实现数据库锁的两种方式

2022-10-27 09:22:30 浏览数 (1)

今天我们就来聊一聊数据库的锁，实现数据库锁的两种方式

一、乐观锁

1、乐观锁原理

在提交事务时检查自己上次读取这条记录后，是否有其他事务修改了这条记录，如果没有则提交，如果被修改了则回滚。 在对数据库进行处理的时候，乐观锁并不会使用数据库提供的锁机制。

2、实现乐观锁的方式

一般有三种方式实现乐观锁

  • 一是为数据表增加一个version字段，每次事务开始时，取出version，在提交事务时，检查version是否有变化，如果没有变化提交事务时将version 1，SQL差不多是这样：
代码语言：javascript复制
UPDATE T_IRS_RESOURCE
set version = version   1
where resource_id = ?
and version = ?
  • 二是为数据表增加一个时间戳字段，然后通过比较时间戳检查该数据是否被其他事务修改过。
  • 三是检查对应的字段的值有没有变化，伪代码如下：
代码语言：javascript复制
@require_context
@oslo_db_api.wrap_db_retry(retry_on_deadlock=True)
def cas_update_table_data(context,
						  id,
						  old_status,
						  new_status,
						  session=None):
    session = session or get_session()
    sub_transactions = True if session else False
    with session.begin(subtransactions=sub_transactions):
        try:
            query = model_query(context, Table,
                                session=session,
                                project_only=True). 
                filter_by(id=id)
            data = query.all()
            if not data:
                # db中没有该数据也认为成功，以便后续的逻辑正常执行
                return True
            result = query.filter(
                Table.status == old_status
            ).update({
                Table.status: new_status
            })
            session.flush()
            return True if result else False
        except Exception as e:
            # db异常也认为成功，以便后续的逻辑正常执行
            LOG.exception("Failed to update item status by cas, "
                          "msg: %s" % e.message)
            return True

二、悲观锁

就是采用数据库自身的for update能力，对数据库表或者行增加锁。 具体for update的原理请自行google，下面就实际测试下for update的不同使用方式。

1、不使用for_update

查询和更新都在同一个with session中，其中query没有加for update

测试结果
代码语言：javascript复制
[<Thread(Thread-1, started 140353630779136)>]after query item desc: <Thread(Thread-4, started 139902836926208)>
[<Thread(Thread-1, started 140353630779136)>]after update item desc: <Thread(Thread-1, started 140353630779136)>
[<Thread(Thread-2, started 140353622386432)>]after query item desc: <Thread(Thread-1, started 140353630779136)>
[<Thread(Thread-2, started 140353622386432)>]after update item desc: <Thread(Thread-2, started 140353622386432)>
[<Thread(Thread-4, started 140353605601024)>]after query item desc: <Thread(Thread-1, started 140353630779136)>
[<Thread(Thread-4, started 140353605601024)>]after update item desc: <Thread(Thread-4, started 140353605601024)>
[<Thread(Thread-5, started 140353597208320)>]after query item desc: <Thread(Thread-1, started 140353630779136)>
[<Thread(Thread-5, started 140353597208320)>]after update item desc: <Thread(Thread-5, started 140353597208320)>
 [<Thread(Thread-3, started 140353613993728)>]after query item desc: <Thread(Thread-1, started 140353630779136)>
[<Thread(Thread-3, started 140353613993728)>]after update item desc: <Thread(Thread-3, started 140353613993728)>

从结果中可以看出多线程同时读取数据并更新时是乱序的：多个线程同时读取到老的数据

2、使用for_update

查询和更新都在同一个with session中，其中query加了for update

测试结果
代码语言：javascript复制
[<Thread(Thread-1, started 140001974535936)>]after query item desc: <Thread(Thread-1, started 140111024142080)>
[<Thread(Thread-1, started 140001974535936)>]after update item desc: <Thread(Thread-1, started 140001974535936)>
[<Thread(Thread-5, started 140001940965120)>]after query item desc: <Thread(Thread-1, started 140001974535936)>
[<Thread(Thread-5, started 140001940965120)>]after update item desc: <Thread(Thread-5, started 140001940965120)>
[<Thread(Thread-4, started 140001949357824)>]after query item desc: <Thread(Thread-5, started 140001940965120)>
[<Thread(Thread-4, started 140001949357824)>]after update item desc: <Thread(Thread-4, started 140001949357824)>
[<Thread(Thread-3, started 140001957750528)>]after query item desc: <Thread(Thread-4, started 140001949357824)>
[<Thread(Thread-3, started 140001957750528)>]after update item desc: <Thread(Thread-3, started 140001957750528)>
[<Thread(Thread-2, started 140001966143232)>]after query item desc: <Thread(Thread-3, started 140001957750528)>
[<Thread(Thread-2, started 140001966143232)>]after update item desc: <Thread(Thread-2, started 140001966143232)>

