9 Databases

9.4 Security And Integrity Issues

9.4.1 Multi-Access Databases

If the database is installed on a shared network drive then more than one person can work with the data.

9.4.2 Database Integrity

The aim is to ensure that no data is accidentally lost or corrupted.

• Locking
  • Operation in exclusive mode.
  • Lock other users from tables been modified.
  • Lock out the record been edited.
  • No locks. Users are informed of conflicts.
  • Ordinary users can only open tables in read-only mode.
• Refreshing
• Resolving a deadly embrace

Deadly Embrace

If two users attempt to update two records at the same time then the following can happen:

User 1	User 2
Accesses and locks record 1	Accesses and locks record 2
Attempts to access record 2	Attempts to access record 1
waits...	waits...
and waits...	and waits...
and waits...	and waits...

The DBMS must resolve this conflict.

9.4.3 Database Performance

When database queires are made it is important to give users a fast response time. Furthermore if users are locked out of records they will become frustrated and disgruntled.

Improving Database Performance

• Use the least restrictive locking viable.
• Educate the users to: schedule disk intensive operations and update records promptly.
• Index attributes that are frequently used in searches and queries.
• Install the database software onto local machines.
• Re-schedule other network jobs.
• Upgrade the hardware.

9.4.4 Security Issues

Database security can be promoted by:

• Regular backups
• Transaction logging
• Checkpoints
  • All updates are stored in transaction log.
  • When the database is permantly updated a checkpoint is placed in the transaction log and the old version of the database is kept as a backup.
  • If failure occurs then all the updates up to the last check point can be applied to the old database.
• Passwords
• The DBMS can assign priviledges to users or groups of users.
• Encrypting the data.

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