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Trouble with transactions

Trouble with transactions. Evan Jones http:// evanjones.ca. A love story. A short love story. I fell in love …. A short love story. …with transactions. …with transactions.

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Trouble with transactions

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

  2. A love story

  3. A short love story

  4. I fell in love… A short love story

  5. …with transactions

  6. …with transactions 💖 💖 💖

  7. Transactions = correct programs

  8. A four year romance

  9. A four year romance 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

  13. Applications,not databases

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

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

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

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

  18. 💔#1: Weak defaults Transactions = Serializability Postgres: Read committed MySQL/InnoDB: “Repeatable read” (reality: something weird)

  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) Set SERIALIZABLE by default

  21. Example 2: Withdrawal 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()

  22. Example 2: Withdrawal 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() = $1000

  23. Example 2: Withdrawal 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() = $1000 = okay!

  24. Example 2: Withdrawal 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() = $1000 = okay! Exception

  25. Example 2: Withdrawal 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() = $1000 = okay! Exception = $500

  26. Example 2: Withdrawal 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() = $1000 = okay! Exception = $500 💔

  27. Example 2: Withdrawal 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() = $1000 = okay! Exception = $500 💔

  28. 💔#2: Implicit begin some_function committed (my fault) DB automatically started new txn

  29. 💔#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

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

  31. 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/

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