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Mysql并發(fā)保證數(shù)據(jù)一致性——實(shí)例

1. 背景

最近的項(xiàng)目中遇到一項(xiàng)問(wèn)題，并發(fā)更新某一單據(jù)的時(shí)候，出現(xiàn)了更新失效的情況。比如：

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@Transactional(rollbackFor = Exception.class)
public void update(Integer id){
    //1.按id查詢
    //2.更新某一字段的值
}

生成的SQL大概是這樣的：

UPDATE  table 
SET field = #{field,jdbcType=INTEGER}
WHERE id= 1

那么以上代碼產(chǎn)生的問(wèn)題就是：
對(duì)于同一個(gè)id=1來(lái)說(shuō)，請(qǐng)求A與請(qǐng)求B都進(jìn)到了update方法中，此時(shí)按id查詢得到的信息是相同的，那么請(qǐng)求A更新了id=1的這條記錄之后，此時(shí)，請(qǐng)求B又對(duì)id=1的記錄進(jìn)行更新，此時(shí)注意請(qǐng)求B更新id=1的記錄的時(shí)候，請(qǐng)求B按id查詢到的數(shù)據(jù)已經(jīng)是舊數(shù)據(jù)了，請(qǐng)求B執(zhí)行完之后，請(qǐng)求A的更新被請(qǐng)求B覆蓋了。

如果更新的是狀態(tài)，那倒無(wú)所謂，如果庫(kù)存量呢？
如果更新的是庫(kù)存，是在這條記錄的原始值上進(jìn)行+或者-操作，是不是就出問(wèn)題了？

2. 分析產(chǎn)生的原因

事后寫(xiě)了個(gè)小demo來(lái)復(fù)現(xiàn)問(wèn)題，代碼大致如下：

需求： 某個(gè)分類每被訪問(wèn)一次，排序就+1；
開(kāi)啟兩個(gè)客戶端同時(shí)訪問(wèn)

@Transactional(rollbackFor = Exception.class)
public void updateStatusTest(Integer id) {
    System.out.println("start:"+Thread.currentThread().getName());
    Category category = categoryMapper.selectById(id);
    try {
        Thread.sleep(8000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    System.out.println(Thread.currentThread().getName()+":"+category);
    category.setSort(category.getSort()+1);
    categoryMapper.updateById(category);

    category = categoryMapper.selectById(id);
    System.out.println(Thread.currentThread().getName()+":"+category);
    System.out.println("end:"+Thread.currentThread().getName());
}

DAO層使用的mybatis，生成日志大致如下：
需要更新的sort值數(shù)據(jù)庫(kù)初始為：0

start:http-nio-8062-exec-1
Creating a new SqlSession
Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@2d5c951c] will be managed by Spring
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
 Time：27 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

start:http-nio-8062-exec-2
Creating a new SqlSession
Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046]
JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@700c5b36] will be managed by Spring
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
 Time：2 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046]
http-nio-8062-exec-1:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9] from current transaction
==>  Preparing: UPDATE t_admin_category SET category_name=?, parent_id=?, image_url=?, icon_url=?, sort=?, status=? WHERE cid=? 
==> Parameters: 1(String), 0(Integer), 11111(String), 1(String), 1(Integer), 1111(String), 6(Integer)
 Time：16 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.updateById
Execute SQL：
    UPDATE
        t_admin_category 
    SET
        category_name='1',
        parent_id=0,
        image_url='11111',
        icon_url='1',
        sort=1,
        status='1111' 
    WHERE
<==    Updates: 1
        cid=6

Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9] from current transaction
    SELECT
        cid,
        category_name,
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
        parent_id,
==> Parameters: 6(Integer)
        image_url,
        icon_url,
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
        sort,
        status 
<==        Row: 6, 1, 0, 11111, 1, 1, 1111
    FROM
<==      Total: 1
        t_admin_category 
    WHERE
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
        cid=6

http-nio-8062-exec-1:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=1, status='1111'}
end:http-nio-8062-exec-1
Transaction synchronization committing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
Transaction synchronization deregistering SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
Transaction synchronization closing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6df7e3e9]
http-nio-8062-exec-2:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046] from current transaction
==>  Preparing: UPDATE t_admin_category SET category_name=?, parent_id=?, image_url=?, icon_url=?, sort=?, status=? WHERE cid=? 
 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.updateById
Execute SQL：
    UPDATE
        t_admin_category 
    SET
        category_name='1',
        parent_id=0,
        image_url='11111',
        icon_url='1',
        sort=1,
        status='1111' 
    WHERE
        cid=6

==> Parameters: 1(String), 0(Integer), 11111(String), 1(String), 1(Integer), 1111(String), 6(Integer)
<==    Updates: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046]
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046] from current transaction
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
 Time：16 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
<==      Total: 1
        parent_id,
        image_url,
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@7af43046]
        icon_url,
        sort,
        status 
    FROM
http-nio-8062-exec-2:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
        t_admin_category 
end:http-nio-8062-exec-2
    WHERE
        cid=6

