1. Optimistic Concurrency Internals
Title
Month Year
Optimistic Concurrency Internals
James Rowland-Jones
@jrowlandjones

2. What we will cover in the next hour
Concurrency Basics
Flavours of Optimistic Concurrency
The Internals

3. “the ability of multiple processes to act on the same data at the same time”
Concurrency

4. Read Committed Snapshot Isolation
Essentially same isolation level as Read Committed in Pessimistic Mode
No shared locks when reading data
Once set - default transaction isolation level for that SQL Server database.
The data that the user can read is the data provided at the time the statement inside the transaction starts.

5. Snapshot Isolation Completely New Isolation Level
Designed for Higher degrees of Isolation level than RCSI
Not Good if concurrent Updates are occurring
Fair number of restrictions

6. Row Versioning Used to implement
Online Index Rebuilds
Triggers (inserted and deleted tables use row versioning)
MARS
Prior to updating a row SQL Server copies the row and stores it in the version store
Held till no longer required
Marked by the Transaction ID of the row that caused its creation
Writers bear the brunt of the cost for setting up the version
Readers pay navigating pointer chains
BLOBs aren’t moved into the version store in their entirety
Only fragment that was changed is held.

7. Title
Month Year