快照隔离与可序列化

DBMS 中的事务隔离级别有助于克服脏读、幻读和不可重复读等并发问题。快照和可序列化隔离级别都被认为是最高的隔离级别，因为它们都避免了所有并发问题，但方式却截然不同。

快照隔离：

顾名思义，“快照”允许同时发生的事务看到与事务开始时相同的数据库快照或副本。因此，允许第二个事务对另一个并发事务要读取的数据进行更改。这个其他事务不会观察到第二个事务所做的更改，并将继续处理数据库的前一个快照。

可序列化：

可序列化执行之后的事务虽然并发运行，但看起来好像它们以串行顺序运行。可序列化使用锁来对事务进行读写操作。锁确保在完成之前，不允许其他并发事务修改持有锁的事务使用的数据。

快照隔离和可序列化的区别：

S.No.	Snapshot Isolation	Serializable
1.	In Snapshot, the SQL server avoids locks by making use of row versioning.	In Serializable, the SQL server makes use of locks and holds them until the transaction is committed or aborted.
2.	Follows optimistic concurrency control.	Follows pessimistic concurrency control.
3.	The concurrency level is high in comparison to serializable.	Low-level of concurrency is achieved as one transaction needs to wait for the completion of another transaction.
4.	Since no locks are imposed on data when it is read, other concurrent transactions are allowed to write data at the same time without any conflicts.	If two transactions try to read and write data at the same time then a deadlock occurs and one of the transactions is killed or rolled back as a deadlock victim.
5.	If two transactions try to update the same record at the same time then an update conflict occurs and the SQL server had to kill one of the transactions.	If two transactions try to update the same record then the second transaction will wait for the first transaction to either rollback or commit.