MySQL: Transaction

1. Serializable ( locks all data used by current transaction)

2. repeatable read ( locks all existing records that are/might be scanned by current transaction, blocks all insert/update op, blocks all non-repeatable reads relative to current scanned records)

3. read committed ( only see changes that are committed, no locks. Thus each transaction may change rows based on obsoleted info, resulting a non-trustable state.)

4. read uncommitted ( can see changes that are not yet committed by other transactions)