2. Concurrency and locking Pertemuan 8 Matakuliah: T0413 Tahun: 2009

3. Bina Nusantara University 3 What is a Transaction? Your bank account Your Mom’s bank account Balance = $1000 Balance = $200 Transfer $100 from your account to your Mom’s account: - Debit $100 from Savings account - Credit $100 to Checking account

4. Bina Nusantara University 4 IDNameAge 3Peter33 5John23 22Mary22 35Ann55 App A App B App C App D Concurrency overview

5. Bina Nusantara University 5 IDNameAge 3Peter33 5John23 22Mary22 35Ann55 App A App B App C App D Locking overview

6. Bina Nusantara University 6 Concurrency DB2 was designed as a multi-user database Access to data must be coordinated properly and transparently using a mechanism to ensure data integrity and consistency Without some form of concurrency control, the following problems may be encountered –Lost update –Uncommitted read –Non-repeatable read –Phantom read

7. Bina Nusantara University 7 Lost Update Reservations Flight Seat P-Name 512 7C 7B... 512 7C Instruct Manager... ? 7C Update Reservations Set P-name = 'Instruct' Where Flight = 512 and Seat = '7C' and P_name is NULL Update Reservations Set P-name = 'Manager' Where Flight = 512 and Seat = '7C' and P_Name is NULL

8. Bina Nusantara University 8 Uncommitted Read Reservations Flight Seat P_Name 512 7C 7B 7C Instruct 1 3 Update Reservations Set P-name = 'Instruct' Where Flight = 512 and Seat = '7C' and P_Name is NULL Roll back 2 4 Select seat From Reservations Where P-name is NULL Incorrect results set...

9. Bina Nusantara University 9 Non-repeatable Read FLIGHT SEAT NAME DESTINATION ORIGIN 512 814 134 7B 8A 1C DENVER SAN JOSE HONOLULU DALLAS DENVER SAN JOSE.... Book a flight from Dallas to Honolulu....

10. Bina Nusantara University 10 Phantom Read Reservations Flight Seat P-name 512 7B 7A 7B 1 Update Reservations Set P-name = 'NULL' Where Flight = 512 and Seat = '7A' and P-name = ' Susan Liu ' 2 Select seat From Reservations Where P-name is NULL Repeat 1 now seat 7A is available... Susan Liu 3

11. Bina Nusantara University 11 Locking DB2 uses locking to maintain data integrity Locks are acquired automatically as needed to support a transaction and are released when the transaction terminates (COMMIT/ROLLBACK) Locks can be acquired on tables or rows Two basic types of locks: –Share locks (S locks) – acquired when an application wants to read and prevent others from updating the same row –Exclusive locks (X locks) – acquired when an application updates, inserts, or deletes a row

12. Bina Nusantara University 12 Isolation Levels DB2 provides different levels of protection to isolate data –Uncommitted Read (UR) –Cursor Stability (CS) –Read Stability (RS) –Repeatable Read (RR) Isolation level can be specified at many levels –Session (application). Defaults to CS –Connection –Statement For embedded SQL, the level is set at bind time For dynamic SQL, the level is set at run time

13. Bina Nusantara University 13 Comparing isolation levels

14. Bina Nusantara University 14 Isolation Levels – Uncommitted Read Uncommitted Read is also known as DIRTY READ –Lowest level of isolation –Provides highest degree of concurrency No row locks are obtained on read operations –unless other application attempts to drop or alter table Update operations act as if using Cursor Stability Possible Situations –Uncommitted Read –Non-repeatable Read –Phantom Read Situations Prevented –Loss of Update

15. Bina Nusantara University 15 Isolation Levels – Cursor Stability Cursor Stability is the default isolation level –Minimal degree of locking –Locks the "current" row of a cursor –If the row is only read the lock is held until a new row is fetched or the unit of work is terminated –If the row is updated the lock is held until the unit of work is terminated Possible Situations –Non-repeatable Read –Phantom Read Prevented Situations –Loss of Update –Uncommitted Read

16. Bina Nusantara University 16 Isolation Levels – Read Stability Locks all the rows an application retrieves within a unit of work –For a given cursor, it locks all rows that qualify for the result set –Moderate degree of locking Possible Situations –Phantom Read Prevented Situations –Loss of Update –Uncommitted Read –Non-repeatable Read

17. Bina Nusantara University 17 Isolation Levels – Repeatable Read Highest isolation level, least concurrency –Same query issued by the application more than once in a unit of work will give the same result each time –High degree of locking –Locks held on all rows processed to build the result set i.e. rows not necessarily in the final result set may be locked –No other application can update, delete, or insert a row that would affect the result set until the unit of work completes Possible Situations –none Prevented Situations –Loss of Update –Uncommitted Read –Non-repeatable Read –Phantom Read

18. Bina Nusantara University 18 Comparison of Isolation Level Terminolgy DB2.NETJDBC Uncommitted Read (UR) ReadUncommittedTRANSACTION_RE AD_UNCOMMITTE D Cursor Stability (CS)ReadCommittedTRANSACTION_RE AD_COMMITTED Read Stability (RS)RepeatableReadTRANSACTION_RE PEATABLE_READ Repeatable Read (RR) SerializableTRANSACTION_SE RIALIZABLE

