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

• 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• i.e. Two or more users making updates to database at the

same time– Single vs. Multi-user 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

• 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• Problem: Data is temporarily out of date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.

• 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

SHARED UPDATE - SOLUTIONS

– 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

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• Restriction of access to authorised users only.

1. Passwords

2. Encryption

3. Views

4. Authorisation Levels• read only• edit• delete• create

Data Integrity

• DBMS provides a mechanism to enforce specific rules. – E.g.

*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

• 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

• 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

• 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

• Compact data files• 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)