CORE 2: Information systems and Databases CENTRALISED AND DISTRIBUTED DATABASES

It is a rare occurrence, within a large organisation, that ALL data is stored within a single centralised location. The reality is there a typically multiple databases in multiple locations on multiple computers. That said, these databases are linked so that to the user it appears to be one single large database. We call this a DISTRIBUTED database and the controlling software is call a Distributed Database Management System or DDBMS. CENTRALISED AND DISTRIBUTED DATABASES

CENTRALISED DATABASE  A centralised database is a single database under the control of a single DBMS. All users and client applications connect directly to the DBMS. CENTRALISED AND DISTRIBUTED DATABASES

DISTRIBUTED DATABASE  A distributed database is a set of connected databases stored on multiple computers that appears to users as a single database. CENTRALISED AND DISTRIBUTED DATABASES

Fragmentation :  Fragmentation is when different parts of the database are stored at different locations. When using this system individual data items are physically stored once only at one single location. To execute a query require all to be physically retrieved from a remote server. It requires a fast and secure connection between all DDMBS servers. Horizontal fragmentation stores different records of the same table at different locations. Vertical fragmentation stores different attributes at different locations. TYPES OF DISTRIBUTED DATABASES

HORIZONTAL FRAGMENTATION

VERTICAL FRAGMENTATION

Downloading  Downloading is when each server downloads copies of data as it is required from remote databases and stores the data within its local database. This suits types of data that rarely changes. For example, DNS (Domain Name Service) systems for the internet. When you type in a URL, you request a domain name. This domain name is searched for through multiple DNS servers until a match is found and the IP is accessed for the computer to open the site. Once it is found, the IP is then stored in your local database. This is why sites you use often load faster than ones you don't. TYPES OF DISTRIBUTED DATABASES

Replication  This involves a master server and various replicant servers. The replicant servers hold copies of most of the data from the master server. Replicants are synchronised with the master server at regular intervals to avoid data inaccuracies. The master server is kept as the most secure server. In this type of database the aim is for local servers to hold copies of all data all of the time, making changes and updating records according to the associated master server. TYPES OF DISTRIBUTED DATABASES