2013 07 Transaction Isolation Level
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教學用，關於Isolation Level的說明。在不同的Isolation Level下會遇到什麼樣的問題等等。
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2013 07 Transaction Isolation Level
- 1. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Transaction Isolation Levels of Database System Aecho Penpower, Inc aecho.liu@penpower.com.tw 2013, 07 1 / 31
- 2. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Overview Overview Anomalies and Isolation Levels Anomalies Dirty Reads Unrepeatable Reads Phantoms Isolation Levels Default level Locks of Isolation Levels FAQ 2 / 31
- 3. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ References • Database Management Systems, 3rd edition. • Isolation level, @wiki. http://goo.gl/NYSza • Isolation level, @msdn. http://goo.gl/deqhV • Acid, @wiki. https://en.wikipedia.org/wiki/ACID 3 / 31
- 4. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Brief ACID properties of database system. • Atomic Each transaction is regarded as atomic. • Consistency The consistency property ensures that any transaction will bring the database from one valid state to another. • Isolation Users should be able to understand a transaction without considering the effect of other concurrently executing transactions. • Durability Once the DBMS informs the user that a transaction has been successfully completed, its effects should persist even if the system crashes before all its changes are reflected on disk. 4 / 31
- 5. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Brief Strict 2-Phase Locking1 . Rule 1 . • Shared lock Allow another transaction to read. . • Exclusive lock No allow another transaction to read or write. . Rule 2 . All locks held by a transaction are released when the transaction is completed. . 1 Strict 2PL 5 / 31
- 6. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Brief About Isolation Level • Most DBA System offers a number of transaction isolation levels, which control the degree of locking when selecting data. • The higher isolation level, the more locks needed. It is trade-off. • For concurrency control, with multiple transactions. 6 / 31
- 7. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Brief Level/Locks Read Uncommitted Read Committed Repeatable Read Serializable Write Lock X X X Read Lock S X X Range Lock X • X → Exclusive Lock • S → Shared Lock • - → Nothing 7 / 31
- 8. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Brief Level/Anomaly Read Uncommitted Read Committed Repeatable Read Serializable Dirty Read Maybe No No No Unrepetable read Maybe Maybe No No Phantom Maybe Maybe Maybe No 8 / 31
- 9. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Anomalies • Dirty Reads • Unrepeatable Reads • Phantoms 9 / 31
- 10. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Anomalies • Dirty Reads • Unrepeatable Reads • Phantoms A Sample User Table id 1 2 name Joe Jill age 20 25 10 / 31
- 11. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Dirty Reads . When a transaction is allowed to read data from a row that has been modified by another running transaction and not yet committed. . 11 / 31
- 12. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Dirty Reads Level/Anomaly Read Uncommitted Read Committed Repeatable Read Serializable Level/Locks Read Uncommitted Read Committed Repeatable Read Serializable Dirty Read Maybe No No No Unrepetable read Maybe Maybe No No Write Lock X X X Read Lock S X X Phantom Maybe Maybe Maybe No Range Lock X 12 / 31
- 13. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Dirty Reads Transaction 1 . /∗ Query 1 ∗/ SELECT ag e FROM u s e r s WHERE i d = 1; /∗ w i l l r e a d 20 ∗/ Transaction 2 . /∗ Query 2 ∗/ UPDATE u s e r s SET age = 21 WHERE i d = 1; /∗ No commit h e r e ∗/ . . . /∗ Query 1 ∗/ SELECT ag e FROM u s e r s WHERE i d = 1; /∗ w i l l r e a d 21 ∗/ . ROLLBACK ; . . 13 / 31
- 14. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Unrepeatable Reads . A row is retrieved twice and the values within the row differ between reads. . 14 / 31
- 15. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Unrepeatable Reads Level/Anomaly Read Uncommitted Read Committed Repeatable Read Serializable Level/Locks Read Uncommitted Read Committed Repeatable Read Serializable Dirty Read Maybe No No No Unrepetable read Maybe Maybe No No Write Lock X X X Read Lock S X X Phantom Maybe Maybe Maybe No Range Lock X 15 / 31
- 16. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Unrepeatable Reads Transaction 1 . /∗ Query 1 ∗/ SELECT ∗ FROM u s e r s WHERE i d = 1 ; Transaction 2 . /∗ Query 2 ∗/ UPDATE u s e r s SET age = 21 WHERE i d = 1; COMMIT; . . . /∗ Query 1 ∗/ SELECT ∗ FROM u s e r s WHERE i d = 1 ; COMMIT; . 16 / 31
- 17. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Phantoms . when two identical queries are executed, and the collection of rows returned by the second query is different from the first. . 17 / 31
- 18. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Phantoms Level/Anomaly Read Uncommitted Read Committed Repeatable Read Serializable Level/Locks Read Uncommitted Read Committed Repeatable Read Serializable Dirty Read Maybe No No No Unrepetable read Maybe Maybe No No Write Lock X X X Read Lock S X X Phantom Maybe Maybe Maybe No Range Lock X 18 / 31
- 19. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Phantoms Transaction 1 . /∗ Query 1 ∗/ SELECT ∗ FROM u s e r s WHERE age BETWEEN 10 AND 3 0 ; Transaction 2 . /∗ Query 2 ∗/ INSERT INTO u s e r s ( i d , name , a g e ) VALUES ( 3 , ’ Bob ’ , 27 ) ; COMMIT; . . . /∗ Query 1 ∗/ SELECT ∗ FROM u s e r s WHERE age BETWEEN 10 AND 3 0 ; . 19 / 31
- 20. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Isolation Levels • Read Uncommitted The lowest level. • Read Committed • Repeatable Read • Serializable The highest level. 20 / 31
- 21. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Isolation Levels The default isolation level. • Sqlite • Serializable by default.2 • Able to switch to Read uncommitted. • Mssql • Read Committed by default.3 • Serializable → WCC’s Category tree. 2 Sqlite pragma statements. http://goo.gl/pYNwN 3 Isolation level, @msdn. http://goo.gl/deqhV 21 / 31
- 22. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Isolation Levels Level/Locks Read Uncommitted Read Committed Repeatable Read Serializable Write Lock X X X Read Lock S X X Range Lock X • X → Exclusive Lock • S → Shared Lock • - → Nothing 22 / 31
- 23. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Read Committed • Exclusive locks • Obtains before writing objects. • Released until the end of transaction. 23 / 31
- 24. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Read Committed • Exclusive locks • Obtains before writing objects. • Released until the end of transaction. • Shared locks • Obtained before reading objects. • Released immediately. 24 / 31
- 25. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Repeatable Reads and Serializable • Obtains exclusive locks before reading or writing. • All locks released until the end of transaction, according to Strict 2-PL. 25 / 31
- 26. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Repeatable Reads and Serializable • Obtains exclusive locks before reading or writing. • All locks released until the end of transaction, according to Strict 2-PL. . . • Repeatable Reads → locks individual object. • Serializable → locks set of objects. 26 / 31
- 27. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Transaction and locks When the transaction waits for lock resources ... • Wait to gain the locks. • Commit or Rollback 27 / 31
- 28. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Transaction and locks When the transaction waits for lock resources ... • Wait to gain the locks. • Commit or Rollback • Dead lock... ? 28 / 31
- 29. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Transaction and locks When the transaction waits for lock resources ... • Wait to gain the locks. • Commit or Rollback • Dead lock... ? . Set up time-out to prevent dead lock. . 29 / 31
- 30. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ Thinking In practice, what isolation level do we need ? 30 / 31
- 31. Overview ... Anomalies .. .. .. Isolation Levels . ..... FAQ The End FAQ, any questions ? 31 / 31