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SqlSaturday199 - Deadlocks

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Денис Резник
Денис Резник

The document discusses database locks and transaction isolation levels. It begins by defining shared, exclusive, and update locks. It then explains different isolation levels including read uncommitted, read committed, repeatable read, and serializable. Read uncommitted allows dirty reads while read committed does not. Repeatable read prevents non-repeatable reads and serializable prevents phantom records. The document also covers snapshot isolation and how to avoid deadlocks through proper database design and transaction ordering.

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Deadlock detected! All is lost or it's too early to sound the alarm? Denis Reznik
Sponsors
About me      3 | Denis Reznik Kiev, Ukraine Database Architect at The Frayman Group Microsoft MVP Community enthusiast
Agenda  Locks  Lock Types  Transaction Isolation Levels  Deadlocks  Classic Deadlocks  Not obvious deadlocks  Deadlock detecting and analyzing
Lock Types- Shared X S S
Lock Types - Exclusive S X X
Lock Types - Update S U X U S
Lock Types – Intent locks IS S IS
READ UNCOMMITTED  The less restricted Isolation Level  Allow all collisions, which READ COMMITTED allow  Allow Dirty Reads  Doesn’t set Shared locks on read operations
Demo READ UNCOMMITTED
DIRTY READS SELECT * FROM Users WHERE City = 'Kiev' ID Sofia Kiev 2 Sofia 3 Sofia Sofia SELECT * FROM Users WHERE City = 'Kiev' 5 ROLLBACK 1 4 BEGIN TRAN X UPDATE Users SET City = 'Sofia' WHERE City = 'Kiev' City Moscow 0 Records 6 New York 7 New York SELECT * FROM Users WHERE City = 'Kiev'
READ COMMITTED  Default Isolation Level  Doesn’t allow Dirty Reads  Shared locks released after the read
NO DIRTY READS ID BEGIN TRAN UPDATE Users SET City = 'Sofia' WHERE City = 'Kiev' X City 1 Sofia Kiev 2 Sofia 3 Sofia 4 Sofia 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Kiev' S SELECT * FROM Users WHERE City = 'Kiev' Wait for Shared lock on the row
Demo READ COMMITTED
NON-REPEATABLE READS BEGIN TRAN ID UPDATE Users SET City = 'Kiev' WHERE Id = 2 1 X City Kiev 2 Sofia Kiev S 3 Sofia 4 Sofia S ... S 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Sofia' SELECT * FROM Users WHERE City = 'Sofia'
REPEATABLE READ     More restricted than READ COMMITTED Doesn’t allow Dirty Reads Doesn’t allow Non-Repeatable reads Shared locks are hold to the end of the transaction
NO NON-REPEATABLE READS BEGIN TRAN ID UPDATE Users SET City = ‘Kiev' WHERE Id = 2 1 X City Kiev 2 Sofia Kiev S 3 Sofia 4 Sofia S ... S 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Sofia' SELECT * FROM Users WHERE City = 'Sofia' COMMIT
Demo REPEATABLE READ
PHANTOM RECORDS BEGIN TRAN ID UPDATE Users SET City = 'Sofia' WHERE Id = 'Kiev' X City 1 Sofia Kiev S 2 Sofia S 3 Sofia 4 Sofia S ... S 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Sofia' SELECT * FROM Users WHERE City = 'Sofia'
SERIALIZABLE      The most restricted Isolation Level Doesn’t allow Dirty Reads Doesn’t allow Non-Repeatable reads Doesn’t allow Phantom Records Lock range of keys
NO PHANTOM RECORDS BEGIN TRAN UPDATE Users SET City = 'Sofia' WHERE Id = 'Kiev' X City 1 Sofia Kiev 2 Sofia 3 Sofia 4 Sofia 5 Moscow 6 New York 7 New York RANGE S-S ID SELECT * FROM Users WHERE City = 'Sofia' ... SELECT * FROM Users WHERE City = 'Sofia' COMMIT
Demo SERIALIZABLE
READ COMMITTED SNAPSHOT     Optimistic concurrency for reads Use row-version store in tempdb No shared locks on reads The same collisions as in READ COMMITTED
READ COMMITTED SNAPSHOT ID BEGIN TRAN X UPDATE Users SET City = 'Sofia' WHERE City = 'Kiev' Version Store ID City 1 Kiev tempdb City 1 Sofia Kiev 2 Sofia 3 Sofia 4 Sofia 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Kiev' SELECT * FROM Users WHERE City = 'Kiev'
Demo SNAPSHOT READ COMMITTED
SNAPSHOT        Optimistic concurrency for reads Use row-version store in tempdb No shared locks on reads Doesn’t allow Dirty Reads Doesn’t allow Non-Repeatable reads Doesn’t allow Phantom Records Update conflict detection
Demo SNAPSHOT
How to avoid?  Design database so, that it will be no possibility for a deadlock occur   Modify tables in the same order  Choose appropriate Transaction Isolation Level and check possibility of a deadlock in your Isolation Level  There is no unsolvable deadlocks.. But there can be solution, which will not suit you completely
Sponsors
Thank you!  Denis Reznik      Twitter: @denisreznik Email: denisreznik@live.ru Blog (in russian): http://reznik.uneta.com.ua Facebook: https://www.facebook.com/denis.reznik.5 LinkedIn: http://ua.linkedin.com/pub/denis-reznik/3/502/234

