The weird and the wonderful world of SQL Server Concurrency model looking at transactions, isolation levels and locking.

1. READPAST & Furious:
Transactions, Locking and Isolation
Mark Broadbent
Principal Consultant
SQLCloud

2. About
Mark Broadbent
“Possibly the hardest drinking DBA on the planet”
Email: mark.broadbent@sqlcambs.org.uk
Twitter: retracement
Blog: http://tenbulls.co.uk
Event Lead to the UK’s first ever SQLSaturday (Cambridge)
http://www.sqlsaturday.com/events.aspx
Cambridgeshire PASS Chapter UG Leader http://sqlcambs.org.uk
DBA-309

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11. Agenda
TRANSACTIONS
Structure, Scope, Management, the weird and the wonderful!
LOCKING
Compatibility, Multi Granularity and Escalation
NOLOCK, READPAST and UPDLOCK
ISOLATION
Isolation Levels
Snapshot Isolation and Read Committed Snapshot
Rolling Database Snapshots
AlwaysOn Readable Secondaries

12. TRANSACTIONS
Part I
March 16, Lisbon, Portugal

13. Transactions can be…
Batch Scope
Auto-Commit
UPDATE …
INSERT … MSDTC MSDTC INSERT …
DELETE … DELETE …
GO
Distributed
Explicit
BEGIN TRAN
… Binding Token
spid 115 spid 162
Implicit (evil muthas!) BEGIN TRAN BEGIN TRAN
UPDATE … INSERT …
INSERT …
BEGIN TRAN
DELETE …
Session Binding
and provide Atomicity (all or nothing) …or do they?!
They effect the duration of locks. It can be really confusing!

14. Transactions are easy-peasy…
Transaction Name Transaction Mark
BEGIN TRAN transaction_1 WITH MARK ‘restorepoint’
BEGIN TRAN
--do something
COMMIT TRAN Savepoint
SAVE TRAN savepoint
BEGIN TRAN transaction_3
BEGIN TRAN Nested
--do something else Transaction
COMMIT
IF {something_wrong} THEN ROLLBACK TRAN savepoint
COMMIT
COMMIT

15. TRANSACTIONS
Demo
March 16, Lisbon, Portugal

16. LOCKING
Part II
March 16, Lisbon, Portugal

17. Locks, the Lock Manager and Locking
Locks are ONLY memory structures
• They can be converted or escalated
• Lock escalation will occur for performance and memory savings
• Are compatible OR incompatible with other locks
• Will wait if they are incompatible
• Are taken depending upon the Isolation level
• Can cause blocking or deadlocks
Lock Manager compares locks ONLY on same resource
AND lock partition
• So that’s why intent locks are needed (for granularity)

19. NL SCH-S SCH-M S U X IS IU IX SIU SIX UIX BU RS-S RS-U RI-N RI-S RI-U RI-X RX-S RX-U RX-X
NL
SCH-S
SCH-M
S
U
X
IS
IU
IX SQL Server
SIU
Lock Compatibility Chart
SIX
UIX
BU
RS-S
RS-U
RI-N
RI-S No Conflict
RI-U Conflict
RI-X
RX-S Illegal
RX-U
RX-X
NL No Lock IX Intent Exclusive RI-S Insert Range-Shared
SCH-S Schema Stability Lock SIU Share with Intent Update RI-U Insert Range-Update
SCH-M Schema Modification Lock SIX Share with Intent Exclusive RI-X Insert Range-Exclusive
S Shared UIX Update with Intent Exclusive RX-S Exclusive Range-Shared
U Update BU Bulk Update RX-U Exclusive Range-Update
X Exclusive RS-S Shared Range-Shared RX-X Exclusive Range-Exclusive
IS Intent Shared RS-U Shared Range-Update
IU Intent Update RI-N Insert Range-Null

20. READPAST & Furious: Locking Blocking and Isolation ·
Mark Broadbent · sqlcloud.co.uk
NL SCH-S SCH-M S U X IS IU IX SIU SIX UIX BU RS-S RS-U RI-N RI-S RI-U RI-X RX-S RX-U RX-X
NL
SCH-S
SCH-M
S
U
X
IS
IU
IX SQL Server
SIU
Lock Compatibility Chart
SIX
UIX
BU
RS-S
RS-U
RI-N
RI-S Compatible
RI-U Incompatible
RI-X
RX-S Illegal
RX-U
RX-X
NL No Lock IX Intent Exclusive RI-S Insert Range-Shared
SCH-S Schema Stability Lock SIU Share with Intent Update RI-U Insert Range-Update
SCH-M Schema Modification Lock SIX Share with Intent Exclusive RI-X Insert Range-Exclusive
S Shared UIX Update with Intent Exclusive RX-S Exclusive Range-Shared
U Update BU Bulk Update RX-U Exclusive Range-Update
X Exclusive RS-S Shared Range-Shared RX-X Exclusive Range-Exclusive
IS Intent Shared RS-U Shared Range-Update
IU Intent Update RI-N Insert Range-Null
http://bit.ly/sc9f7l

