1. Managing Multi-User Databases

2. Mutli-User Issues n Concurrency Control n Database Reliability n Database Security n Database Administration

3. Common Multi-User Databases n Windows 2000 – Access 2000 – SQL Server – ORACLE n UNIX – ORACLE – Cybase – Informix

4. Concurrency Control “Making sure that one user’s work does not inappropriately influence another’s” The need for atomic transactions “logical work performed as a unit”

5. Concurrency Processing Problems n Lost update problem n Inconsistent read problem

6. Remedy for inconsistencies caused by concurrent processing n Resource Locking “Disallow sharing by locking data that are retrieved for update”

7. Lock Terminology n Implicit locks placed by the DBMS n Explicit locks placed by command n Lock granularity the size of the lock n Exclusive lock from access of any type n Shared lock from change but not read

8. Deadlock “Deadly embrace”; each transaction waiting for a resource that the other person has locked

9. Database Recovery n Via Reprocessing n Via Rollback/Rollforward

10. Recovery Terminology Log records of the data changes in chronological order Before-images/After-images copy of every record before and after it was changed Checkpoint a point of synchronization between the database and the transaction log