Functions of database management systems
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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

  • 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


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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


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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.


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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  • Problem: Data is temporarily out of date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.


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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


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  • 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.


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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.


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SECURITY

  • Restriction of access to authorised users only.

    1. Passwords

    2. Encryption

    3. Views

    4. Authorisation Levels

    • read only

    • edit

    • delete

    • create


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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.


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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.


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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)


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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


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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)