21. Intent Locks
IX Table
orders
Data Page
IX
X Row
Free Space
X

22. Intent Locks
Lock must wait!
IX S Table
orders
Data Page
IX
X Row
Free Space
X

23. Escalation
Don’t escalate by TF1211
or take artificial IS on table Table
Ignore # locks by TF1224 IX
X
orders
Data Page
IX
Partition
X Row
Free Space
X ALTER <table> SET
(LOCK_ESCALATION
= auto| table| disable)

24. NOLOCK (a Wolf in Sheep’s Clothing?)
Should use READUNCOMMITTED hint instead –but ONLY in very
specialist situations.
Doesn’t acquire Shared locks so is able to perform DIRTY READS,
NON-REPEATABLE READS, PHANTOMS
…it can also return DUPLICATE READS!
It DOES NOT give “Oracle style” concurrency
When can this ever be acceptable?

25. Well since you mention it…
AlwaysOn Readable Secondaries
Scalable Shared Database
“There is SQL Server
NOLOCK” Instance A
SQL Server
Instance B
Instance C
SQL Server
Instance B
SQL Server
Instance A
Filegroup1
Filegroup2
Filegroup3
Readonly Database
Database Snapshot Readonly Filegroup/s
Blog post: “When should you use NOLOCK?“ http://bit.ly/rdGzow

26. READPAST & (your boss) may be
Furious!
Is an alternative to NOLOCK/ READUNCOMMITTED.
Skips over resources holding incompatible (to S) locks.
…does not therefore cause dirty or duplicate reads. (yay!)
DOES NOT give “Oracle style” concurrency
…and can return INCOMPLETE DATA SETS!
When can this ever be acceptable?

27. Using READPAST and Table Queues
--Get the first record
orders --possible and update
UPDATE u SET …
X X FROM 2
U
(
SELECT TOP(1) *
FROM orders
U WITH (READPAST
,UPDLOCK)
WHERE … 1
) AS U

28. LOCKING
Demo
March 16, Lisbon, Portugal

29. ISOLATION
Part III
March 16, Lisbon, Portugal

30. BAD DEPENDENCIES
Pessimistic
Non-
Optimistic Update
Dirty Read Repeatable Phantoms
Conflict
Read
1
READ
UNCOMMITTED Warning! Warning! Warning! OK
2 READ
COMMITTED OK Warning! Warning! OK
…WITH SNAPSHOT OK Warning! Warning! OK
3
REPEATABLE
READ OK OK Warning! OK
4
SERIALIZABLE OK OK OK OK
5
SNAPSHOT OK OK OK Warning!

31. Read Committed Snapshot vs
Snapshot Isolation
Read Committed Snapshot Snapshot
Isolation at the statement level. Isolation at the transaction level.
Becomes the new default Must be explicitly SET in each
Isolation. connection.
Requires Exclusive Transaction Requires no active transactions in
Workspace lock (i.e. no other order to transition.
connections to DB).
Is allowed master, tempdb &
Not allowed on master, tempdb & msdb.
msdb.
Implements automatic update
Conflict detection not required. conflict detection.

32. Optimistic Concurrency #FAIL
Snapshot Isolation introduces update conflicts!
TempDB overhead for version store.
Writers STILL block writers and their escalation can still cause a
concurrency problem!
RCSI (and SI) are not a silver bullet for Concurrency.

33. Database Snapshots spid 115
5
COMMIT or
COMMIT or
spid 115
ROLLBACK
ROLLBACK
1
BEGIN TRAN
UPDATE spid 162 2
orders CREATE DATABASE ssdb
SET … …
WHERE … Buffer Cache AS SNAPSHOT OF db
3
4
Dirty Pages
Automatic Recovery
CHECKPOINT
Datafile Transaction Log
Blog post: “Lifting the lid on database snapshots“ http://bit.ly/NS64qA
DBA-309

34. Readable Secondaries
Great for reporting, but…
Cause a 14 byte pointer to be added to Primary database –since
Secondary uses SNAPSHOT isolation under the covers.
Beware sizing considerations and page splitting.
Data latency from redo thread can cause unexpected results if your
application does not expect it.
More administration overhead and knowledge.
You cannot write to them (name is the giveaway!).
DBA-309

35. ISOLATION
Demo
March 16, Lisbon, Portugal

36. Summary
Concurrency is a very complicated topic, ensure you understand the
effects of running different transactional modes and Isolation levels.
Understand the duration and compatibility of common locks and how
they are effected.
Isolation can be achieved through scalability mechanisms and
techniques, your ultimate aim is to separate writers from readers,
readers from writers and writers from writers. In short maximize lock
compatibility.

37. References and Thanks
Kalen Delaney, SQLPASS Summit 2011
DBA301P - Locking and Blocking
and Row Versions, Oh My! – DVD
Jose Barreto's Blog http://bit.ly/nPJGjv
Paul White http://sqlblog.com/blogs/paul_white
Benjamin Neverez http://www.benjaminnevarez.com/

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March 16, Lisbon, Portugal

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