1. Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
DEEP INTO
ISOLATION LEVELS
Boris Hristov
SQLUG Sweden, September 2015

2. That’s not a Hekaton Talk!
(In-Memory OLTP)

3. So who’s Boris?
@BorisHristov

4. Module 4: Managing Security
Course 2786B
Session’s Timeline
Module 4: Managing Security
complexity ?
Fundamentals
Isolation Levels
time

5. Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
The Fundamentals

6. Module 4: Managing Security
Course 2786B
Pessimistic Concurrency
Optimistic Concurrency
Module 4: Managing Security
Locks
Locking
Blocking
Deadlocks
Lock Escalations
Versions
Version store
Locks
Blocking
Update Conflicts

7. Module 4: Managing Security
Course 2786B
Common lock types
Module 4: Managing Security
Shared (S)
Used for: Reading
Duration: Released almost immediately (depends on the isolation level)
Update (U) Used for: Preparing to modify Duration: End of the transaction or until converted to exclusive (X)
Intent Used for: Preventing incompatible locks Duration: End of the transaction
Exclusive (X)
Used for: Modifying Duration: End of the transaction

8. Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Course 10776A
Lock Compatibility
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Lock
Shared
Update
Exclusive
Shared (S)  X
Update (U)
Exclusive (X)

9. Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Lock Hierarchy
Database
Table
Page
Row

10. Let’s update a row. What do we need?
Course 10776A
Let’s update a row. What do we need?
Module 13: Creating Highly Concurrent SQL Server 2012 Applications S
A query!  IX
USE AdventureWorks2012 GO
UPDATE [Person].[Address]
SET AddressLine1=‘Stockholm, Sweden'
WHERE AddressID=2 IX
Header
Row X

11. Methods to View Locking Information
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Methods to View Locking Information
Dynamic Management Views
SQL Server Profiler or Extended Events
Performance monitor or Activity Monitor

12. Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
DEMO Locking and Locking Hierarchies

13. Transaction isolation levels (pessimistic concurrency)
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Transaction isolation levels (pessimistic concurrency)

14. Module 4: Managing Security
Course 2786B
Module 4: Managing Security
Read Uncommitted (pessimistic concurrency control)
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED (NOLOCK?)
Transaction 1
eXclusive lock
Update
Select
Transaction 2
Dirty read
Suggestion: Better offload the reads or go with optimistic level concurrency!

15. Module 4: Managing Security
Course 2786B
Module 4: Managing Security
Read Committed (pessimistic concurrency control)
SET TRANSACTION ISOLATION LEVEL READ COMMITTED
Transaction 1
rows S S
Transaction 2 S S
Updates and Inserts
Non-repeatable reads possible (updates during Transaction 1)
Phantom records possible (inserts during Transaction 1)
What else? (that’s a Pluralsight voucher right here!)

16. Module 4: Managing Security
Course 2786B
Module 4: Managing Security
Repeatable Read (pessimistic concurrency control)
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
Transaction 1
S(hared) lock
select
Transaction 2
Update
No non-repeatable reads possible (updates during Transaction 1)
Phantom records still possible (inserts during Transaction 1)

17. Module 4: Managing Security
Course 2786B
Serializable (pessimistic concurrency control)
Module 4: Managing Security
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
S(hared) lock
Transaction 1
select
Transaction 2
Insert
Even phantom records are not possible!
Highest pessimistic level of isolation, lowest level of concurrency
Oh, TransactionScope in C# and MSDTC transactions default to this level too

18. Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
DEMO Playing with Isolation levels (Pessimistic Concurrency)

19. Transaction isolation levels (optimistic concurrency)
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Transaction isolation levels (optimistic concurrency)
-14 bytes – XSN transaction + pointer

20. Read Committed and Snapshot Isolation levels
Course 2786B
Read Committed and Snapshot Isolation levels
Module 4: Managing Security
Transaction 1 V1 V2
Select
Select in RCSI
Transaction 2
Select in SI
RCSI – Read Committed Snapshot Isolation Level
Statement level versioning
Requires ALTER DATABASE SET READ_COMMITTED_SNAPSHOT ON
Snapshot Isolation Level
Transaction level versioning
Requires ALTER DATABASE SET ALLOW_SNAPSHOT_ISOLATION ON
Requires SET TRANSACTION ISOLATION LEVEL SNAPSHOT

21. Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
DEMO Playing with Isolation levels (Optimistic Concurrency)
And Azure DB too

22. Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Just before we end…

23. Module 4: Managing Security
Course 2786B
Module 4: Managing Security
Summary
Isolation levels can have dramatic impact on your application
They must also be a business decision
Only the behavior of the readers changes in the various levels
And one more thing – please be careful with ORMs 

24. Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Course 10776A
Module 13: Creating Highly Concurrent SQL Server 2012 Applications
Thanks a ton, SQLug.se!
Contacts:
@BorisHristov