1. 1099 Why Use InterBase? Bill Todd The Database Group, Inc.

2. Two Methods of Concurrency Control Locking Versioning

3. Locking Schemes Your locks restrict other user’s access to the locked object Lock granularity –Database –Table –Page –Row –Index

4. Pessimistic Locking Row is locked before you begin changing it. Guarantees that you can commit your changes. Locks can persist for a long time which reduces concurrency. Larger number of locks increases load on database server.

5. Optimistic Locking Most common method among database servers. Lock is placed when row is updated and released when transaction commits. Locks exist for a shorter time. Another user can update the row while you are making changes.

6. Optimistic Locking Example 1 John reads row and makes changes Jane reads row and makes changes John updates row causing lock Jane attemps update & gets error

7. Optimistic Locking Example 2 John reads row and makes changes Jane reads row and makes changes John updates row causing lock John commits releasing lock Jane updates row overwriting John’s changes Jane commits releasing lock

8. To Prevent Overwriting Changes Use UPDATE statement with original value of all fields in WHERE clause –Does not work if only blob field is changed Use UPDATE that includes timestamp or counter field in WHERE clause –Works for all field types

9. Transactions All changes within a transaction succeed or fail as a single unit

10. Transaction Isolation Level Your transaction’s isolation level controls when your transaction sees changes made by other transactions

11. SQL Standard Isolation Levels Read uncommitted Read committed Repeatable read Serializable

12. Locking & Concurrency Repeatable read isolation requires locks to stop other users from updating rows you have read Serializable requires locks to stop other users from updating rows you have read or inserting new rows

13. Locking Example Inventory valuation by warehouse –User A starts valuation query –User B moves material from warehouse 1 to warehouse 20 while user A’s query is reading warehouse 10’s records –User A’s query counts the material twice.

14. Solution Use serializable isolation for the valuation query Problem - User B cannot update rows until the valuation query ends

15. Versioning Each transaction gets a unique number Each row version contains the number of the transaction that created it

16. Versioning Example Inventory valuation by warehouse –User A starts valuation query –User B moves material from warehouse 1 to warehouse 20 while user A’s query is reading warehouse 10’s records. Because there is an active transaction with a lower number new row versions are created for all rows that are updated

17. Versioning Example (continued) User A’s transaction only reads the rows that were committed at the time it started The transaction gets the correct count User B is not blocked

18. Snapshot Isolation Level Unique to InterBase All the benefits of serializable Does not place any locks Does not block updates by others

19. How Versioning Works When a snapshot transaction starts it gets: –The next transaction number –A copy of the Transaction Inventory Pages (TIP)

20. The TIP Shows the state of all transactions –Active –Committed –Rolled back –Limbo Only occurs if a two-phase commit fails

21. Which Row Version is Read? The most recent version whose transaction was committed at the time the reading transaction started is the version that is read

22. Example Transaction 90 reads a row with the following versions Tran 100 status = committed Tran 80 status = active when reader started Tran 60 status = rolled back Tran 40 status = committed when reader started

23. How Locking & Versioning Compare A series of examples What happens with no concurrency control What happens with locking What happens with versioning

24. Example 1 Husband and wife go to two different ATM’s at the same time to withdraw money from their checking account.

25. Ex 1 – No Concurrency

26. Example 1 - Locking

27. Example 1 - Versioning

28. Ex 2 – No Concurrency

29. Example 2 - Locking

30. Example 2 - Versioning

31. Ex 3 – No Concurrency

32. Example 3 - locking

33. Example 3 - Versioning

34. Ex 4 – No Concurrency SELECT with a WHERE clause Another user inserts a row that satisfies the WHERE clause Rerun the query and you see the new row

35. Example 4 - Locking Place a table lock to prevent the insert Place an index range lock to prevent the insert

36. Example 4 - Versioning The insert is allowed The second SELECT does not see the inserted row because its transaction number is higher than that of the reading transaction

37. Example 5 John and Jane are told to make their salaries equal

38. Ex 5 – No Concurrency

39. Example 5 - Locking

40. Example 5 - Versioning

41. Example 5 – Versioning Fix

42. How Do They Compare Versioning provides better concurrency in most cases involving mixed reads and writes

43. Recovery Database server must roll back all active transactions on restart after a crash For locking database this means processing transaction log For versioning only the TIP must be updated –Status bits for all active transactions changed from Active to Rolled Back –Garbage collection cleans up rolled back row versions

44. Garbage Collection Record versions cause database to grow Old versions are deleted each time a row is accessed A sweep removes all row versions that are no longer needed

45. InterBase Features Transaction support –Read committed isolation –Snapshot isolation Benefits of serializable isolation without blocking updaters –SQL 92 entry level support with extnesions –User define functions –Cost base query optimizer

46. InterBase Features Row level locking No lock escalation No conversion deadlocks Two phase commit Multi-user and single user versions SMP support Hyperthreading support

47. InterBase Features Triggers –Before –After –Old & new values available –Multiple triggers per event –Control of trigger firing order –Post events to clients from triggers Make any view updateable with triggers

48. InterBase Features Stored procedures Temporary tables –ON COMMIT PRESERVE | DELETE –ALTER TABLE allows changing ON COMMIT clause Generators for incrementing keys Null state for all data types Declarative referential integrity

49. InterBase Features Server level security Database level security The levels can be mixed on a single server On-line backup On-line metadata changes Instantaneous crash recovery Replication

50. InterBase Features Zero maintenance Small disk footprint Small memeory footprint Does not require certified DBA Does not require custom installer Multi-platform support

51. IB vs. Oracle & DB2 Much less expensive Less complex – easier to learn Easier to deploy Does not require highly trained DBA Much lower resource requirements –Memory –Disk –CPU

52. IB vs. Microsoft SQL Server Less expensive Less complex – easier to learn Easier to deploy No lock escalation No conversion deadlocks Does not require highly trained DBA Lower resource requirements Multi-platform support Instant crash recovery

53. IB vs. Advantage & DBISAM Snapshot isolation for stable view of data without blocking updates On-line backups Shadowing Transactions can span databases Triggers can post events to clients Complex views are updateable with triggers

54. Applications Made For IB Mixed long reads and updates Embedded applications Widely deployed applications No on-site support Multi-platform applications

55. Questions?

56. Thank You Please fill out the speaker evaluation You can contact me further at … bt2@dbginc.com