1.3 Exchanging Data

• Define the term Hash

  a result generated by applying an algorithm to a value
• Advantages of Hashing

  hash functions cannot be reversed
• Symmetric Encryption


  Single key is used to encrypt and decrypt a message and both parties need to know the key
• Disadvantage of symmetric encryption

  messages can be cracked if:

  • key is intercepted
  • key is duplicated
• Asymmetric Encryption

  two different keys one is used as a public key while the other a private key
• What is the purpose of transaction processing?


  to ensure data integrity

  all data management systems must conform to a set of rules
• Atomicity


  a change to a database is either completely performed or not at all. a half-completed change must not e saved back to the database
• Consistency


  any change in the database must retain the overall state of the database
• Isolation


  a transaction must not be interrupted by another transaction
• Why is isolation important?

  so other users or processes cannot access data concerned
• Durability


  once a change has been made to a database it must not be lost due to system failure
• dictionary based encoding

  Lossless data compression

  Searches for matches between the text to be compressed and a set of strings contained in a dictionary
• hashing


  The process of calculating a numeric value from a data item, usually an index from a unique key
• Lossless


  Partial reduction in file size but none of the data is lost

  The original file can be recreated from the compressed data
• Lossy


  Greatly reduces file size but some data is lost
• What is lossy used in?

  images, audio and video where a certain level of quality loss is acceptable in exchange for smaller file sizes
• Examples of uses of lossless


  ZIP, PNG, FLAC for audio
• What type of compression does run length encoding use?

  lossless
• Explain why asymmetric encryption is more suited to transactions over the internet than symmetric encryption

  One key encrypts the data which can be publicly distributed and a different key to decrypt it which is kept private whereas in symmetric encryption one key is shared to both parties, this can get intercepted
• Dictionary based encoding (how it works)

  • Frequently occurring pieces of data or sets of characters are replaced by symbols/tokens
  • Which are then matched to a dictionary against it's groups of characters.
  • This makes it so that when decompressing the dictionary can be used to replace the symbols/tokens with the original text
• What type of compression is dictionary based encoding?

  lossless
• Uses of hashing


  Hashing for security:

  hash passwords in database to make sure they cannot be read if they are stolen

  Hashing for direct access:

  Records can be quickly accessed by using hash of index as address
• Lossy

  • permanently removes data
  • not able to recreate the original file
  • reduces quality of videos
  • file size is smaller than if lossless were used
  • compression ratio may be adjusted depending on bandwidth
• Lossless

  • rewrites original data in more efficient format
  • able to recreate the original file
  • keeps original quality
• Disadvantage of record locking

  Can cause delays or use deadlock
• Explain how record locking can be used to ensure that the ACID principle of isolation is achieved when carrying out multiple transactions

  allows one user/process to access and modify record level data at any one time

  So data that is being used elsewhere cannot be modified
• Two methods of exchanging data

  • SQL
  • JSON
  • XML
  • APIs
• Where is javascript executed?

  client-side