Welcome to the nightmare of locking, blocking and isolation levels!

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So who am I?

@BorisHristov

Agenda…

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

Locks. What is there for us?

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…

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

Common lock types

Intent

Used  for:    Preven-ng  incompa-ble  locks  Dura-on:  End  of  the  transac-on  

Shared  (S)    

Used  for:  Reading  Dura-on:  Released  almost  immediately  

(depends  on  the  isola-on  level)

Update  (U)    

Used  for:  Preparing  to  modify  Dura-on:  End  of  the  transac-on  or  un-l  

converted  to  exclusive  (X)

Exclusive  (X)    

Used  for:  Modifying  Dura-on:  End  of  the  transac-on

Lock Compatibility

Not all locks are compatible with other locks.

Lock Shared Update Exclusive Shared (S) P P X Update (U) P X X Exclusive (X) X X X

Lock Hierarchy

Database

Table

Page

Row

Let’s update a row! What do we need?

USE AdventureWorks2012 GO UPDATE [Person].[Address] SET AddressLine1=’Vienna, Austria' WHERE AddressID=2

S

IX

Header Row

Row

Row

Row

Row

IX

X

Methods to View Locking Information

Dynamic  Management  

Views  

SQL  Server  Profiler  or  

Extended  Events  

Performance  monitor  or  

Ac-vity  Monitor  

Troubleshooting locking problems

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

Lock escalation S

S

X

>= 5000

IX

Header Row

Row

Row

Row

Row

X

X

X

IX

X

1. Switch the escalation level (per table) 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

Controlling Lock escalation

SELECT lock_escalation_desc FROM sys.tables WHERE name = 'Person.Address' ALTER  TABLE  Person.Address  SET  (LOCK_ESCALATION  =  {AUTO  |  TABLE  |  DISABLE}

What Are Deadlocks?

Task A

Task B

Resource 1

Resource 2

Who is victim? •  Cost for Rollback

•  Deadlock priority – SET DEADLOCK_PRIOIRTY

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

DEMO

Monitor for locks with xEvents

Lock escalation – both to table and partition

Deadlock and the SET DEADLOCK_PRIORITY option

Transaction isolation levels

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED (NOLOCK?)

Transaction 1

Transaction 2

  Suggestion: Better offload the reads or go with optimistic level concurrency!

Select

Update

eXclusive lock

Read Uncommitted (pessimistic concurrency control)

Dirty read

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ

Transaction 1 S(hared) lock

select

  No non-repeatable reads possible (updates during Transaction 1)

  Phantom records still possible (inserts during Transaction 1)

Update

Transaction 2

Repeatable Read (pessimistic concurrency control)

Transaction 1 S(hared) lock

select

  Even phantom records are not possible!   Highest pessimistic level of isolation, lowest level of concurrency

Insert

Transaction 2

Serializable (pessimistic concurrency control)

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE

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

Optimistic Concurrency

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

RCSI and SI (optimistic concurrency control)

V1 V2 Transaction 1

Transaction 2

Select in RCSI Select

Select in SI

DEMO

Playing around with the Isolation levels

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!

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

THANK YOU!