1. A Masters view on Locking and blocking
Mikael Wedham
A Masters view on Locking and blocking

2. Thank you to our AWESOME sponsors!

3. Mikael Wedham
Microsoft Certified
Master
Trainer
Developer
Database Administrator
Solution Developer
Systems Engineer
and so on…
Växjö,

4. Locking, the problem
Reading, sometimes Writing Multiple writes Consistent analysis DEMO: Classic lock.

5. Finding the issue - with sys.dm_os_wait_stats
The hard way: Write your own
The easy way:
DEMO…

6. Finding lock details
sp_lock sp_who sp_who2 OR more ”modern” sys.dm_tran_locks sys.dm_exec_sessions

7. Transactions
A C I D

8. Transactions
Always Check It, Dumba$$

9. Transactions
Atomicity Consistency Isolation Durability

10. Isolation levels Different transaction levels for different cases
Reads set Shared (S) locks
Writes set Exclusive (X) locks
Busy Writes sets Update (U) locks, transitions to (X) when alone
Check BOL for compatibility matrix

11. READ COMMITTED (default)
All reads occur on commited data. Two identical SELECT statements, even when encapsulated in a transaction, can return different data.

12. READ UNCOMMITTED Catastrophic, due to data movement mid-select.
Reads without honoring locks.
Problem because rows are not committed
Bigger problem because data may be inconsistent
Catastrophic, due to data movement mid-select.

13. REPEATABLE READ
Locks read data until end of transaction. All read rows will stay the same New rows can appear

14. SERIALIZABLE
All rows and their ”before” and ”after” rows are locked. Prevents new rows from being inserted.

15. ALLOW SNAPSHOT Turning it on makes all the difference in performance
READ COMMITTED SNAPSHOT
Changes only Reads.
Requires no code change
TRANSACTION ISOLATION LEVEL SNAPSHOT
Must be set – and coded for.

16. SNAPSHOT
DEMO

17. InMemory tables
Uses row-versioning similar to snapshot. DEMO

18. Thoughts about solutions for blocking…
Short(er) transactions
Snapshot
In-Memory OLTP
Read-only copy
”Real” BI

19. Thoughts about solutions for blocking…
Short(er) transactions
Snapshot
In-Memory OLTP
Read-only copy
”Real” BI
Questions??

20. Thank You for listening!
Mikael