The document discusses transaction locking and isolation levels in SQL Server. It outlines the types of locks, the difference between locking and blocking, as well as how to troubleshoot locking issues. Key recommendations include minimizing transaction duration and strategically selecting isolation levels to enhance concurrency control.

1. Welcome to the nightmare of
locking, blocking and isolation levels!
28.06.2014 SQLSaturday Rheinland 2014

2. Thank you to our sponsors!

3. So who am I?
So who am I?
@BorisHristov

4. Agenda…
Locks. What is there for us?
Troubleshooting locking problems
Transaction Isolation Levels

5. Locks. What is there for us?

6. Methods of Concurrency Control
1. Pessimistic
– SQL Server uses locks, causes blocks and who said deadlocks?
2. Optimistic
– SQL Server generates versions for everyone, but the updates…

7. What Are Locks and what is locking?
Lock – internal memory structure that “tells” us what we all do with the
resources inside the system
Locking – mechanism to protect the resources and guarantee consistent data

8. Common lock types
Intent
Used for: Preventing incompatible locks
Duration: End of the transaction
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)
Exclusive (X)
Used for: Modifying
Duration: End of the transaction

9. Lock Compatibility
Not all locks are compatible with other locks.
Lock Shared Update Exclusive
Shared
  X
(S)
Update
(U)
 X X
Exclusive
(X) X X X

10. Lock Hierarchy
Database
Table
Page
Row

11. Let’s update a row!
What do we need?
USE AdventureWorks2012
GO
UPDATE [Person].[Address]
SET AddressLine1=’Oslo, Norway'
WHERE AddressID=2
S
IX
Header
Row
Row
Row
Row
Row
IX
X

12. Methods to View Locking Information
Dynamic
Management
Views
SQL Server
Profiler or
Extended Events
Performance
monitor or
Activity Monitor

13. Troubleshooting locking problems

14. Locking and blocking
Locking and blocking are often confused!
Locking
• The action of taking and potentially holding locks
• Used to implement concurrency control
Blocking is result of locking!
• One process needs to wait for another process to release locked
resources
• In a multiuser environment, there is always, always blocking!
• Only a problem if it lasts too long

15. Lock escalation
S
S
X
>= 5000
IX
Header
Row
Row
Row
Row
Row
IX
X
X
X
X

16. Controlling Lock escalation
1. Switch the escalation level (per table)
SELECT lock_escalation_desc
FROM sys.tables
WHERE name = 'Person.Address'
ALTER TABLE Person.Address SET (LOCK_ESCALATION = {AUTO | TABLE | DISABLE}
AUTO – Partition-level escalation if the table is partitioned
TABLE – Always table-level escalation
DISABLE – Do not escalate until absolutely necessary
2. Just disable it (that’s not Nike’s “Just do it!”)
• Trace flag 1211 – disables lock escalation on server level
• Trace flag 1224 – disables lock escalation if 40% of the memory used is consumed

17. What Are Deadlocks?
Task A
Task B
Resource 1
Resource 2
Who is victim?
• Cost for Rollback
• Deadlock priority – SET DEADLOCK_PRIOIRTY

18. Resolve blocking a.k.a live locking
1. Keep the transactions as short as possible
2. No user interactions required in the middle of the transaction
3. Use indexes (proper ones)
4. Consider a server to offload some of the workloads
5. Choose isolation level

19. DEMO
Monitor for locks with xEvents
Lock escalation – both to table and partition
Deadlock and the SET DEADLOCK_PRIORITY option

20. Transaction isolation levels

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

22. Repeatable Read
(pessimistic concurrency control)
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
Transaction 1 S(hared) lock
select
Update
Transaction 2
No non-repeatable reads possible (updates during Transaction 1)
Phantom records still possible (inserts during Transaction 1)

23. (pessimistic concurrency control)
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
Transaction 1 S(hared) lock
select
Serializable
Insert
Transaction 2
Even phantom records are not possible!
Highest pessimistic level of isolation, lowest level of concurrency

24. Optimistic Concurrency
Based on Row versioning (stored inside tempdb’s version store area)
• No dirty, non-repeatable reads or phantom records
• Every single modification is versioned even if not used
• Adds 14 bytes per row
Readers do not block writers and writers do not block readers
Writers can and will block writers, this can cause conflicts

25. RCSI and SI
(optimistic concurrency control)
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

26. DEMO
Playing around with the Isolation levels

27. Summary
1. Blocking is something normal when it’s not for long
2. There are numerous of ways to monitor locking and blocking
3. Be extremely careful for lock escalations
4. Choosing the Isolation level is also a business decision!

28. Resources
MCM Readiness videos on locking lecture and demo
MCM Readiness video on Snapshot Isolation Level
http://blogs.msdn.com/b/bartd/archive/tags/sql+locking
http://www.sqlskills.com/blogs/paul/category/locking/
Lock hints -
http://www.techrepublic.com/article/control-sql-server-locking-
with-hints/5181472

29. Thank you to our sponsors!