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System Design - 2 (Security and Control (data (data type, visibility,…

- - - - verification
        
        double keying (2x email address)
        
        self-checking codes
        
        repeating back what the user entered
      - validation
        
        existence checks
        
        range checks
        
        cross-field validation
    - - Control totals
        
        right no of outputs produced -> any missing?
      - spot checks
        
        sample of payslips
      - Pre-numbering
        
        pre-numbered invoices etc?
    - - serial number on batch
      - count of batch contents
      - serial numbers of forms in the batch
      - batch total
      - hash totals
  - - - locks and barriers
      - safes
      - security personnel
      - backup & recovery
    - - Access
        
        PIN
      - firewalls
      - Encryption
      - anti-malware software
      - system audit trail
        
        what happened, who did it, on what equipment
        
        record of compliance
        
        recover from failed system -> maintain data integrity
        
        identify unauthorised activity
        
        typically includes
        
        unique sequence number
        
        date of
        
        time of
        
        user ID
        
        machine ID
        
        type of transaction
        
        transaction value(s)
        
        data values before & after
- - - - Interface -> create families without specifying classes
    - - same construction process to create various representations
    - - define interface, sub-classes decide which class to instantiate
    - - delay creation of object/calculation/something expensive until first time it's needed
    - - prototypical instance, create new objects by copying prototype
    - - class has one instance, global point of access
  - - - convert interface into one client expects
    - - de-couple abstraction from its implementation
    - - objects -> tree structure. treat objects and compositions uniformly
    - - attach additional responsibilities to an object, keep same interface
    - - unified interface to set of interfaces
    - - sharing to support large numbers of similar objects efficiently
    - - provide surrogate or placeholder for another object
- - - - Become potential classes or entities
    - - Agree technology used -> external data files / db management system
        
        DB management system
        
        Hierarchical - records stored in hierarchy structure
        
        Permanently store data
        
        Network DB
        
        Relational DB
        
        Object Oriented
        
        Physical files
        
        How to organise
        
        Serial
        
        Order they are created - not logical
        
        Simple to store, more difficult to retrieve
        
        Sequential
        
        Alphabetical/numerical sort
        
        Simple to retrieve, more difficult to store
        
        How to access/retrieve
        
        Index using key
      - Normalisation
        
        Derive flexible, robust, redundancy free structure
        
        Do not contain data that can be derived
        
        Contain one copy of each logical data item
        
        Contain the latest value for each data item
        
        accurately reflect values per original source/consistent with changes
        
        combine items into logical groups
        
        good data integrity
  - - - Uniqueness
      - Stability
      - Expandability
      - Length
    - - 12345678-0
        Sequence-check digit
        
        Algorithm to determine check digit