SQLintersection Locks, Blocks, and Deadlocks Oh My! Randy Knight Wednesday, 2:15-3:15

© SQLintersection. All rights reserved. Randy Knight  Microsoft Certified Master in SQL Server  20+Years of database experience, focusing on SQL Server since  Worked in a variety of settings, including six years as a Database Architect for match.com  Founder, SQL Solutions Group

© SQLintersection. All rights reserved. Overview  Review  ACID Properties  Transaction Isolation Levels  Defined  Locking  Blocking  Deadlocks  Detection and Prevention  Excessive Blocking  Deadlocks

© SQLintersection. All rights reserved. ACID  Atomic  Consistent  Isolated  Durable

© SQLintersection. All rights reserved. Isolation Levels  ANSI Standard Levels  Level 0: Read Uncommitted  Level 1: Read Committed  Level 2: Repeatable Read  Level 3: Serializable  Snapshot / Versioning

© SQLintersection. All rights reserved. Locking Defined  This is how SQL Server handles Isolation  Read Locks  Shared  Concurrent  Write Locks  Exclusive  Isolated  Lock Levels  Table, Page, Row

© SQLintersection. All rights reserved. Blocking Defined  Definition  User A has a lock on Resource X  User B needs a lock on Resource X  Locks are incompatible  This is a Block  This is normal behavior!  What we care about is EXCESSIVE blocking  Blocking Chains  User A block B, B blocks C, and on and on

Demo Locks and Blocks

© SQLintersection. All rights reserved. Preventing Excessive Blocking  Short transactions  Appropriate Indexes  SNAPSHOT isolation level

© SQLintersection. All rights reserved. Deadlocks Defined  User A and User B both have locks that prevent the other’s transaction from completing  Will never clear itself  SQL server handles automatically  Never normal

© SQLintersection. All rights reserved. Deadlock Detection  Lock Monitor Thread  Every 5 seconds (normally)  Looks at blocks  Can the block clear itself  Cyclic blocks would last forever  Chooses a victim if needed  How is the victim chosen  DEADLOCK_PRIORITY  If they are the same priority, the least expensive transaction to rollback

© SQLintersection. All rights reserved. Viewing Deadlock Information  SQL Server Error Log  Trace Flags  1204  1222  SQL Server Profiler  Extended Events  Already on in the default session  XML is different so doesn’t show up as a deadlock graph

Demo Deadlocks

© SQLintersection. All rights reserved. Common Deadlock Causes  Table Order  Key Lookup  Serializable Isolation Level  Parallelism

© SQLintersection. All rights reserved. Recommendations  Minimize blocking  Short transactions  SNAPSHOT when appropriate  Locking is normal  Blocking is normal  Excessive Blocking is not  Deadlocks are not normal behavior

© SQLintersection. All rights reserved. Review  Review  ACID Properties  Transaction Isolation Levels  Defined  Locking  Blocking  Deadlocks  Detection and Prevention  Excessive Blocking  Deadlocks

© SQLintersection. All rights reserved. References  Forums  MSDN  SQLServerCentral  pluralsight.com  SQL Server: Deadlock Analysis and Prevention  Twitter  #sqlhelp, #sqlblog

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