19. Bina Nusantara University 19 Statement Level Isolation SELECT... WITH {UR | CS | RS | RR} UR = Uncommitted Read CS = Cursor Stability RS = Read Stability RR = Repeatable Read Example Scenario: –Application needs to get a "rough" count of how many rows are in table. Performance is of utmost importance. Cursor Stability isolation level is required with the exception of one SQL statement: SELECT COUNT(*) FROM tab1 WITH UR

20. Bina Nusantara University 20 Lock Escalation

21. Bina Nusantara University 21 Lock Escalation When optimizer thinks it is better to have one lock on the entire table, rather than multiple row locks Database configuration parameters that affect lock escalation: –LOCKLIST – the amount of memory (4k pages) to manage locks for all connected applications Default is 50 * 4K pages on Windows –MAXLOCKS –Max percentage of the entire lock list that a single application can use up Default is 22 percent

22. Bina Nusantara University 22 Lock Escalation (continued) If the default values are used, lock escalation occurs when a single application requires more than 44K of lock memory If lock escalation, increase the value of LOCKLIST and MAXLOCKS Avoid lock escalations: Performance bottleneck Check the DB2 diagnostic log file (db2diag.log). The location is shown below. On Windows XP, 2003: –C:\Documents and Settings\All Users\Application Data\IBM\DB2\DB2COPY1\ On Windows Vista: –ProgramData\IBM\DB2\ On Linux/UNIX: –INSTHOME/sqllib/db2dump (INSTHOME is the home directory of the instance owner)

23. Bina Nusantara University 23 2007-01-02-23.04.43.699000 Instance:DB2 Node:000 PID:984(db2syscs.exe) TID:1720 Appid:*LOCAL.DB2.011003030417 data_management sqldEscalateLocks Probe:1 Database:SAMPLE -- Start Table Lock Escalation. -- Lock Count, Target : 28, 14 7570 6461 7465 2065 6d70 6c6f 7965 6520 update employee 7365 7420 6669 7273 746e 6d65 3d27 6162 set firstnme='ab 6327 c' 2007-01-02-23.04.43.699001 Instance:DB2 Node:000 PID:984(db2syscs.exe) TID:1720 Appid:*LOCAL.DB2.011003030417 data_management sqldEscalateLocks Probe:2 Database:SAMPLE -- Lock Count, Target : 28, 14 -- Table (ID) Name : (2;5) ADMINISTRATOR.EMPLOYEE -- Locks, Request Type : 25, X -- Result (0 = success): 0 Lock Escalation indication in the db2diag.log

24. Bina Nusantara University 24  View locks currently held by an application  UPDATE MONITOR SWITCHES USING LOCK ON  GET SNAPSHOT FOR LOCKS FOR APPLICATION AGENT ID Application Lock Snapshot Snapshot timestamp = 11-05-2002 00:09:08.672586 Application handle = 9 Application ID = *LOCAL.DB2.00B9C5050843 Sequence number = 0001 Application name = db2bp.exe Authorization ID = ADMINISTRATOR Application status = UOW Waiting Status change time = Not Collected Application code page = 1252 Locks held = 4 Total wait time (ms) = 0 List Of Locks Lock Name = 0x05000700048001000000000052 Lock Attributes = 0x00000000 Release Flags = 0x40000000 Lock Count = 255 Hold Count = 0 Lock Object Name = 98308 Object Type = Row Tablespace Name = TEST4K Table Schema = ADMINISTRATOR Table Name = T2 Mode = X Lock Snapshot

25. Bina Nusantara University 25 Lock Wait By default, an application waits indefinitely to obtain any needed locks LOCKTIMEOUT (db cfg): –Change to specify the number of seconds to wait for a lock –Default value is -1 or infinite wait A database connection also has a user-definable CURRENT LOCK TIMEOUT register –By default it inherits its value from the LOCKTIMEOUT parameter –Use the SET LOCK TIMEOUT statement to change its value –Once it is set for a connection, it persists across transactions –e.g. SET LOCK TIMEOUT=WAIT n

26. Bina Nusantara University 26 Deadlock Causes and Detection MILK USER A USER B RAISIN BRAN within a UNIT OF WORK (UOW) INSERT CEREAL AND MILK into BOWL

27. Bina Nusantara University 27 Deadlock Settings Deadlocks are an application design issue most of the time DLCHKTIME (db cfg) sets the time interval for checking for deadlocks –It defines the frequency that the database manager checks for deadlocks among all the applications connected to a database If you are experiencing many deadlocks, you should re-examine your existing transactions and see if any re-structuring is possible

28. Bina Nusantara University 28 Best Practices Keep transactions as short as possible –Issue frequent COMMIT statements – even for read-only transactions Log transaction information only when required –Purge data quick using: ALTER TABLE ACTIVATE NOT LOGGED INITIALLY WITH EMPTY TABLE –Perform data modifications in batches/groups DELETE FROM ( SELECT * FROM tedwas.t1 WHERE c1 = … FETCH FIRST 3000 ROWS ONLY ) –Use concurrency features in DB2 data movement tools Set the database level LOCKTIMEOUT parameter (usually between 30- 120 seconds) –Can also use session-based lock timeout Do not retrieve more data than is required –Use the FETCH FIRST n ROWS ONLY clause in SELECT statements