数据库操作的隔离级别 Transaction Isolation Levels

by yan 20220223

• ANSI/ISO Transaction Isolation Levels(ANSI/ISO标准的隔离级别)
  （1）未提交读（read uncommitted）
  （2）提交读 或 不可重复读（read committed）
  （3）重复读（repeatable read）
  （4）序列化（Serializable）

• isolation levels 的一些表现
  （1）Dirtyreads（脏读）
  Atransaction reads data that has been written by another transaction that hasnot been committed yet.
  –一个事务读取另一个事务已经修改但为提交的数据
  （2）Nonrepeatable (fuzzy) reads（模糊读）
  Atransaction rereads data it has previously read and finds that anothercommitted transaction has modified or deleted the data. For example, a userqueries a row and then later queries the same row, onlyto discover that the data has changed.
  –当事务重读之前的数据时，发现这个事务被另一个事务修改，如modified 或者deleted，并且已经提交。
  （3）Phantomreads
  Atransaction reruns a queryreturning a set of rows that satisfies a search condition and finds thatanother committed transaction has inserted additional rows that satisfy thecondition.
  –同一查询在同一事务中多次进行，由于其他提交事务所做的插入操作，每次返回不同的结果集，此时发生幻像读。

• 隔离级别与并发性
  隔离级别与并发性是互为矛盾的：隔离程度越高，数据库的并发性越差；隔离程度越低，数据库的并发性越好。
  

• java里的定义

//java.sql.Connection.java

 /**
     * A constant indicating that transactions are not supported.
     */
    int TRANSACTION_NONE             = 0;

    /**
     * A constant indicating that
     * dirty reads, non-repeatable reads and phantom reads can occur.
     * This level allows a row changed by one transaction to be read
     * by another transaction before any changes in that row have been
     * committed (a "dirty read").  If any of the changes are rolled back,
     * the second transaction will have retrieved an invalid row.
     */
    int TRANSACTION_READ_UNCOMMITTED = 1;

    /**
     * A constant indicating that
     * dirty reads are prevented; non-repeatable reads and phantom
     * reads can occur.  This level only prohibits a transaction
     * from reading a row with uncommitted changes in it.
     */
    int TRANSACTION_READ_COMMITTED   = 2;

    /**
     * A constant indicating that
     * dirty reads and non-repeatable reads are prevented; phantom
     * reads can occur.  This level prohibits a transaction from
     * reading a row with uncommitted changes in it, and it also
     * prohibits the situation where one transaction reads a row,
     * a second transaction alters the row, and the first transaction
     * rereads the row, getting different values the second time
     * (a "non-repeatable read").
     */
    int TRANSACTION_REPEATABLE_READ  = 4;

    /**
     * A constant indicating that
     * dirty reads, non-repeatable reads and phantom reads are prevented.
     * This level includes the prohibitions in
     * {@code TRANSACTION_REPEATABLE_READ} and further prohibits the
     * situation where one transaction reads all rows that satisfy
     * a {@code WHERE} condition, a second transaction inserts a row that
     * satisfies that {@code WHERE} condition, and the first transaction
     * rereads for the same condition, retrieving the additional
     * "phantom" row in the second read.
     */
    int TRANSACTION_SERIALIZABLE     = 8;