More Related Content

SqlSaturday199 - Deadlocks

  • 1. Deadlock detected! All is lost or it's too early to sound the alarm? Denis Reznik
  • 3. About me      3 | Denis Reznik Kiev, Ukraine Database Architect at The Frayman Group Microsoft MVP Community enthusiast
  • 4. Agenda  Locks  Lock Types  Transaction Isolation Levels  Deadlocks  Classic Deadlocks  Not obvious deadlocks  Deadlock detecting and analyzing
  • 6. Lock Types - Exclusive S X X
  • 7. Lock Types - Update S U X U S
  • 8. Lock Types – Intent locks IS S IS
  • 9. READ UNCOMMITTED  The less restricted Isolation Level  Allow all collisions, which READ COMMITTED allow  Allow Dirty Reads  Doesn’t set Shared locks on read operations
  • 11. DIRTY READS SELECT * FROM Users WHERE City = 'Kiev' ID Sofia Kiev 2 Sofia 3 Sofia Sofia SELECT * FROM Users WHERE City = 'Kiev' 5 ROLLBACK 1 4 BEGIN TRAN X UPDATE Users SET City = 'Sofia' WHERE City = 'Kiev' City Moscow 0 Records 6 New York 7 New York SELECT * FROM Users WHERE City = 'Kiev'
  • 12. READ COMMITTED  Default Isolation Level  Doesn’t allow Dirty Reads  Shared locks released after the read
  • 13. NO DIRTY READS ID BEGIN TRAN UPDATE Users SET City = 'Sofia' WHERE City = 'Kiev' X City 1 Sofia Kiev 2 Sofia 3 Sofia 4 Sofia 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Kiev' S SELECT * FROM Users WHERE City = 'Kiev' Wait for Shared lock on the row
  • 15. NON-REPEATABLE READS BEGIN TRAN ID UPDATE Users SET City = 'Kiev' WHERE Id = 2 1 X City Kiev 2 Sofia Kiev S 3 Sofia 4 Sofia S ... S 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Sofia' SELECT * FROM Users WHERE City = 'Sofia'
  • 16. REPEATABLE READ     More restricted than READ COMMITTED Doesn’t allow Dirty Reads Doesn’t allow Non-Repeatable reads Shared locks are hold to the end of the transaction
  • 17. NO NON-REPEATABLE READS BEGIN TRAN ID UPDATE Users SET City = ‘Kiev' WHERE Id = 2 1 X City Kiev 2 Sofia Kiev S 3 Sofia 4 Sofia S ... S 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Sofia' SELECT * FROM Users WHERE City = 'Sofia' COMMIT
  • 19. PHANTOM RECORDS BEGIN TRAN ID UPDATE Users SET City = 'Sofia' WHERE Id = 'Kiev' X City 1 Sofia Kiev S 2 Sofia S 3 Sofia 4 Sofia S ... S 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Sofia' SELECT * FROM Users WHERE City = 'Sofia'
  • 20. SERIALIZABLE      The most restricted Isolation Level Doesn’t allow Dirty Reads Doesn’t allow Non-Repeatable reads Doesn’t allow Phantom Records Lock range of keys
  • 21. NO PHANTOM RECORDS BEGIN TRAN UPDATE Users SET City = 'Sofia' WHERE Id = 'Kiev' X City 1 Sofia Kiev 2 Sofia 3 Sofia 4 Sofia 5 Moscow 6 New York 7 New York RANGE S-S ID SELECT * FROM Users WHERE City = 'Sofia' ... SELECT * FROM Users WHERE City = 'Sofia' COMMIT
  • 23. READ COMMITTED SNAPSHOT     Optimistic concurrency for reads Use row-version store in tempdb No shared locks on reads The same collisions as in READ COMMITTED
  • 24. READ COMMITTED SNAPSHOT ID BEGIN TRAN X UPDATE Users SET City = 'Sofia' WHERE City = 'Kiev' Version Store ID City 1 Kiev tempdb City 1 Sofia Kiev 2 Sofia 3 Sofia 4 Sofia 5 Moscow 6 New York 7 New York SELECT * FROM Users WHERE City = 'Kiev' SELECT * FROM Users WHERE City = 'Kiev'
  • 26. SNAPSHOT        Optimistic concurrency for reads Use row-version store in tempdb No shared locks on reads Doesn’t allow Dirty Reads Doesn’t allow Non-Repeatable reads Doesn’t allow Phantom Records Update conflict detection
  • 28. How to avoid?  Design database so, that it will be no possibility for a deadlock occur   Modify tables in the same order  Choose appropriate Transaction Isolation Level and check possibility of a deadlock in your Isolation Level  There is no unsolvable deadlocks.. But there can be solution, which will not suit you completely
  • 30. Thank you!  Denis Reznik      Twitter: @denisreznik Email: denisreznik@live.ru Blog (in russian): http://reznik.uneta.com.ua Facebook: https://www.facebook.com/denis.reznik.5 LinkedIn: http://ua.linkedin.com/pub/denis-reznik/3/502/234