are you optimistic about concurrency - wordpress.com · 2016. 10. 8. · 2016-10-08 2 08.10.2016...

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08.10.2016 SQLSaturday #555 Munich 2016

Are You OPTIMISTIC About Concurrency?

Isolation Levels with SQL Server In-Memory OLTP

Kalen Delaneywww.SQLServerInternals.com


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Kalen DelaneyBackground:

MS in Computer Science from UC Berkeley Working exclusively with SQL Server for 28 years

Server In-Memory OLTP Internals (RedGate 2014) Primary Author: SQL Server 2012 Internals (MS Press/O’Reilly, 2013) Author: SQL Server Concurrency (RedGate 2010) Primary Author: SQL Server 2008 Internals (MS Press, 2009) Primary Author: Inside SQL Server 2005: Query Tuning and Optimization (MS

Press, 2007) Author: Inside SQL Server 2005: The Storage Engine (MS Press, 2006) SQL Server Magazine columnist and contributing editor

Website: www.SQLServerInternals.comTwitter: @sqlqueenBlog: www.SQLBlog.com


AbstractWhat exactly does it mean to have optimistic concurrency? What is the alternative? Is SQL Server 2012's SNAPSHOT Isolation optimistic? How can SQL Server 2014's In-Memory OLTP provide truly optimistic concurrency?

In this session, we'll look at what guarantees the various isolation levels provide, the difference between pessimistic and optimistic concurrency, and the new data structures in SQL Server 2014 that allow the enormous benefits of having totally in-memory storage with no waiting!

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Main Topics

Isolation Levels and Concurrency Models

Pessimistic Concurrency

Optimistic Concurrency with In-Memory OLTP



Consistency Guarantees

Bad Dependencies

Isolation Levels

Concurrency Models

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Consistency Guarantees

Read Stability

Phantom Avoidance


Bad Dependencies

Lost Updates

Dirty Reads

Non-repeatable Reads

Phantoms

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Isolation LevelsPhenomena Allowed

Isolation LevelsDirty Read

Non-Repeatable Read Phantoms Update

Conflict

READ UNCOMMITTED Yes Yes Yes No

READ COMMITTED

1 Locking2 Snapshot

NoNo

YesYes

YesYes

NoNo

SNAPSHOT No No (but not read stability)

Yes (but not visible) Yes

REPEATABLE READ No No Yes No

SERIALIZABLE No No No No


Concurrency Models


Optimistic Concurrency

SQL Server 2005-2012 SNAPSHOT ISOLATION options are not fully optimistic

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Locks are used to guarantee consistency Changes to isolation level affect: Lock conflicts Duration of locks Type of locks


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Optimistic Concurrency

No locks needed Data in existing rows is never changed Updates are implemented as DELETE + INSERT DELETE changes end-timestamp of row

No latches needed for In-memory OLTP No pages used! Internal lock table not needed


In-Memory OLTP Table Storage

Hash Table for Name

E Elizabeth San Francisco $80K20 ∞

John Charlotte $50K∞10J

Jane N.Y. $75K30 ∞

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Multiversioning

E Elizabeth San Francisco $80K20 ∞

ElizabethElizabeth San FranciscoSan Francisco $90K$90KElizabeth San Francisco $90K∞

Xb

Xb

Hash Table for Name


Consistency ValidationOnly three isolation levels possible Snapshot Isolation No validation required Fail immediately if conflict occurs

Repeatable Read Verify Read Stability

Serializable Verify Read Stability Confirm Phantom Avoidance

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Validation Requirements

During Processing, In-Memory OLTP maintains: Read-set Write-set Scan-set

End-timestamp of transaction acquired before validation starts


Validating Read Stability

Confirm that each row version in read-set is still visible during validation using end-timestamp

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Confirming Phantom Avoidance

Repeat each scan checking whether new versions have become visible since transaction began

Not as expensive as it might seem: All rows are in memory Re-resolving of scan-set can be simplified Only done for transactions in serializable isolation Less expensive than range locking


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Isolation LevelsPhenomena Allowed

Isolation LevelsDirty Read

Non-Repeatable Read Phantoms Update

Conflict

READ UNCOMMITTED Yes Yes Yes No

READ COMMITTED

1 Locking2 Snapshot

NoNo

YesYes

YesYes

NoNo

SNAPSHOT No No (but not read stability)

Yes (but not visible) Yes

REPEATABLE READ No No Yes No

SERIALIZABLE No No No No


Summary



Optimistic Concurrency with In-Memory OLTP

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Save the Dates!

PASS Camp 2016 - 06. to 09. December 2016

SQL Konferenz 2017 - 14. to 16. February 2017

are you optimistic about concurrency - wordpress.com · 2016. 10. 8. · 2016-10-08 2 08.10.2016...

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