1. © 2002 by Prentice Hall 1 SI 654 Database Application Design Winter 2004 Dragomir R. Radev

2. © 2002 by Prentice Hall 2 David M. Kroenke Database Processing Eighth Edition Chapter 11 Managing Multi-User Databases

3. © 2002 by Prentice Hall 3 Multi-User Databases Serving the needs of multiple users and multiple applications adds complexity in… –design, –development, and –migration (future updates)

4. © 2002 by Prentice Hall 4 Multi-User Database Issues include… Interdependency –Changes required by one user may impact others Concurrency –People or applications may try to update the same information at the same time

5. © 2002 by Prentice Hall 5 Multi-User Database Issues include… (continued) Record Retention –When information should be discarded Backup/Recovery –How to protect yourself from losing critical information

6. © 2002 by Prentice Hall 6 Common Multi-User DBMS Windows 2000 –Access 2000 –SQL Server –ORACLE UNIX –ORACLE –Sybase –Informix

7. © 2002 by Prentice Hall 7 Role of the Database Administrator Organizations typically hire a database administrator (DBA) to handle the issues and complexities associated with multi-user databases. A DBA facilitates the development and use of one or more databases.

8. © 2002 by Prentice Hall 8 Data Administrator versus Database Administrator Data Administrator –Handle the database functions and responsibilities for the entire organization. –Data administrator responsibilities are discussed in Chapter 17. Database Administrator (DBA) –Handle the functions associated with a specific database, including those applications served by the database. –This chapter describes the responsibilities of the DBA.

9. © 2002 by Prentice Hall 9 The Characteristics of a DBA Technical –The DBA is responsible for the performance and maintenance of one or more databases. Diplomatic –The DBA must coordinate the efforts, requirements, and sometimes conflicting goals of various user groups to develop community-wide solutions.

10. © 2002 by Prentice Hall 10 Technical Skills of the DBA Managing the database structure Controlling concurrent processing Managing processing rights and responsibilities Developing database security Providing database recovery Managing the database management system (DBMS) Maintaining the data repository

11. © 2002 by Prentice Hall 11 Managing the Database Structure Managing the database structure includes configuration control and documentation regarding: –The allocation of space –Table creation –Indices creation –Storage procedures –Trigger creation

12. © 2002 by Prentice Hall 12 Configuration Control The database configuration must reflect changes in organizational and user requirements Structural changes to the database often effect most, if not all, applications and users Sometimes configuration changes have unanticipated consequences Consequently, broad perspectives, careful analysis, and effective communication are essential. As well, the DBA must be prepared to debug and repair unforeseen issues.

13. © 2002 by Prentice Hall 13 The Need for Documentation When altering a databases structure, unanticipated issues are inevitable In recording the specific changes, dates, and times, it is easier to determine the root cause of issues and to resolve the issues When historical data is restored, it must be reformatted with all the changes in the database structure since the data was originally saved.

14. © 2002 by Prentice Hall 14 Documentation All structural changes must be carefully documented with the following: –Reason for change –Who made the changes –Specifically what was changed –How and when the changes were implemented –How were the changes tested and what were the results

15. © 2002 by Prentice Hall 15 Documentation Aids Version Control and Computer Assisted Software Engineering (CASE) tools automate and/or manage many tedious documentation tasks. Printing the data dictionaries after structural changes also helps eliminate many tedious documentation tasks

16. © 2002 by Prentice Hall 16 Controlling Concurrency Processing Concurrency control ensures that one user’s actions do not adversely impact another user’s actions At the core of concurrency is accessibility. In one extreme, data becomes inaccessible once a user touches the data. This ensures that data that is being considered for update is not shown. In the other extreme, data is always readable. The data is even readable when it is locked for update.

17. © 2002 by Prentice Hall 17 Aspects of Concurrency Control Rollback/Commit: Ensuring all actions are successful before posting to the database Multitasking: Simultaneously serving multiple users Lost Updates: When one user’s action overwrites another user’s request

18. © 2002 by Prentice Hall 18 Rollback/Commit A database operation typically involves several transactions. These transactions are atomic and are sometimes called logical units of work (LUW). Before an operation is committed to the database, all LUWs must be successfully completed. If one or more LUW is unsuccessful, a rollback is performed and no changes are saved to the database.

19. © 2002 by Prentice Hall 19 Lost Update Problem If two or more users are attempting to update the same piece of data at the same time, it is possible that one update may overwrite another update. Resource locking scenarios are designed to address this problem

20. © 2002 by Prentice Hall 20 Resource Locking A resource lock prevents a user from reading and/or writing to a piece of data The size of the piece of data (e.g., database, table, field) is termed the lock granularity

21. © 2002 by Prentice Hall 21 Types of Resource Locks Implicit versus Explicit –Implicit locks are issued automatically by the DBMS based on an activity –Explicit locks are issued by users requesting exclusive rights to the data Exclusive versus Shared –An exclusive lock lock prevents others from reading or updating the data –A shared lock allows others to read, but not update the data

22. © 2002 by Prentice Hall 22 Two-Phased Resource Locking Two-phased locking, whereby locks are obtained as they are needed –A growing phase, whereby the transaction continues to request additional locks –A shrinking phase, whereby the transaction begins to release the locks

23. © 2002 by Prentice Hall 23 Deadlocks As a transaction begins to lock resources, it may have to wait for a particular resource to be released by another transaction. On occasions, two transactions maybe indefinitely waiting on one another to release resources. This condition is known as a deadlock or a deadly embrace.

