EF Core 基礎知識

時間 2019-12-05

標籤 core 基礎知識简体版

原文原文鏈接

數據庫鏈接字符串

在 ASP.NET Core 添加配置片斷：sql

{
  "ConnectionStrings": {
    "BloggingDatabase": "Server=(localdb)\\mssqllocaldb;Database=EFGetStarted.ConsoleApp.NewDb;Trusted_Connection=True;"
  }
}

而後，配置對應的DbContext：數據庫

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<BloggingContext>(options =>
        options.UseSqlServer(Configuration.GetConnectionString("BloggingDatabase")));
}

日誌記錄

EF Core 默認會與 ASP.NET Core的日誌提供程序一塊兒工做，只須要使用AddDbContext或AddDbContextPool添加服務便可。多線程

除此以外，還能夠手工添加日誌記錄。async

首先，建立LoggerFactory的單例：ide

public static readonly LoggerFactory MyLoggerFactory
    = new LoggerFactory(new[] {new ConsoleLoggerProvider((_, __) => true, true)});

而後，經過DbContextOptionsBuilder註冊此單例：函數

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    => optionsBuilder
        .UseLoggerFactory(MyLoggerFactory) // Warning: Do not create a new ILoggerFactory instance each time
        .UseSqlServer(
            @"Server=(localdb)\mssqllocaldb;Database=EFLogging;Trusted_Connection=True;ConnectRetryCount=0");

若是你只想記錄想要的日誌，例如數據操做語句，能夠在ILoggerProvider中進行配置：ui

public static readonly LoggerFactory MyLoggerFactory
    = new LoggerFactory(new[]
    {
        new ConsoleLoggerProvider((category, level)
            => category == DbLoggerCategory.Database.Command.Name
               && level == LogLevel.Information, true)
    });

配置彈性鏈接

EF Core 能夠根據不一樣的數據庫失敗，制定不一樣的執行策略，例如故障自動重試等。線程

針對SQL Server，它知道能夠重試的異常類型，而且具備合理的默認值的最大重試，重試次數等之間的延遲。日誌

配置以下：code

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder
        .UseSqlServer(
            @"Server=(localdb)\mssqllocaldb;Database=EFMiscellanous.ConnectionResiliency;Trusted_Connection=True;ConnectRetryCount=0",
            options => options.EnableRetryOnFailure());
}

也可在Startup中配置：

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<PicnicContext>(
        options => options.UseSqlServer(
            "<connection string>",
            providerOptions => providerOptions.EnableRetryOnFailure()));
}

你也能夠自定義執行策略：

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder
        .UseMyProvider(
            "<connection string>",
            options => options.ExecutionStrategy(...));
}

自動重試與事物

對於執行自動重試策略來講，每一次調用context.SaveChanges()方法將會當作一個重試單元。若是你的事物中有多個SaveChanges操做，配置的自動重試策略將會拋出異常，解決方法是使用委託來手動調用執行策略。代碼以下：

using (var db = new BloggingContext())
{
    var strategy = db.Database.CreateExecutionStrategy();

    strategy.Execute(() =>
    {
        using (var context = new BloggingContext())
        {
            using (var transaction = context.Database.BeginTransaction())
            {
                context.Blogs.Add(new Blog {Url = "http://blogs.msdn.com/dotnet"});
                context.SaveChanges();

                context.Blogs.Add(new Blog {Url = "http://blogs.msdn.com/visualstudio"});
                context.SaveChanges();

                transaction.Commit();
            }
        }
    });
}

此方法一樣適用於環境事物：

using (var context1 = new BloggingContext())
{
    context1.Blogs.Add(new Blog { Url = "http://blogs.msdn.com/visualstudio" });

    var strategy = context1.Database.CreateExecutionStrategy();

    strategy.Execute(() =>
    {
        using (var context2 = new BloggingContext())
        {
            using (var transaction = new TransactionScope())
            {
                context2.Blogs.Add(new Blog { Url = "http://blogs.msdn.com/dotnet" });
                context2.SaveChanges();

                context1.SaveChanges();

                transaction.Complete();
            }
        }
    });
}

自動重試策略須要考慮冪等性問題，防止數據添加劇復等誤操做。EF Core 引入了一種狀態檢查機制，能夠幫助咱們實現是否執行成功的檢測：

using (var db = new BloggingContext())
{
    var strategy = db.Database.CreateExecutionStrategy();

    var blogToAdd = new Blog {Url = "http://blogs.msdn.com/dotnet"};
    db.Blogs.Add(blogToAdd);

    strategy.ExecuteInTransaction(db,
        operation: context =>
        {
            context.SaveChanges(acceptAllChangesOnSuccess: false);
        },
        verifySucceeded: context => context.Blogs.AsNoTracking().Any(b => b.BlogId == blogToAdd.BlogId));

    db.ChangeTracker.AcceptAllChanges();
}

DbContext配置項

DbContext必須有DbContextOptions實例能，Options的做用以下：

配置數據庫提供程序
鏈接字符串
數據庫提供程序級別的可選項
EF Core級別的可選項

能夠經過構造函數添加Options：

public class BloggingContext : DbContext
{
    public BloggingContext(DbContextOptions<BloggingContext> options)
        : base(options)
    { }

    public DbSet<Blog> Blogs { get; set; }
}

也能夠經過OnConfiguring方法進行配置：

public class BloggingContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlite("Data Source=blog.db");
    }
}

使用依賴注入DbContext時，須要構造函數的方式進行配置，並在Startup中配置DbContext：

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<BloggingContext>(options => options.UseSqlite("Data Source=blog.db"));
}