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Functions of Database Management Systems. Data storage retrieval and update facilities A user-accessible catalogue or data dictionary Support for shared update Backup and recovery services Security services Integrity services Connectivity Utilities. Support for Logical Transactions.

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functions of database management systems
Functions of Database Management Systems
  • Data storage retrieval and update facilities
  • A user-accessible catalogue or data dictionary
  • Support for shared update
  • Backup and recovery services
  • Security services
  • Integrity services
  • Connectivity
  • Utilities
support for logical transactions
Support for Logical Transactions
  • logical transaction = many separate physical transactions (reading, updating, writing records)
  • if transaction are interrupted before entire completion "up to date" data is sacrificed for consistent data.
  • If not, transaction is committed - ie written to disk
  • DBMS provides mechanisms that either Commit or Rollback transactions
shared update
SHARED UPDATE
  • i.e. Two or more users making updates to database at the same time
    • Single vs. Multiuser Environment (eg: Networked DBMS)
  • Problem: double update
    • CUSTOMER BALANCE: 418
    • Pat (recording sale: +100) and Jo (recording payment -100):
    • CORRECT: Pat reads, updates and writes (commits: 518). Jo reads (518), updates and writes (commits: 418).
    • VALUE: 418.
    • INCORRECT: Pat reads and updates. Jo reads and updates. Pat writes (commit: 518). Jo writes (commit: 318).
    • VALUE: 318.
shared update solutions
SHARED UPDATE - SOLUTIONS
  • 1. AVOIDANCE:
    • Prohibit shared update,
    • Allow access for retrieval only,
    • Record updates in transaction file and update database periodically using a batch program.
  • Problem: Data is temporarily out of date
  • customer may not be allowed credit because his balance had not been credited with last payment.
shared update solutions5
SHARED UPDATE - SOLUTIONS
  • 2. LOCKING
    • Lock table/record/field from access by other users.
  • TYPES OF LOCK
    • Exclusive Lock
    • Read Only Lock
    • Lock Time-Out
  • Other variables
    • Lock Granularity
    • Deadlock
slide6
Exclusive Lock: Other users can neither read nor update locked table/record/row. Extreme and inflexible.
  • Read Only Lock: Other users can read but not update the locked table/record.
  • Lock Time-Out: If a record is locked, a user could have a long wait for its release. Some DBMS\'s detect lengthy locks and unlock them, undoing any updates made to any records during the transaction.
  • Lock Granularity: Refers to the level of the lock: field, record, page/block, table.
  • Deadlock: Users can have a lock on more than one record at a time. This poses problems when two users require each others locked records.
recovery
RECOVERY

1. Backups or Saves (normal backup of DB files)

2. Journaling / Audit trail / Audit file

  • Keep a log or journal of the activity which updates the database
  • recovery involves: Copying the backup over database and running a special program to update the backup version of the database with the transaction in the log.
security
SECURITY
  • Restriction of access to authorised users only.

1. Passwords

2. Encryption

3. Views

4. Authorisation Levels

      • read only
      • edit
      • delete
      • create
data integrity
Data Integrity
  • DBMS provides a mechanism to enforce specific rules.
    • Examples:

*Customer numbers must be numeric,

  • But programmers must also develop their own

* Credit Limits must be £300, £500 or £1000 only,

* The sales rep for a given customer must exist,

* No customer may be deleted if he/she currently has an order on file.

data independence
Data Independence
  • DBMS must support the isolation of data structure from the programs
  • Users or application programs not affected by changes to the database structure.
  • Logical and Physical Data Independence Usually achieved through Subschema or View type mechanisms.
database schema
Database Schema
  • description of the overall logical structure of a database, expressed / programmed in Data Definition Language (DDL)
  • broken down into sub-schemas: logical description of a user’s view or program’s view of the data used
  • DDL can be very sophisticated on a mainframe or trivial on a PC (queries / views)
connectivity
Connectivity
  • organisations are rarely single site / single entity
  • flows of data transcend the boundaries of organisations - so do information systems
  • data communication must be implemented
  • databases can be used to support the distribution of information resources
database utilities
Database Utilities
  • Compact datafiles
  • Index / re-index data files
  • Repair database (crash)
  • Import/export data from and to other sources
  • Enforce standards (eg: integrity of relationships, NF...)
  • Associated data dictionary
  • Access to remote computers (login, emulation)
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