CORE 2: Information systems and Databases

STORAGE AND RETRIEVAL

Storage and Retrieval of data occurs within all information systems, however it is particularly critical regarding large data stores.

The performance of databases is centred on the effi ciency of storage and retrieval systems. Storing and Retrieval is a two part process…

Storing saves data or information Retrieving reloads data or information

STORAGE AND RETRIEVAL

A database management system is used to provide secure and effi cient methods for storage and retrieval of information from a database.

DATABASE MANAGEMENT SYSTEM  (DBMS)- is a software package that al lows users to access a database so they can enter, maintain and view the data. In a DBMS data is organised into tables, viewed in forms, retrieved using queries and displayed in reports. DBMSs manipulate the data in ways such as searching and sorting. They also perform a number of tasks to help users develop and maintain a database:

organising the data using a data dictionary showing relationships between entities using schematic diagrams checking for identifiable errors in data entry allowing flexibility to change the definition of the database restricting access to the data to authorised people providing information about the performance of a database

STORAGE AND RETRIEVAL

During this section of the course we will explore…

1. Types of Storage Hardware 2. Techniques for Data Security3. Searching and Sorting

STORAGE AND RETRIEVAL

ACCESSING DATA:

Before we look at types of hardware we need to remind ourselves of the diff erent way data is accessed.

SEQUENTIAL ACCESS  occurs when data is accessed in a sequence. Data is accessed in the order it was stored. It does not require the exact location of the data item. Sequential is much slower than direct and impractical when immediate processing is required. It is the only method of accessing data stored on magnetic tape. (THINK CASSETTE TAPE)

DIRECT ACCESS  occurs when data is accessed without accessing previous data items. Data is stored in a particular storage location based on a mathematical procedure or algorithm. Direct access uses this algorithm to calculate the approximate location of the data. I f the data is not found here the computer searches through successive locations unti l i t is found. Direct data often requires the use of an index and this is cal led indexed access. (THINK COMPACT DISK)

ACCESSING DATA

Offl ine refers to storage that cannot be accessed unless it is mounted on a drive. E.g. flash drive, CD’s, DVDs.

Online data is available immediately to connected computers. E.g. A hard drive or H: drive.

Cloud computing refers to storage of data through an external storage medium. For example, the gaming company Steam off ers cloud saving of game data so that if your personal hard-drive crashes your data will be safe in another location.

ONLINE / OFFLINE STORAGE

Magnetic storage is currently the most popular method for maintaining large quantities of data. It provides large storage capacity and can allow for direct access at high speeds for both storage and retrieval. Neither cloud nor optical storage can compete in terms of times required for storage and retrieval. We will look at the following examples…

Hard Disks RAID (Redundant Array of Independent Disks) Cartridge and Tape Tape Libraries

MAGNETIC STORAGE

MAGNETIC STORAGE

Example How it works

Hard drive

Store data magnetically on aluminium or glass. Data is arranged into tracks and sectors. Data is analysed through changes in magnetic fields representing 1 and O. Read/write heads use electromagnets to write data into the disk.

RAID Stands for Redundant Array of Independent Disks. Utilises multiple hard disk drives together with a RAID controller. The computer sees the RAID drive as a single drive, when really the drive is a controller which mirrors or stripes the data onto multiple hard-drive-disks.

• Mirroring involves backing up the same data onto every hard-drive.

• Striping involves saving fragments of data across each hard-drive, allowing for faster access.

Cartridge and Tape

First appeared in 1950. Magnetic tape consists within small cassette cartridges. Can store up to 500gb at only a couple of cents per gigabyte.

Tape libraries

Tape libraries are robotic controllers that automatically cycle through libraries of tape that can back-up information.

Optical Storage processes are based on the reflection of light. The diff erence between a good and a poor reflection is used to represent binary and as such – data. Due to the tightly packed data on optical storage lasers are used for their consistency and high focus. We will look at…

Compact Disc (CD) Digital Versatile Disc (DVD) & Bluray

OPTICAL STORAGE

OPTICAL STORAGE

Example

How it works

CD Optical storage works based on the reflection of light, either the light reflects poorly or effectively. These changes in reflection refer to changes between 1’s and 0’s. CDs contain a single spiral track that commences at the inner portion of the disk and spirals outward towards the edge of the disk. A single track can store up to 680MB of data.

