Isolation (one of ACID) is an important attribute of Database. Understanding it helps us understand the database behavior and trade-offs.

1. Understanding Isolation Levels
By Hieu Nguyen

2. Which isolation
level our database
currently is?

3. ACID

4. Vestibulum congue
ACID
Durable
Isolated
Consistent
Atomic

5. Vestibulum congue
ACID
Durable
Isolated
Consistent
Atomic

6. Isolation ● A transaction is invisible to other
transactions before it is
completed

7. Isolation
● A transaction is invisible to other
transactions before it is
completed
● The statement above is usually
true

8. Isolation Levels

9. Serializable
04
Repeatable Read
(Snapshot Isolation)03
Read committed
02
Read uncommitted
(Dirty Read)01

10. Query 1
BEGIN TRANSACTION
SET balance = balance - 100 FROM users WHERE users.name = 'A';
SET balance = balance + 100 FROM users WHERE users.name = 'B';
END

11. Query 2
BEGIN TRANSACTION
SELECT name, balance FROM users WHERE users.name IN ('A', 'B');
END

13. Serializable
04
Repeatable Read
(Snapshot Isolation)03
Read committed
02
Read uncommitted
(Dirty Read)01
● Has no concurrency control
● No overhead
● Bad data consistency

15. Serializable
04
● Use locks for concurrency control
● High overhead
● Good consistency
Repeatable Read
(Snapshot Isolation)03
Read committed
02
Read uncommitted
(Dirty Read)01
● Has no concurrency control
● No overhead
● Bad data consistency

16. Serializable
04
● Use locks for concurrency control
● High overhead
● Good consistency
Repeatable Read
(Snapshot Isolation)03
Read committed
02
Read uncommitted
(Dirty Read)01
● Has no concurrency control
● No overhead
● Bad data consistency
?

18. Serializable
04
● Use locks for concurrency control
● High overhead
● Good consistency
Repeatable Read
(Snapshot Isolation)03
Read committed
02
● Use committed/uncommitted state for CC
● Very low overhead
● Prevent Dirty Read
● Has Non-repeatable Read issue
Read uncommitted
(Dirty Read)01
● Has no concurrency control
● No overhead
● Bad data consistency

19. Query 1
BEGIN TRANSACTION
INSERT INTO mails (content, read) VALUE (content, FALSE);
SET notifications_count = (SELECT COUNT(*) FROM mails WHERE read IS FALSE)
FROM mailboxes;
END

20. Query 2
BEGIN TRANSACTION
SELECT notifications_count FROM mailboxes;
SELECT * FROM mails WHERE read IS FALSE;
END

24. Multiversion Concurrency Control (MVCC)
● 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

27. Serializable
04
● Use locks for concurrency control
● High overhead
● Good consistency
Repeatable Read
(Snapshot Isolation)03
● Use MVCC
● Low overhead
● Prevent Dirty Read & Non-repeatable Read
● Prevent certain Phantom Read issues
Read committed
02
● Use committed/uncommitted state for CC
● Very low overhead
● Prevent Dirty Read
● Has Non-repeatable Read issue
Read uncommitted
(Dirty Read)01
● Has no concurrency control
● No overhead
● Bad data consistency

28. Prevent data inconsistency for non-serialization
● Application level of concurrency control
● Scheduled audit to check bad data

29. Usage

30. Read
uncommitted
● Not recommend
● Should only be used in very
special cases

31. Read committed
● Not recommend
● Used in application which
required very fast read/write

32. Repeatable Read ● Recommended for most OLTP
application

33. Serializable ● Recommended for data-critical
application like banking

34. Conclusion

35. Conclusion
● Isolation Level is level of
invisibility of a transaction to
another transaction
● There are 4 levels defined by SQL
● Each level has trade-off in
consistency and concurrency
● Aware of data inconsistency
problem of non-serialization
isolation levels

36. Thank you for listening!