Database isolation levels
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Database isolation levels
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Database isolation levels
- 2. ACID
- 3. ACID
- 6. ACID - ATOMIC - CONSISTENT - ISOLATED
- 7. ACID - ATOMIC - CONSISTENT - ISOLATED - DURABLE
- 8. ACID - ATOMIC - CONSISTENT - ISOLATED - DURABLE
- 9. IS THE BELOW CODE CORRECT?
- 10. ANOTHER QUERY
- 11. IT DEPENDS!!!
- 13. Dirty read
- 14. Prevent dirty read with lock
- 15. Prevent dirty read with committed/uncommitted flag
- 16. Prevent dirty read with committed/uncommitted flag READ COMMITTED LEVEL!!!
- 19. MVCC to prevent non-repeatable read
- 20. MVCC - Multiversion Concurrency Control ● When a record is created (using INSERT), it has created_id as the current transaction_id and deleted_id as nil. ● When a record is updated, a new row is created with created_id as the current transaction_id and deleted_id as nil. The old row is updated with deleted_id as the current transaction_id. ● When a record is deleted, deleted_id of the row is updated with current transaction_id. ● When reading, we filter all versions of the row using 2 criterias: ○ row must have created_id <= current transaction id ○ row must have deleted_id > current transaction id or nil
- 21. Lost update?
- 22. Raise error about inconsistency?
- 23. MVCC to prevent non-repeatable read REPEATABLE READ !!!
- 25. Phantom read
- 28. Other solutions?
- 29. Other solutions?
- 30. Summary Isolation Level Attributes Read Uncommitted - Fast - A lot of consistency issues Read Committed - Good performance - Non-repeatable read & Phantom read Repeatable Read - Ok performance - Phantom read Serializable - Slow - Almost no consistency issues
- 31. Why do I need to know? - ACID applies for all storage system, not just databases - Understand limitation and consistency attributes of the tools help: - Design better system: make good trade-off - Design better test: Jepsen test
Notes de l'éditeur
- Many databases claim that they supports ACID, like Postgres, MySQL, MongoDB and others
- However, do they always support it?
- What is ACID, by the way?
- We won’t talk about other attributes today, as we’ll focus only in “ISOLATED” attribute
- Assume that the syntax is correct, the records are created with correct schema and all libraries are loaded
- This query is in another thread, which run concurrently with the thread that run the first query
- Depend on what? Multithread or not Isolation level of database
- Go back to locking solution?
- MVCC is Multiversion Concurrency Control
- This happen in MySQL InnoDB
- Postgres does this
- MVCC is Multiversion Concurrency Control
- This should be a range lock. MySQL InnoDB implements this. MyISAM uses table locks
- Greater concurrency in most cases, but much worse throughput in worst case. Use together with MVCC. Postgres 9.5 or higher supports this.
- Redis uses single thread to write, which eliminates all possibility of conflict with an extremely simple design. Redis cluster doesn’t guarantee any serializable because of this design