G063 - Distributed Databases

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

Database storage:

A Distributed Database is: a single logical database –consisting of many entities –possibly used by many users for different purposes a database is not stored in its entirety at a single physical location database is spread physically across a number of computers –computers could be in multiple locations  buildings or sites, –computers connected by a data communications link  LAN and/or WAN

Why distribute a database: allows faster local queries –faster searching speeds up other network operations –due to some data queries being handled locally  reduces network traffic improved reliability –data may be replicated at multiple sites allows for modular growth of the database –can easily add new sites and/or uses user does not need to know where data is stored physically –looks like a single, location, centralized system to the user

Types of Distributed Database Replicated Centralised Partitioned

Replicated Database complete database is duplicated at each centre exact copy of the database stored & accessed locally duplicated versions are usually read only –transaction files created of changes at each centre updates allowed made on a master database –a ‘new’, updated copy of database sent to each centre  at regular intervals

Replicated Database Advantages: reliability –data is always available locally –not reliant on the network or central server –work carries on even if some nodes are down fast response to searches –local access will be faster than WAN access  data does not have to be transmitted over the network reduced network traffic at prime time –faster access to network if required

Replicated Database Disadvantages additional local requirements for storage space additional time for update operations complexity and cost of updating data integrity issues –if replicated data is not updated simultaneously –local copies of data may be different

Centralised Database single database held centrally (possibly at Head Office) each node accesses database through a network (WAN) –access available to all branches or offices, an index to the central database is held locally at each node –speeds up queries/transactions booking systems need distributed access to a central database if they are to work effectively –sharing of up-to-date information important, –avoids double bookings.

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

Centralised Database Advantages (from June 2011 Q13 mark scheme): storage is only required at the central location for the centralised database (1) the local indexes stored at each site take up far less memory (1) queries are processed locally(1) this speeds up searches as only the required data is retrieved from the central location (1) less data traffic than complete centralisation (1) as only data is sent and not the additional information /forms/reports structure (1) increased security (1) only central database needs increased security as that is where the data is stored (1) integrity of data not compromised (1) as it is stored in only one location and one database to update (1) centralised back-up of data (1) management backup easier as it is just one person’s responsibility (1)

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 database is split into sections each node or site on the network stores local data –i.e the section of the database that relates to that site,  e.g. the section 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,

Horizontal partitioning involves putting different rows into different tables. splitting the table into number of smaller tables –on the basis of rows (records)  i.e. specific field contents Example: branch offices in an organization deal mostly with a set of local customers –Euston Road branch stores the fragment where contents of the Branch field = 'Euston Road'

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 dividing the table based on the different columns. involves creating tables with fewer columns –using additional tables to store the remaining columns. different columns of a table located at different sites –e.g. stock descriptions (country of origin, supplier name at one site and prices at another site)

Vertical partitioning From June 2011 Q13 mark scheme: only certain people see certain fields –e.g. financial matters not revealed to all (1) to conform to the law/DPA (1) –keeping personal information private (1) reduces amount of data being sent between locations (1) –in order to speed up data transfer (1) –allowing faster reaction time (1) –meaning rescue reaches emergency quicker (1)

Partitioned Database Advantages: speed: –faster access to local data  less network access required local control over local data scalability –can add new sites as required not reliant on network or server for day-to-day tasks each partition can have its own transaction log –local reporting (access/sales)

Partitioned Database Drawbacks: data inconsistency –possibility of different data being held centrally to that on partition –regular batch update required to maintain consistency 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