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.NET中關(guān)于臟讀不可重復(fù)讀與幻讀的代碼示例

并發(fā)可能產(chǎn)生的三種問(wèn)題

臟讀

定義：A事務(wù)執(zhí)行過(guò)程中B事務(wù)讀取了A事務(wù)的修改，但是A事務(wù)并沒(méi)有結(jié)束（提交），A事務(wù)后來(lái)可能成功也可能失敗。

比喻：A修改了源代碼并且并沒(méi)有提交到源代碼系統(tǒng)，A直接通過(guò)QQ將代碼發(fā)給了B，A后來(lái)取消了修改。

代碼示例

復(fù)制代碼代碼如下:

[TestMethod]
         public void 臟讀_測(cè)試()
         {
             //前置條件
             using (var context = new TestEntities())
             {
                 Assert.AreEqual(1, context.Tables.Count());
             }

             var autoResetEvent = new AutoResetEvent(false);

             var transactionOptions1 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted };
             var transactionOptions2 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadUncommitted };

             using (var ts1 = new TransactionScope(TransactionScopeOption.Required, transactionOptions1))
             {
                 //添加數(shù)據(jù)
                 using (var context = new TestEntities())
                 {
                     context.Tables.Add(new Table() { Id = Guid.NewGuid(), Name = "段光偉" });
                     context.SaveChanges();
                 }

                 ThreadPool.QueueUserWorkItem(data =>
                 {
                     using (var ts2 = new TransactionScope(TransactionScopeOption.Required, transactionOptions2))
                     {
                         //臟讀測(cè)試
                         using (var context = new TestEntities())
                         {
                             Assert.AreEqual(2, context.Tables.Count());
                         }
                     }

                     autoResetEvent.Set();
                 });

                 autoResetEvent.WaitOne();
             }

             //前置條件
             using (var context = new TestEntities())
             {
                 Assert.AreEqual(1, context.Tables.Count());
             }
         }

不可重復(fù)讀

定義：A事務(wù)讀取了兩次數(shù)據(jù)，在這兩次的讀取過(guò)程中B事務(wù)修改了數(shù)據(jù)，A事務(wù)的這兩次讀取出來(lái)的數(shù)據(jù)不一樣了（不可重復(fù)讀）。

比喻：A在做源代碼審查，在審查的過(guò)程中獲取了兩次源代碼，在這兩次獲取期間B修改了源代碼，B修改的很可能是A審查過(guò)的代碼，而這部分代碼可能不符合規(guī)范了。

代碼示例

復(fù)制代碼代碼如下:

[TestMethod]
         public void 不可重復(fù)讀_測(cè)試()
         {
             var autoResetEvent = new AutoResetEvent(false);

             var transactionOptions1 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted };
             var transactionOptions2 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted };

             using (var ts1 = new TransactionScope(TransactionScopeOption.Required, transactionOptions1))
             {
                 //前置條件
                 using (var context = new TestEntities())
                 {
                     Assert.AreEqual("李妞妞", context.Tables.First().Name);
                 }

                 ThreadPool.QueueUserWorkItem(data =>
                 {
                     using (var ts2 = new TransactionScope(TransactionScopeOption.Required, transactionOptions2))
                     {
                         //修改數(shù)據(jù)
                         using (var context = new TestEntities())
                         {
                             context.Tables.First().Name = "段光偉";
                             context.SaveChanges();
                         }

                         ts2.Complete();    
                     }

                     autoResetEvent.Set();
                 });

                 autoResetEvent.WaitOne();

                 //不可重復(fù)讀測(cè)試
                 using (var context = new TestEntities())
                 {
                     Assert.AreEqual("段光偉", context.Tables.First().Name);
                 }
             }
         }

幻讀

定義：A事務(wù)讀取了兩次數(shù)據(jù)，在這兩次的讀取過(guò)程中B事務(wù)添加了數(shù)據(jù)，A事務(wù)的這兩次讀取出來(lái)的集合不一樣了（幻讀）。

比喻：A在統(tǒng)計(jì)文件數(shù)據(jù)，為了統(tǒng)計(jì)精確A統(tǒng)計(jì)了兩次，在這兩次的統(tǒng)計(jì)過(guò)程中B添加了一個(gè)文件，A發(fā)現(xiàn)這兩次統(tǒng)計(jì)的數(shù)量不一樣（幻讀），A會(huì)感覺(jué)自己的腦袋有點(diǎn)頭疼。

代碼示例

復(fù)制代碼代碼如下:

[TestMethod]
         public void 幻讀_測(cè)試()
         {
             var autoResetEvent = new AutoResetEvent(false);

             var transactionOptions1 = new TransactionOptions { IsolationLevel = IsolationLevel.RepeatableRead };
             var transactionOptions2 = new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted };

             using (var ts1 = new TransactionScope(TransactionScopeOption.Required, transactionOptions1))
             {
                 //前置條件
                 using (var context = new TestEntities())
                 {
                     Assert.AreEqual(1, context.Tables.Count());
                 }

                 ThreadPool.QueueUserWorkItem(data =>
                 {
                     using (var ts2 = new TransactionScope(TransactionScopeOption.Required, transactionOptions2))
                     {
                         //添加數(shù)據(jù)
                         using (var context = new TestEntities())
                         {
                             context.Tables.Add(new Table() { Id = Guid.NewGuid(), Name = "段光偉" });
                             context.SaveChanges();
                         }

                         ts2.Complete();
                     }

                     autoResetEvent.Set();
                 });

                 autoResetEvent.WaitOne();

                 //幻讀測(cè)試
                 using (var context = new TestEntities())
                 {
                     Assert.AreEqual(2, context.Tables.Count());
                 }
             }
         }

四種隔離級(jí)別如何處理并發(fā)問(wèn)題

	臟讀	不可重復(fù)讀	幻讀
讀未提交	允許	允許	允許
讀已提交	不允許	允許	允許
可重復(fù)讀	不允許	不允許	允許
串行化	不允許	不允許	不允許

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