OPTIMISTIC AND MULTIVERSION CONCURRENCY CONTROL

Transcription

1 OPTIMISTIC AND MULTIVERSION CONCURRENCY CONTROL With PostgreSQL Seminar DBS I, Presentation 2 Gian Poltéra Student, Master of Science in Engineering MRU Software & Systems Rapperswil, 06. June 2014

2 Agenda Introduction MVCC MVCC in PostgreSQL Practical Part Demo Conclusions and Outlook 2

3 Introduction Optimistic Concurrency Control (OCC) Parallel access of all users with read access Privileges the user who first modified a record Mainly suitable for read only accesses Pessimistic Concurrency -> read and write access locked Multiversion Concurrency Control (MVCC) Expansion of OCC No blocking No endanger the consistency of the database Never waiting for a database object Internally, different versions of the object are kept Mainly suitable for heavily used read only databases 3

4 MVCC MVCC Behavior 4

5 MVCC Snapshot Timeline Highest committed Transaction-ID: 100 Open Transactions: 25, 50, 75 5

6 MVCC Snapshot Row Visability 6

7 MVCC Advantages Readers never block writers Writers never block readers Reading at any time, without waiting Disadvantages Intensive memory usage for storing multiple data versions Needs clean up Xid «only» 32-bit (4 billion transactions) 7

8 MVCC Speed Measurement with MSSQL 2008 (MVCC ON/OFF) 8

9 MVCC in PostgreSQL Insert Delete 9

10 MVCC in PostgreSQL Update Rollback 10

11 MVCC in PostgreSQL Status Flags 11

12 MVCC in PostgreSQL Multi-Statement Transactions 12

13 Practical Part Dirty Read Nonrepeatable Read Phantom Read Lost Update Read Skew Write Skew 13

14 Practical Part Dirty Read Time Transaction T 1 Transaction T 2 1 READ x 2 UPDATE x 3 READ x 4 ROLLBACK Nonrepeatable Read Time Transaction T 1 Transaction T 2 1 READ x 2 UPDATE x 3 COMMIT 4 READ x 14

15 Practical Part Phantom Read Time Transaction T 1 Transaction T 2 1 READ (row count) 2 INSERT 3 COMMIT 4 READ (row count) Lost Update Time Transaction T 1 Transaction T 2 1 READ x 2 READ x 3 UPDATE x 4 UPDATE x 5 COMMIT 6 COMMIT 15

16 Practical Part Read Skew Time Transaction T 1 Transaction T 2 1 READ x 2 UPDATE x, y 3 COMMIT 4 READ y Write Skew Time Transaction T 1 Transaction T 2 1 READ x, y 2 READ x, y 3 UPDATE x 4 COMMIT 5 UPDATE y 6 COMMIT 16

17 Practical Part Anomalies Isolation Level Dirty Read Lost Updates Nonrepeatable Read Phantom Read Read uncommitted Possible Possible Possible Possible Read committed Not Possible Possible Possible Possible Repeatable read Not Possible Not Possible Not Possible Possible Serializable Not Possible Not Possible Not Possible Not Possible MVCC Isolation Level Dirty Read Nonrepeatable Read Phantom Read Lost Updates Read committed Not Possible Possible Possible Possible Repeatable read Not Possible Not Possible Not Possible Possible Serializable Not Possible Not Possible Not Possible Not Possible 17

18 Demo 18

19 Conclusions and Outlook MVCC is ideal for many reads and few writes. Reading is every time possible. Speed of updating is about 20 times slower. Increased memory space and maintenance requirements. Four anomalies were successfully implemented. Adaptation of isolation level in PostgreSQL very easily possible Concept for MVCC in 1978 and Oracle since Microsoft since PostgreSQL since version 6.5 in No new technology to replace the MVCC predictable. Trend non-relational databases (NoSQL), many also use MVCC. 19

20 References B. Momjian, MVCC Unmasked, Pennsylvania, H. Berenson, P. Bernstein, J. Gray, J. Melton, E. O'Neil und P. O'Neil, A critique of ANSI SQL isolation levels, Microsoft Corporation, Redmond, K. Douglas und D. Susan, PostgreSQL: a comprehensive guide to building, programming, and administering PostgresSQL databases, SAMS publishing, B. Momjian, SQL Script, [Online]. Available: B. Helmle, VACUUM FULL, [Online]. Available: PostgreSQL, Transaction Isolation, [Online]. Available: PostgreSQL, SSI, [Online]. Available: L. Frank, Countermeasures against isolation anomalies, Department of Informatics Copenhagen Business School, Copenhagen, R. David P., Naming and synchronization in a decentralized computer system, Massachusetts Institute of Technology, Cambridge, P. A. Bernstein und N. Goodman, Concurrency control in distributed database systems, (CSUR), ACM Computing Surveys, pp ,