大致意思：
請(qǐng)求A與請(qǐng)求B，同時(shí)操作id=6的這條記錄時(shí)，查詢出來(lái)id=6的這條記錄，然后請(qǐng)求A對(duì)其進(jìn)行了修改 SET　sort=1操作，完事之后，請(qǐng)求A的事務(wù)提交了。而此時(shí)請(qǐng)求B，通過(guò)id=6條件查詢到的category的信息是請(qǐng)求A提交之前的，此時(shí)請(qǐng)求B執(zhí)行了SET　sort=1，覆蓋了請(qǐng)求B的修改操作。

3. 解決思路

解決方法其它很簡(jiǎn)單，我們上面可以看到，update語(yǔ)句是直接set sort=1的，我們只需要改為set sort = sort +1 就可以了。你不信？那我改一下試試：

@Transactional(rollbackFor = Exception.class)
public void updateStatusTest(Integer id) {
    System.out.println("start:"+Thread.currentThread().getName());
    Category category = categoryMapper.selectById(id);
    try {
        Thread.sleep(8000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    System.out.println(Thread.currentThread().getName()+":"+category);
//        category.setSort(category.getSort()+1);
    categoryMapper.updateStatusById(category);

    category = categoryMapper.selectById(id);
    System.out.println(Thread.currentThread().getName()+":"+category);
    System.out.println("end:"+Thread.currentThread().getName());
}

Mapper文件：


      UPDATE
        t_admin_category
      SET
        sort =sort + 1
    WHERE
        cid= #{cid,jdbcType=INTEGER}

我們執(zhí)行試一下，看下日志：

start:http-nio-8062-exec-1
Creating a new SqlSession
Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@6c17839b] will be managed by Spring
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
 Time：43 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

start:http-nio-8062-exec-2
Creating a new SqlSession
Registering transaction synchronization for SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022]
JDBC Connection [com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl@195f8b5a] will be managed by Spring
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 0, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022]
 Time：2 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

http-nio-8062-exec-1:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f] from current transaction
==>  Preparing: UPDATE t_admin_category SET sort =sort + 1 WHERE cid= ? 
==> Parameters: 6(Integer)
<==    Updates: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f] from current transaction
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.updateStatusById
Execute SQL：
    UPDATE
        t_admin_category 
    SET
        sort =sort + 1 
    WHERE
        cid= 6

==> Parameters: 6(Integer)
 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
Execute SQL：
<==        Row: 6, 1, 0, 11111, 1, 1, 1111
    SELECT
<==      Total: 1
        cid,
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
        category_name,
        parent_id,
http-nio-8062-exec-1:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=1, status='1111'}
        image_url,
end:http-nio-8062-exec-1
        icon_url,
        sort,
Transaction synchronization committing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
        status 
    FROM
Transaction synchronization deregistering SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
        t_admin_category 
    WHERE
Transaction synchronization closing SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@5e925f3f]
        cid=6

 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.updateStatusById
Execute SQL：
    UPDATE
        t_admin_category 
    SET
        sort =sort + 1 
    WHERE
        cid= 6

 Time：0 ms - ID：com.sxl.simple.shop.admin.category.mapper.CategoryMapper.selectById
Execute SQL：
    SELECT
        cid,
        category_name,
        parent_id,
        image_url,
        icon_url,
        sort,
        status 
    FROM
        t_admin_category 
    WHERE
        cid=6

http-nio-8062-exec-2:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=0, status='1111'}
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022] from current transaction
==>  Preparing: UPDATE t_admin_category SET sort =sort + 1 WHERE cid= ? 
==> Parameters: 6(Integer)
<==    Updates: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022]
Fetched SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022] from current transaction
==>  Preparing: SELECT cid,category_name,parent_id,image_url,icon_url,sort,status FROM t_admin_category WHERE cid=? 
==> Parameters: 6(Integer)
<==    Columns: cid, category_name, parent_id, image_url, icon_url, sort, status
<==        Row: 6, 1, 0, 11111, 1, 2, 1111
<==      Total: 1
Releasing transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6c88f022]
http-nio-8062-exec-2:Category{cid=6, categoryName='1', parentId=0, imageUrl='11111', iconUrl='1', sort=2, status='1111'}
end:http-nio-8062-exec-2

看到了吧，保證數(shù)據(jù)一致了吧

我之前的文章有對(duì)MySQL事務(wù)這塊做過(guò)詳解，想知道真正原理的小伙伴請(qǐng)閱讀： 深入理解mysql事務(wù)
注意，我這里使用的數(shù)據(jù)庫(kù)是mysql8以上，應(yīng)用服務(wù)器是單機(jī)版的。


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