Unit - 4, Exchanging data

Lossy Compression

Non-essential data is permanently removed, for example, different shades of the same colour in an image or frequencies of sound outside the range of human hearing

Lossless compression

Patterns in the data are spotted and summarised in a shorter format without permanently removing any information.

Run length encoding

A basic method of compression that sumarises consecutive patterns of the same data.

Dictionary compression

Spots regular occurring data and stores it separately to be referenced

Encryption

A way of making sure data cannot be understood if you are not meant to see it.

Caesar cipher

A basic for of encryption that shifts letters of the alphabet by a set amount.

Brute force attack

Try every possible combination till you find the correct one.

Vernam cipher

Uses a one-time pad to encrypt, then destroy it. The one form of cipher proven to be unbreakable

Symmetric encryption

The same key is used to encrypt and decrypt the data/message

Asymmetric encryption

This uses two different keys, one to encrypt and one to decrypt.

Hashing

One way encryption that using a formula, commonly used to store passwords and pins.

ACID

Atomicity, Consistency, Isolation, Durability, This is a set of properties to ensure that the integrity of the database

Atomicity

This property requires that a transaction is processed in its entirety or not at all

Consistency

This property ensures that no transaction can violate any of the defined validation rules.

Isolation

This property ensures that concurrent execution of transactions leads to the same result as if transactions were processed one after the other

Durability

This ensures that once a transaction has been committed, it will remain so, even in the even of a power cut.

Multi -user databases

Allows multiple users to simultaneously access a database

Record Locking

This prevents simultaneous access to objects in a database in order to prevent updates being lost or inconsistencies in the data arising

Deadlock

If two users are attempting to update each others records so record locking prevents them from proceeding

Serialisation

This ensures that transactions do not overlap in time, so cannot interfere with each other or lead to loss of updates. e.g. Timestamp ordering; Commitment ordering

Timestamp ordering

Each object in the database will have a read and write marker. If user tries to save an update with the read markers differing they will be denied.

Redundancy

The duplication of hardware, and servers in different geographical locations to protect against a possible failure

Normalisation

A process used to come up with the best possible database design.

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First normal form

• No repeating attributes

• All attributes are atomic

Second normal form

• Is in first normal form

• contains no partial dependencies

Third normal form

All attributes are dependent on the key, the whole key and nothing but the key

• Is in second normal form

• contains no non-key dependencies

Link tables

A table that links a primary key in one table to many primary keys in another table.

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A table that makes a many to many relationship possible.

Entities

A table in a database. Each one needs an identifier that will uniquely identify a specific record that they hold.

Natural Primary keys

Unique identifiers that occur naturally in the data. e.g. phone numbers, national security number and email

Composite primary key

When two or more attributes are used to uniquely identify a record.

Secondary key

A field that isn’t the primary key that is often search and therefore will be indexed for faster look up.

Entity relationship diagrams

A way of representing how tables are connected in a database