24. © 2002 by Prentice Hall 24 Avoiding Deadlocks Strategy 1: –Wait until all resources are available, then lock them all before beginning Strategy 2: –Establish and use clear locking orders/sequences Strategy 3: –Once detected, the DBMS will rollback one transaction

25. © 2002 by Prentice Hall 25 Resource Locking Strategies Optimistic Locking –Read data –Process transaction –Issue update –Look for conflict –If conflict occurred, rollback and repeat or else commit Pessimistic Locking –Lock required resources –Read data –Process transaction –Issue update –Release locks

26. © 2002 by Prentice Hall 26 Consistent Transactions Consistent transactions are often referred to by the acronym ACID –Atomic –Consistent –Isolated –Durable

27. © 2002 by Prentice Hall 27 ACID: Atomic A transaction consists of a series of steps. Each step must be successful for the transaction to be saved. This ensures that the transaction completes everything it intended to do before saving the changes.

28. © 2002 by Prentice Hall 28 ACID: Consistent No other transactions are permitted on the records until the current transaction finishes This ensures that the transaction integrity has statement level consistence among all records

29. © 2002 by Prentice Hall 29 ACID: Isolation Within multi-user environments, different transactions may be operating on the same data. As such, the sequencing of uncommitted updates, rollbacks, and commits continuously change the data content. The 1992 ANSI SQL standards define four isolation levels and specify respective issues.

30. © 2002 by Prentice Hall 30 Summary of Isolation Levels

31. © 2002 by Prentice Hall 31 ACID: Durable Durable transactions are saved to the data permanently Interim calculations, views, and sub- queries are temporal rather than durable; that is to say that these temporal results are not saved

32. © 2002 by Prentice Hall 32 Set-at-a-Time Versus Row-at-a-Time SQL statements act as filters for the entire data set. A cursor may be defined within a SQL statement to point to a particular record. Several types of cursors have been defined. The cursor type defines how the cursor behaves.

33. © 2002 by Prentice Hall 33 Types of Cursors

34. © 2002 by Prentice Hall 34 Database Security Database security strives to ensure… –Only authorized users perform authorized activities at authorized times

35. © 2002 by Prentice Hall 35 Managing Processing Rights and Responsibilities Processing rights define who is permitted to do what, when The individuals performing these activities have full responsibility for the implications of their actions Individuals are identified by a username and a password

36. © 2002 by Prentice Hall 36 Granting of Processing Rights Database users are known as an individual and as a member of one or more role Access and processing rights/privileges may be granted to an individual and/or a role Users possess the compilation of rights granted to the individual and all the roles for which they are members

37. © 2002 by Prentice Hall 37 Granting Privileges

38. © 2002 by Prentice Hall 38 Providing Database Recovery Common causes of database failures… –Hardware failures –Programming bugs –Human errors/mistakes –Malicious actions Since these issues are impossible to completely avoid, recovery procedures are essential

39. © 2002 by Prentice Hall 39 Database Recovery Characteristics Continuing business operations (Fall- back procedures/Continuity planning) Restore from backup Replay database activities since backup was originally made

40. © 2002 by Prentice Hall 40 Fall-back Procedures/ Continuity Planning The business will continue to operate even when the database is inaccessible The fall-back procedure defines how the organization will continue operations Careful attention must be paid to… –saving essential data –continuing to provide quality service

41. © 2002 by Prentice Hall 41 Restoring from Backup In the event that the system must be rebuilt or reloaded, the database is restored from the last full backup. Since it is inevitable that activities occurred since the last full backup was made, subsequent activities must be replayed/restored.

42. © 2002 by Prentice Hall 42 Recovery via Reprocessing This is a brunt-force technique. Simply re-type all activities since the backup was performed. This procedure is costly because of the effort involved in re-entering the data. This procedure is risky in that human error is likely and in that paper record-keeping may not be accurate.

43. © 2002 by Prentice Hall 43 Recovery via Rollback/Rollforward Most database management systems provide a mechanism to record activities into a log file.

44. © 2002 by Prentice Hall 44 Rollforward Activities recorded in the log files may be replayed. In doing so, all activities are re- applied to the database. This procedure is used to resynchronize restored database data. This procedure is termed a Rollforward.

45. © 2002 by Prentice Hall 45 Rollback Since log files save activities in sequence order, it is possible to undo activities in reserve order that they were originally executed. This is performed to correct/undo erroneous or malicious transaction(s). This procedure is known as a Rollback.

46. © 2002 by Prentice Hall 46 Managing the Database Management System (DBMS) In addition to controlling and maintaining the users and the data, the DBA must also maintain and monitor the DBMS itself. –Performance statistics (performance tuning/optimizing) –System and data integrity –Establishing, configuring, and maintaining database features and utilities

47. © 2002 by Prentice Hall 47 Maintaining the Data Repository The data repository contains metadata. Metadata is data about data. The data repository specifies the name, type, size, format, structure, definitions, and relationships among the data. They also contain the details about applications, users, add-on products, etc.

48. © 2002 by Prentice Hall 48 Types of Data Repositories Active data repository –The development and management tools automatically maintain and upkeep the metadata. Passive data repository –People manually maintain and upkeep the metadata