G063 - Distributed Databases “Sharing is good!”

Learning Objectives: By the end of this topic you should be able to: explain how databases may be stored in more than one physical location explain the methods by which this distribution may be carried out explain reasons why distribution would be carried out explain the security issues of distributed databases

A Distributed Database is: a database not stored in its entirety at a single location database spread across a number of computers in multiple locations computers connected by a data communications link LAN and/or WAN e.g. Tesco - stock & sales databases Thomas Cook travel agent – holiday database

3 Types of Distributed Database Replicated Centralised Partitioned

Replicated database: = a copy of the complete database at each site

Replicated Database a copy of the complete database at each site exact copy of the database stored & accessed locally transaction files created of changes at each centre updates made on a master database a ‘new’, updated copy of database sent to each centre

Replicated Database Advantages: reliability fast response to searches data is always available locally not reliant on the network or central server fast response to searches local access will be faster than WAN access

Replicated Database Disadvantages: data integrity issues local copies of data may be different to each other need additional local storage space requirements additional time required for update operations

Centralised database:

Centralised Database single database held centrally (possibly at Head Office) an index to the central database is held locally speeds up queries each site accesses database through a WAN Example: Booking systems need distributed access to a central database sharing of up-to-date information important, avoids double bookings.

Centralised Database Advantages: better security of data one copy rather than several (replicated copies) good data integrity one copy rather than several always sharing the same data data always up-to-date data is updated in real time centralised backup can be automated

Centralised Database Drawbacks: a virus in the central system could spread throughout all sites possibility of update clashes two sites trying to modify the same record at the same time

Partitioned database:

Partitioned Database not every site needs to have all the data → give each site just the data that is relevant to that site database is split into sections (partitions) each site on the network stores its own data i.e. the part of the database that relates to that site, e.g. the part of the database that relates to a single supermarket’s stock is stored at that site, other (global) data is held centrally changes to central data can be dealt with overnight by a batch update from the sites,

Partitioned Database Advantages: speed: local control over local data faster access to local data local control over local data scalability can add new sites as required not reliant on network or server for day-to-day tasks

Partitioned Database Drawbacks: data inconsistency possibility of different data being held centrally to that on partition unsuitable for certain applications if data changes at one node must be instantly seen by all nodes e.g. holiday bookings high network usage during update process will slow down other network processes

Partitioned Database Two types: Horizontal partitioning Vertical partitioning

Horizontal partitioning Example: branch offices deal mostly with a set of local customers e.g. Northampton branch stores the fragment where contents of the Branch field = ‘Northampton' So: split the table into number of smaller tables on the basis of rows (records) i.e. specific field contents each site (branch) stores just the table relevant to them

Horizontal partitioning this table represents the database for an estate agency with 3 branches

Horizontal partitioning the database is horizontally partitioned so that the data for each branch is stored on the server in that branch: this will speed up local queries Boldmere staff searching for properties in Boldmere

Horizontal partitioning this means that the data is stored like this:

Horizontal partitioning this means that the data is stored like this:

Vertical partitioning data is separated across sites based upon fields dividing the table based on the different columns different columns of a table located at different sites e.g. stock descriptions: item descriptions & prices at sales outlet item’s country of origin, supplier name at Head Office e.g. employee wages: staff name & hours worked at individual shop personal data stored at Head Office

Vertical partitioning Advantages: faster searching reduced amount of data being sent between sites access rights ensures that only certain people see certain fields help to conform to the DPA personal information kept separate from sales records

Vertical partitioning Disadvantages: potential exists for inconsistency in the data stored complex & time consuming to set up and modify regular backups essential since there is no data replication