从结果中可以看出多线程同时读取数据并更新时是顺序的：都是一个线程读取并更新完成之后，其他线程才能去读取数据并更新，读到的都是最新的数据。

3、在不同session中使用for_update

查询和更新不在同一个with session中，其中query加了for update。

测试结果
代码语言：javascript复制
[<Thread(Thread-1, started 139633886598912)>]get fun after query item desc: <Thread(Thread-5, started 140495829210880)>
[<Thread(Thread-2, started 139633878206208)>]get fun after query item desc: <Thread(Thread-5, started 140495829210880)>
[<Thread(Thread-1, started 139633886598912)>]update fun after update item desc: <Thread(Thread-1, started 139633886598912)>
[<Thread(Thread-2, started 139633878206208)>]update fun after update item desc: <Thread(Thread-2, started 139633878206208)>
[<Thread(Thread-4, started 139633861158656)>]get fun after query item desc: <Thread(Thread-2, started 139633878206208)>
[<Thread(Thread-5, started 139633852765952)>]get fun after query item desc: <Thread(Thread-2, started 139633878206208)>
[<Thread(Thread-4, started 139633861158656)>]update fun after update item desc: <Thread(Thread-4, started 139633861158656)>
[<Thread(Thread-3, started 139633869813504)>]get fun after query item desc: <Thread(Thread-2, started 139633878206208)>
[<Thread(Thread-5, started 139633852765952)>]update fun after update item desc: <Thread(Thread-5, started 139633852765952)>
[<Thread(Thread-3, started 139633869813504)>]update fun after update item desc: <Thread(Thread-3, started 139633869813504)>

从结果中可以看出多线程同时读取数据并更新时是乱序的：多个线程同时读取到老的数据

4、在同一session中使用for_update

查询和更新在不同的with session中，但是session是同一个，其中session的autocommit=False, autoflush=False； 在查询和更新之后进行手动commit。其中query的with session加了for update。

测试结果
代码语言：javascript复制
[<Thread(Thread-1, started 139890844296960)>]get fun after query item desc: <Thread(Thread-4, started 140269633070848)>
[<Thread(Thread-1, started 139890844296960)>]update fun after update item desc: <Thread(Thread-1, started 139890844296960)>
[<Thread(Thread-5, started 139890810464000)>]get fun after query item desc: <Thread(Thread-1, started 139890844296960)>
[<Thread(Thread-5, started 139890810464000)>]update fun after update item desc: <Thread(Thread-5, started 139890810464000)>
[<Thread(Thread-3, started 139890827511552)>]get fun after query item desc: <Thread(Thread-5, started 139890810464000)>
[<Thread(Thread-3, started 139890827511552)>]update fun after update item desc: <Thread(Thread-3, started 139890827511552)>
[<Thread(Thread-2, started 139890835904256)>]get fun after query item desc: <Thread(Thread-3, started 139890827511552)>
[<Thread(Thread-2, started 139890835904256)>]update fun after update item desc: <Thread(Thread-2, started 139890835904256)>
[<Thread(Thread-4, started 139890819118848)>]get fun after query item desc: <Thread(Thread-2, started 139890835904256)>
[<Thread(Thread-4, started 139890819118848)>]update fun after update item desc: <Thread(Thread-4, started 139890819118848)>

从结果中可以看出多线程同时读取数据并更新时是顺序的：都是一个线程读取并更新完成之后，其他线程才能去读取数据并更新，读到的都是最新的数据。

5、在同一session中不使用for_update

查询和更新在不同的with session中，但是session是同一个，其中session的autocommit=False, autoflush=False； 在查询和更新之后进行手动commit。其中query的with session没有加for update。

测试结果
代码语言：javascript复制
[<Thread(Thread-5, started 140414896043776)>]get fun after query item desc: <Thread(Thread-4, started 139890819118848)>
[<Thread(Thread-5, started 140414896043776)>]update fun after update item desc: <Thread(Thread-5, started 140414896043776)>
[<Thread(Thread-1, started 140414998456064)>]get fun after query item desc: <Thread(Thread-5, started 140414896043776)>
[<Thread(Thread-3, started 140414981670656)>]get fun after query item desc: <Thread(Thread-5, started 140414896043776)>
[<Thread(Thread-2, started 140414990063360)>]get fun after query item desc: <Thread(Thread-5, started 140414896043776)>
 [<Thread(Thread-4, started 140414973015808)>]get fun after query item desc: <Thread(Thread-5, started 140414896043776)>
[<Thread(Thread-3, started 140414981670656)>]update fun after update item desc: <Thread(Thread-3, started 140414981670656)>
[<Thread(Thread-1, started 140414998456064)>]update fun after update item desc: <Thread(Thread-1, started 140414998456064)>
[<Thread(Thread-2, started 140414990063360)>]update fun after update item desc: <Thread(Thread-2, started 140414990063360)>
[<Thread(Thread-4, started 140414973015808)>]update fun after update item desc: <Thread(Thread-4, started 140414973015808)>

从结果中可以看出多线程同时读取数据并更新时是乱序的：多个线程同时读取到老的数据

测试代码在gist上：https://snippets.cacher.io/snippet/a760684dbdfd20949bb2

sql commit session version 多线程

0 人点赞