EFCore 5 新特性 Savepoints
EFCore 5 中的 Savepoints
Intro
EFCore 5中引入了一個新特性,叫做 Savepoints,主要是事務(wù)中使用,個人感覺有點類似于 Windows 上的系統(tǒng)還原點,如果事務(wù)發(fā)生了異常,可以回滾到某一個還原點。
Savepoints
當(dāng)我們在一個事務(wù)里執(zhí)行 SaveChanges 的時候,EF Core 會在保存數(shù)據(jù)之前自動的創(chuàng)建一個 savepoint,Savepoints 有點類似于系統(tǒng)還原點的概念,我們可以回滾到指定的 savepoint,
當(dāng)事務(wù)發(fā)生錯誤的時候,會自動回滾到事務(wù)創(chuàng)建的 savepoint 回滾到事務(wù)開始之前的狀態(tài),以便于我們做重試或可能的修復(fù)錯誤或其他邏輯。
我們可以通過 CreateSavepoint 來手動創(chuàng)建一個 savepoint,使用 RollbackToSavepoint 來回滾到某一個 savepoint
來看一個微軟的示例代碼吧:
using?var?context?=?new?BloggingContext(); using?var?transaction?=?context.Database.BeginTransaction();try {context.Blogs.Add(new?Blog?{?Url?=?"https://devblogs.microsoft.com/dotnet/"?});context.SaveChanges();transaction.CreateSavepoint("BeforeMoreBlogs");context.Blogs.Add(new?Blog?{?Url?=?"https://devblogs.microsoft.com/visualstudio/"?});context.Blogs.Add(new?Blog?{?Url?=?"https://devblogs.microsoft.com/aspnet/"?});context.SaveChanges();transaction.Commit(); } catch?(Exception) {//?If?a?failure?occurred,?we?rollback?to?the?savepoint?and?can?continue?the?transactiontransaction.RollbackToSavepoint("BeforeMoreBlogs");//?TODO:?Handle?failure,?possibly?retry?inserting?blogs }Sample
我們自己來動手一試,示例代碼如下:
var?services?=?new?ServiceCollection(); services.AddDbContext<TestDbContext>(options?=> {options.UseSqlite("Data?Source=Application.db;Cache=Shared").LogTo(Console.WriteLine,?LogLevel.Warning); }); using?var?provider?=?services.BuildServiceProvider(); using?var?scope?=?provider.CreateScope(); var?dbContext?=?scope.ServiceProvider.GetRequiredService<TestDbContext>(); dbContext.Database.EnsureCreated(); Console.WriteLine($"Posts?count:{dbContext.Posts.Count()}"); using?var?transaction?=?dbContext.Database.BeginTransaction(); try {dbContext.Posts.Add(new?Post()?{?Author?=?"Tom",?Title?=?"Date?changed",?PostedAt?=?DateTime.UtcNow,?});dbContext.Posts.Add(new?Post()?{?Author?=?"Tom",?Title?=?"Date?changed",?PostedAt?=?DateTime.UtcNow,?});dbContext.SaveChanges();transaction.CreateSavepoint("Stage1");Console.WriteLine($"Posts?count:{dbContext.Posts.Count()}");dbContext.Posts.Add(new?Post()?{?Author?=?"Alice",?Title?=?"Test",?PostedAt?=?DateTime.UtcNow,?});dbContext.SaveChanges();transaction.CreateSavepoint("Stage2");Console.WriteLine($"Posts?count:{dbContext.Posts.Count()}");throw?new?InvalidOperationException();transaction.Commit(); } catch?(Exception) {Console.WriteLine("Exception?throw");transaction.RollbackToSavepoint("Stage1"); }Console.WriteLine($"Posts?count:{dbContext.Posts.Count()}");示例代碼中創(chuàng)建了兩個 savepoint,然后拋出了一個異常,捕獲異常后回滾到第一個 savepoint
輸出結(jié)果如下:
output可以看到,只有第一個 savepoint 之前的數(shù)據(jù)保存了下來,第二個 savepoint 雖然數(shù)據(jù)成功保存了,但是又被回滾了,最終只有第一個 savepoint 之前的數(shù)據(jù)變更被保存了下來
More
通過 savepoint 我們就可以使得事務(wù)控制更加精細,可以更能夠好的控制事務(wù)中的數(shù)據(jù)變更
但是需要注意的是,這個功能不要和 Sql Server 中的 Multiple Active Result Sets 一起使用,一旦發(fā)生了錯誤,事務(wù)控制可能會發(fā)生不可預(yù)期的情況。
Savepoints are incompatible with SQL Server's Multiple Active Result Sets, and are not used. If an error occurs during SaveChanges, the transaction may be left in an unknown state.
References
https://docs.microsoft.com/en-us/ef/core/saving/transactions#savepoints
https://github.com/WeihanLi/SamplesInPractice/blob/master/EF5Samples/SavePointsTest.cs
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