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DEEP INTO
ISOLATION LEVELS
Boris HristovSQLBits, Superheroes, 2015
That’s not a Hekaton Talk! (In-Memory OLTP)
So who am I?
@BorisHristov
Isolation Levels time
com
ple
xity
Fundamentals
?
Session’s Timeline
The Fundamentals
Pessimistic Concurrency Optimistic Concurrency
Locks
Blocking
Versions
Version Store
Update Conflicts
Locks
Locking
Blocking
Deadlocks
Lock Escalations
Common lock types
Intent
Used for: Preventing incompatible locksDuration: End of the transaction
Shared (S)
Used for: ReadingDuration: Released almost immediately
(depends on the isolation level)
Update (U)
Used for: Preparing to modifyDuration: End of the transaction or until
converted to exclusive (X)
Exclusive (X)
Used for: ModifyingDuration: End of the transaction
Lock Compatibility
Lock Shared Update Exclusive
Shared (S) X
Update (U) X X
Exclusive (X) X X X
Lock Hierarchy
Database
Table
Page
Row
Let’s update a row. What do we need?
USE AdventureWorks2012GOUPDATE [Person].[Address]SET AddressLine1=‘London, UK'WHERE AddressID=2
S
IX
Header
Row
Row
Row
Row
Row
IX
X
A query!
Methods to View Locking Information
Dynamic Management
Views
SQL Server Profiler or
Extended Events
Performance monitor or Activity
Monitor
DEMO Locking and Locking Hierarchies
Transaction isolation levels(pessimistic concurrency)
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 READ COMMITTED
Transaction 1 rows
Non-repeatable reads possible (updates during Transaction 1)
Phantom records possible (inserts during Transaction 1)
What else? (that’s a Pluralsight voucher right here!)
Updates and Inserts
Transaction 2
Read Committed(pessimistic concurrency control)
S
S
S
S
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
Oh, yeah, TransactionScope in C# and MSDTC transactions default to this level too
Insert
Transaction 2
Serializable(pessimistic concurrency control)
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
DEMO Playing with Isolation levels (Pessimistic Concurrency)
Transaction isolation levels(optimistic concurrency)
Read Committed and Snapshot Isolation levels
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
V1 V2Transaction 1
Transaction 2
Select in RCSISelect
Select in SI
DEMO Playing with Isolation levels (Optimistic Concurrency)
… and Azure DB too
Just before we end…
Isolation levels can have dramatic impact on your application
As such, isolation levels must also be a business decision
The behavior of the readers changes in the various levels
Writers, though, will always block. No matter what!
And one more thing – please be careful with ORMs
Summary
