so they are pulling the data required from the central location.

Comments, suggestions, ideas to
Stuart Banner