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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.

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Functions of Database Management Systems

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  1. 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

  2. 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

  3. 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.

  4. 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 • Problem: Data is temporarily out of date • customer may not be allowed credit because his balance had not been credited with last payment.

  5. 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

  6. 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.

  7. 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.

  8. SECURITY • Restriction of access to authorised users only. 1. Passwords 2. Encryption 3. Views 4. Authorisation Levels • read only • edit • delete • create

  9. 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.

  10. 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.

  11. 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)

  12. 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

  13. 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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