事务的隔离级别(The isolation level of transaction)

The isolation level of transaction is problem to understand, and what is the isolation, and what is the isolation level? Maybe some document and information talk about this problem, or try to describe the conception to us. But it is still a real problem. As my practice, it puzzle me a long time.

Firstly, let us know what is the isolation, and what is the meaning of the isolation of transaction.

Maybe you know, the character of transaction is ACID. i means the isolation. that point each action in transation must be isolated from each other. if we don't support the isolation in the transaction. we maybe encounter these problem as below.

1. Lose Update - Sometime we have lost the updat after update some data. For example, Person A update a record. and change one filed from 1 to 2. And B person update the same record too at same time, but a bit later than person A. he/her update the data from 1 to 3. Certainly some database add lock for the record when it's updated. therefore avoid this problem rightly.

2. Dirty read -- Sometime we find some data that havn't comminted into our database. That is dirty read. This action is a serious problem.

3. Unrepeat read -- Sometime we find that the reulst is different from between this time and last time in same transation, As some other transaction be done during the period.l

4. Image read --