1. Trouble with transactions Evan Jones http://evanjones.ca

2. A love story

3. A short love story

4. I fell in love…

5. …with transactions

7. Transactions = correct programs

8. A four year romance

9. Main memory transactions

10. A four year romance Main memory transactions Automatic partitioning

11. A four year romance Main memory transactions Consolidating workloads Automatic partitioning

12. A four year romance Main memory transactions Consolidating workloads Automatic partitioning

14. Applications, not databases

15. connection.begin_transaction() print connection.query_balance() #... do some computation... print connection.query_balance() connection.commit() Example: Read bank balance

16. connection.begin_transaction() print connection.query_balance() #... do some computation... print connection.query_balance() connection.commit() = $1000 Example: Read bank balance

17. connection.begin_transaction() print connection.query_balance() #... do some computation... print connection.query_balance() connection.commit() = $1000 = $1500 Example: Read bank balance

18. connection.begin_transaction() print connection.query_balance() #... do some computation... print connection.query_balance() connection.commit() = $1000 = $1500 Example: Read bank balance

19. #1: Weak defaults Transactions = Serializability Postgres: Read committed MySQL/InnoDB: “Repeatable read” (reality: something weird)

20. #1: Weak defaults Transactions = Serializability Postgres: Read committed MySQL/InnoDB: “Repeatable read” (reality: something weird)

21. #1: Weak defaults Transactions = Serializability Postgres: Read committed MySQL/InnoDB: “Repeatable read” (reality: something weird) Set SERIALIZABLE by default

22. try: connection.begin_transaction() balance = connection.query_balance() connection.update_balance(balance – 500) some_function(connection) connection.commit() except e: connection.rollback() print connection.query_balance() Example 2: Withdrawal

23. try: connection.begin_transaction() balance = connection.query_balance() connection.update_balance(balance – 500) some_function(connection) connection.commit() except e: connection.rollback() print connection.query_balance() Example 2: Withdrawal = $1000

24. try: connection.begin_transaction() balance = connection.query_balance() connection.update_balance(balance – 500) some_function(connection) connection.commit() except e: connection.rollback() print connection.query_balance() Example 2: Withdrawal = $1000 = okay!

25. try: connection.begin_transaction() balance = connection.query_balance() connection.update_balance(balance – 500) some_function(connection) connection.commit() except e: connection.rollback() print connection.query_balance() Example 2: Withdrawal = $1000 = okay! Exception

26. try: connection.begin_transaction() balance = connection.query_balance() connection.update_balance(balance – 500) some_function(connection) connection.commit() except e: connection.rollback() print connection.query_balance() Example 2: Withdrawal = $1000 = okay! Exception = $500

27. try: connection.begin_transaction() balance = connection.query_balance() connection.update_balance(balance – 500) some_function(connection) connection.commit() except e: connection.rollback() print connection.query_balance() Example 2: Withdrawal = $1000 = okay! Exception = $500

28. try: connection.begin_transaction() balance = connection.query_balance() connection.update_balance(balance – 500) some_function(connection) connection.commit() except e: connection.rollback() print connection.query_balance() Example 2: Withdrawal = $1000 = okay! Exception = $500

29. #2: Implicit begin some_function committed (my fault) DB automatically started new txn

30. #2: Implicit begin some_function committed (my fault) DB automatically started new txn If you begin, you commit Like memory in C/C++ Nested transactions can help Don’t implicitly start transactions

31. Transactions: Use with care Communication with external systems Accidental long running transactions Retry loops for concurrency errors

32. Transactions: Use with care Communication with external systems Accidental long running transactions Retry loops for concurrency errors Make it hard to use systems incorrectly http://evanjones.ca/