SqlSaturday199 - Deadlocks
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Deadlock detected! All is lost or it's too early to sound the alarm? Denis Reznik
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"Deadlock" is a terrible word, isn't it? Is it as scary as it sounds? Why do they occur and how can they affect an application? Significantly important question is how to solve "Deadlock" issues? The answers to these questions can be found in my session, which is completely dedicated to the fundamental principles of locking and isolation levels.
Transcript of SqlSaturday199 - Deadlocks
Deadlock detected! All is lost or it's too early to sound the alarm?Denis Reznik
Sponsors
About me
Denis Reznik Kiev, Ukraine Database Architect at The Frayman Group Microsoft MVP Community enthusiast
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Agenda
Locks Lock Types Transaction Isolation Levels
Deadlocks Classic Deadlocks Not obvious deadlocks Deadlock detecting and analyzing
Lock Types- Shared
S S
X
Lock Types - Exclusive
X
XS
Lock Types - Update
U
US
SX
Lock Types – Intent locks
S
ISIS
READ UNCOMMITTED
The less restricted Isolation Level Allow all collisions, which READ
COMMITTED allow Allow Dirty Reads Doesn’t set Shared locks on read operations
DemoREAD UNCOMMITTED
DIRTY READS
ID City
1 Kiev
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
SELECT * FROM UsersWHERE City = 'Kiev'
BEGIN TRANUPDATE Users SET City = 'Sofia'WHERE City = 'Kiev'
ID City
1 Sofia
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
SELECT * FROM UsersWHERE City = 'Kiev'0 Records
ROLLBACK
SELECT * FROM UsersWHERE City = 'Kiev'
X
READ COMMITTED
Default Isolation Level Doesn’t allow Dirty Reads Shared locks released after the read
NO DIRTY READS
ID City
1 Kiev
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
SELECT * FROM UsersWHERE City = 'Kiev'
BEGIN TRANUPDATE Users SET City = 'Sofia'WHERE City = 'Kiev'
ID City
1 Sofia
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
SELECT * FROM UsersWHERE City = 'Kiev'Wait for
Shared lock on the row
X S
DemoREAD COMMITTED
NON-REPEATABLE READS
ID City
1 Kiev
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
BEGIN TRAN
SELECT * FROM UsersWHERE City = 'Sofia'UPDATE Users
SET City = 'Kiev'WHERE Id = 2
ID City
1 Kiev
2 Kiev
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
SELECT * FROM UsersWHERE City = 'Sofia'
X
S
S...
S
REPEATABLE READ
More restricted than READ COMMITTED Doesn’t allow Dirty Reads Doesn’t allow Non-Repeatable reads Shared locks are hold to the end of the
transaction
NO NON-REPEATABLE READS
ID City
1 Kiev
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
BEGIN TRAN
SELECT * FROM UsersWHERE City = 'Sofia'UPDATE Users
SET City = ‘Kiev'WHERE Id = 2
ID City
1 Kiev
2 Kiev
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
SELECT * FROM UsersWHERE City = 'Sofia'
X
S
S...
S
COMMIT
DemoREPEATABLE READ
PHANTOM RECORDS
ID City
1 Kiev
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
BEGIN TRAN
SELECT * FROM UsersWHERE City = 'Sofia'UPDATE Users
SET City = 'Sofia'WHERE Id = 'Kiev'
ID City
1 Sofia
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
SELECT * FROM UsersWHERE City = 'Sofia'
X
S
S...
S
S
SERIALIZABLE
The most restricted Isolation Level Doesn’t allow Dirty Reads Doesn’t allow Non-Repeatable reads Doesn’t allow Phantom Records Lock range of keys
NO PHANTOM RECORDS
ID City
1 Kiev
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
BEGIN TRAN
SELECT * FROM UsersWHERE City = 'Sofia'UPDATE Users
SET City = 'Sofia'WHERE Id = 'Kiev'
ID City
1 Sofia
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
SELECT * FROM UsersWHERE City = 'Sofia'
X
RA
NG
E S
-S
...
COMMIT
DemoSERIALIZABLE
READ COMMITTED SNAPSHOT
Optimistic concurrency for reads Use row-version store in tempdb No shared locks on reads The same collisions as in READ
COMMITTED
READ COMMITTED SNAPSHOT
ID City
1 Kiev
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
BEGIN TRANUPDATE Users SET City = 'Sofia'WHERE City = 'Kiev'
SELECT * FROM UsersWHERE City = 'Kiev'
ID City
1 Sofia
2 Sofia
3 Sofia
4 Sofia
5 Moscow
6 New York
7 New York
SELECT * FROM UsersWHERE City = 'Kiev'
X
tempdb
ID City
1 Kiev
Version Store
DemoSNAPSHOT READ COMMITTED
SNAPSHOT
Optimistic concurrency for reads Use row-version store in tempdb No shared locks on reads Doesn’t allow Dirty Reads Doesn’t allow Non-Repeatable reads Doesn’t allow Phantom Records Update conflict detection
DemoSNAPSHOT
How to avoid?
Design database so, that it will be no possibility for a deadlock occur
Modify tables in the same order Choose appropriate Transaction Isolation
Level and check possibility of a deadlock in your Isolation Level
There is no unsolvable deadlocks.. But there can be solution, which will not suit you completely
Sponsors
Thank you!
Denis Reznik Twitter: @denisreznik Email: [email protected] Blog (in russian): http://reznik.uneta.com.ua Facebook: https://www.facebook.com/denis.reznik.5
LinkedIn: http://ua.linkedin.com/pub/denis-reznik/3/502/234