DVD DVD’s contain similar but more densely packed tracks, and each track can store up to 4.7GB of data. DVD’s can also be double sided, and also dual layered. Therefore, a disc like this could contain 4 tracks, with a total of 17GB of storage.

Blu-ray

Blu-ray discs are similar to CD’s and DVD’s but they are able to store 25GB of data per layers. With dual layering, this makes 50GB of storage possible. The name comes from the blue laser which reads and writes the disk. It is much more accurate than on DVDs or CDs, and therefore can compress tracks and sectors more tightly, allowing for more data to be stored on one disc.

Securing data is about achieving two things. Firstly prevent data from being lost or corrupted and secondly it aims to prevent unauthorized access.

2) SECURING DATA

  Protects against

Techniques Data loss Unauthorised

Backup and recovery Yes No

Physical security measures Yes Yes

Usernames and passwords Yes Yes

Encryption and decryption No Yes

Restricting access to databases No Yes

Record locks in databases Yes No

RAID (mirroring only) Yes No

BACKUPS & RECOVERY

Backup refers to copying fi les to a separate secondary storage device as a precaution in case the fi rst device fails or data is lost. There are two types of backup that are commonly used:

Full backups – Includes a backup of all fi les. Synchronises everything, every time that a backup is made.

Partial backups – Only backs up fi les that have been created or edited since the last backup. These are identifi ed using archive bits which are set to true once a fi le has been altered. Partial backups could be Incremental or Diff erential.

Frequency of backups depends on how critical the data is to the organisation and how often data changes.

2) SECURING DATA

PHYSICAL SECURITY MEASURES These measures are used for reducing data loss and

unauthorised access. For example: server rooms should be locked so as to keep out curious or malicious personnel. These rooms should also be air conditioned to prevent overheating and climate control to reduce moisture. Government military computer facilities would be housed within a solid bomb proof concrete bunker.

2) SECURING DATA

USERNAMES and PASSWORDS

User names and passwords secure individual fi les, directories or storage devices. User names and passwords are used by operating systems and network software. User names identify admin levels. Passwords gain access to these levels.

2) SECURING DATA

ENCRYPTION and DECRYPTION

Encryption is the process of making data unreadable by those who do not possess the decryption key. Decryption is the process of decoding encrypted data using a key. The opposite of encryption.

Single key encryption is used within companies etc where all computers are linked. Every computer has a code that is the same code to encrypt and decrypt.

A public key system uses public and private keys or Two key systems . It works in that every user has a public and private key. When someone wants to send them an encrypted code, they tell them their public key. The person encrypts their message with this public key and sends the message. However the message can only be decrypted by the initial user with the private key. This means that it does not matter who knows your public key, so long as your private key is kept safe.

2) SECURING DATA

RESTRICTING ACCESS by DBMS USERS VIEWS

Users Views are essential ly what users see when they access a database. The defi nition for a User view is: The restricted portion of a database made available to a user.

For example: Online games may have scoreboards which show the names and servers of leading players. The databases that hold the names of these players are probably run by the company that owns the games, and therefore knows the players passwords. However, when a player views the leader boards, the only view that they get of the player with top score is their name, when realistically, the database probably knows a lot more. A user view does not change the organisation of layout of a database, it only restricts users from viewing complete attributes etc that are beyond their authorisation.

2) SECURING DATA

RECORD LOCKS in DBMS

This refers to when a databases locks a record so that it cannot be edited by two users at once. The two strategies are pessimistic and optimistic locking. Pessimistic involves the DBMS allowing the fi rst to begin editing to have solo access to the record, while optimistic allows the user to decide to overwrite the edited record with their editing, or discard their editing.

2) SECURING DATA

2) SECURING DATA

  Protects againstTechniques Data loss Unauthorised

Backup and recovery Yes No

Physical security measures Yes Yes

Usernames and passwords Yes Yes

Encryption and decryption No Yes

Restricting access to databases No Yes

Record locks in databases Yes No

RAID (mirroring only) Yes No