1. Copyright © 1998-2008 Curt Hill Database Function What should every database do?

2. Copyright © 1998-2008 Curt Hill Functions of a Database According to EF Codd a database should provide the following services: Data storage and retrieval User-accessible catalog Transaction support Concurrency support Recovery services Authorization services Support for data communication Integrity services Data independence services

3. Copyright © 1998-2008 Curt Hill Data storage and retrieval While hiding the file structure, users may: –Add –Delete –Update in place Duh!

4. Copyright © 1998-2008 Curt Hill Queries A mechanism where a question is asked of the database and it provides an answer: –Not in natural language –Usually in Structured Query Language “Sequel” –Usually state what is wanted rather than how to get it The Data Manipulation Language usually includes both query and update facilities

5. Copyright © 1998-2008 Curt Hill User-accessible catalog The database itself knows the shape of the data –Available records and fields –Attributes of fields –Meaning of the field –Relationships with other fields or records –Applications that access The administrator is the most frequent user of this info

6. Copyright © 1998-2008 Curt Hill Transaction Support The unit of update is a transaction –All or nothing –No partial updates are allowed The DBMS must guarantee the integrity of a transaction, even in the face of: –Program errors –Incorrect input –System crashes

7. Copyright © 1998-2008 Curt Hill Entering an order Gain access to the system or subsystem Find the record of the item requested If there are enough to satisfy the request, then reserve them and decrement the number in stock This reservation is termed committed – no longer available If there are not enough mark this item as back ordered If the request brings the item below the reorder point generate a transaction that sets in motion an order Initiate a shipping order Generate an invoice

8. Copyright © 1998-2008 Curt Hill Transactions: ACID Atomic –Transaction perceived to be indivisible Consistent –Transforms database from one consistent state to another Isolated –Understandable without regards to any other agents Durable –Once committed permanence is guaranteed even with system crashes

9. Copyright © 1998-2008 Curt Hill Concurrency support A mechanism is needed to prevent two concurrent applications from both getting the last airline seat Usually a locking protocol –A transaction may lock a table or fragment of the table –Two types Shared for queries Exclusive for updates Concurrency example

10. Copyright © 1998-2008 Curt Hill Concurrency example Two agents ask how many widgets there are –Both are told 20 Both order 15 –One gets 15 –The other is told they are backordered As soon as the 15 are committed everyone else is told there are 5

11. Copyright © 1998-2008 Curt Hill Recovery services Many dangers: –Power failure –Hardware failure –Software failure How do we get the database back to a consistent state? –Backups –Journaling

12. Copyright © 1998-2008 Curt Hill Recoveries Forward –Use if database is destroyed –Restore from backup –Reapply transactions from the log/journal Backward –Use if database is inconsistent –Process log backward –Unroll transactions that may have been incomplete

13. Copyright © 1998-2008 Curt Hill Recovery with checkpoints At some point delay new transactions until all current transactions finish Save the database This is the point from which a backup may be made

14. Copyright © 1998-2008 Curt Hill Authorization services Verify that those who access the data are authorized Encryption Views or sub-schemas Authorizations User defined procedures Privacy

15. Copyright © 1998-2008 Curt Hill Encryption Transparent to user Data is encrypted before write and decrypted before read Securely prevents access from any other program than DBMS Does the file system perceive the database as one or many files?

16. Copyright © 1998-2008 Curt Hill Views Restrict view to the authority of the viewer If user is not authorized to change or see a value, remove from the view If they may view but not change, mark as read only

17. Copyright © 1998-2008 Curt Hill Authorization rules Subject Often identified by a login or password Object Set of database entities that are subject to access Action View, update, delete, etc Constraint Limitations on what can be done Manager may only change pay of own department workers

18. Copyright © 1998-2008 Curt Hill User Defined Procedures When a particular action is performed a subroutine is executed Subroutine is user-written Vendor supplies exits to these subroutines in the DBMS Checks validity or logs information customized for the organization

19. Copyright © 1998-2008 Curt Hill Privacy A query should not violate the privacy of individuals A query may be attempting to find a statistical value –Find the mean salary of VCSU math professors It may be tailored to violate privacy –Find the mean salary of VCSU math professors with a CS master’s degree

20. Copyright © 1998-2008 Curt Hill Support for data communication Access should not be limited to a single machine From where –Telephone –LAN –Intranet –WAN –Internet

21. Copyright © 1998-2008 Curt Hill Integrity services Guaranteeing that an item satisfies integrity constraints Data type –Integer must be numeric Legal values –A month of 13 is numeric but not legal –Legal values do not always have to be contiguous Format Key constraints

22. Copyright © 1998-2008 Curt Hill Format Rule Example A product serial number must have: –First two characters are digits –Next three digits must be alphabetic –Next must be a hyphen –Next must be six digits –Last character is an R or M Do not confuse with display formatting –Tele is stored as 8457103 and shown as 845-7103

23. Copyright © 1998-2008 Curt Hill Key Constraints (1 of 3) Candidate key –A minimal collection of one or more fields that uniquely identify a record –Removing any makes the key not unique –This may be one field or every field in the record Multiple candidate keys may be possible

24. Copyright © 1998-2008 Curt Hill Key Constraints (2 of 3) Primary key –Any candidate key designated by the DBMS –Inserting a record with an identical primary key must be rejected by the DBMS –Entity integrity Primary key may not be duplicate or null

25. Copyright © 1998-2008 Curt Hill Key Constraints (3 of 3) Foreign key –A field in a record that matches a primary key in another table –Referential integrity A record with a foreign key must have a corresponding record in the other table, where the key is primary

26. Copyright © 1998-2008 Curt Hill Key Example Three tables: –Students, primary key is NAID –Courses, primary key is department, course, section –Grades, primary key is NAID, department, course, section NAID is foreign key as is the other three Must not allow a record where the foreign keys reference non-existing records

27. Data Independence Users should be unaffected by changes in the database structure Physical data independence –Changing the physical structure does not affect the user –Such as inserting or removing an index Logical data independence –Adding/removing a field does not affect any program that does not use the field –Changing the format of a field should also be painless Copyright © 1998-2008 Curt Hill

28. Are there others? Every database offers other services However these services do not distinguish a database from other applications These may include –Accepting SQL or other query language –Report generation Copyright © 1998-2008 Curt Hill