dotnet core系列之Background tasks with hosted services (后台任务)
这篇简单讲asp.net core 中的后台任务
用到的包：
Microsoft.AspNetCore.App metapackage
或者加入
一. Timed background tasks（定时后台任务）
使用到System.Threading.Timer类。定时器触发任务的DoWork方法。定时器在StopAsync上停止，并且释放是在Dispose上
internal class TimedHostedService : IHostedService, IDisposable { private readonly ILogger _logger; private Timer _timer; public TimedHostedService(ILogger<TimedHostedService> logger) { _logger = logger; } public Task StartAsync(CancellationToken cancellationToken) { _logger.LogInformation("Timed Background Service is starting."); _timer = new Timer(DoWork, null, TimeSpan.Zero, TimeSpan.FromSeconds(5)); return Task.CompletedTask; } private void DoWork(object state) { _logger.LogInformation("Timed Background Service is working."); } public Task StopAsync(CancellationToken cancellationToken) { _logger.LogInformation("Timed Background Service is stopping."); _timer?.Change(Timeout.Infinite, 0); return Task.CompletedTask; } public void Dispose() { _timer?.Dispose(); } }
服务是在Startup.ConfigureServices上使用AddHostedService扩展方法注册：
services.AddHostedService<TimedHostedService>();
二. Consuming a scoped service in a background task 在后台任务中运行scoped service
使用IHostService中的scoped services, 创建一个scope. 对于一个hosted service默认没有scope被创建。
这个scoped 后台任务服务包含后台任务逻辑。下面的例子中，一个ILogger被注入到了service中：
internal interface IScopedProcessingService { void DoWork(); } internal class ScopedProcessingService : IScopedProcessingService { private readonly ILogger _logger; public ScopedProcessingService(ILogger<ScopedProcessingService> logger) { _logger = logger; } public void DoWork() { _logger.LogInformation("Scoped Processing Service is working."); } }
这个hosted service 创建了一个scope解析了scoped后台任务服务来调用它的DoWork方法：
internal class ConsumeScopedServiceHostedService : IHostedService { private readonly ILogger _logger; public ConsumeScopedServiceHostedService(IServiceProvider services, ILogger<ConsumeScopedServiceHostedService> logger) { Services = services; _logger = logger; } public IServiceProvider Services { get; } public Task StartAsync(CancellationToken cancellationToken) { _logger.LogInformation( "Consume Scoped Service Hosted Service is starting."); DoWork(); return Task.CompletedTask; } private void DoWork() { _logger.LogInformation( "Consume Scoped Service Hosted Service is working."); using (var scope = Services.CreateScope()) { var scopedProcessingService = scope.ServiceProvider .GetRequiredService<IScopedProcessingService>(); scopedProcessingService.DoWork(); } } public Task StopAsync(CancellationToken cancellationToken) { _logger.LogInformation( "Consume Scoped Service Hosted Service is stopping."); return Task.CompletedTask; } }
服务注册在Startup.ConfigureServices中。IHostedService的实现用AddHostedService扩展方法注册：
services.AddHostedService<ConsumeScopedServiceHostedService>();
services.AddScoped<IScopedProcessingService, ScopedProcessingService>();
三. Queued background tasks 排队的后台任务
public interface IBackgroundTaskQueue { void QueueBackgroundWorkItem(Func<CancellationToken, Task> workItem); Task<Func<CancellationToken, Task>> DequeueAsync( CancellationToken cancellationToken); } public class BackgroundTaskQueue : IBackgroundTaskQueue { private ConcurrentQueue<Func<CancellationToken, Task>> _workItems = new ConcurrentQueue<Func<CancellationToken, Task>>(); private SemaphoreSlim _signal = new SemaphoreSlim(0); public void QueueBackgroundWorkItem( Func<CancellationToken, Task> workItem) { if (workItem == null) { throw new ArgumentNullException(nameof(workItem)); } _workItems.Enqueue(workItem); _signal.Release(); } public async Task<Func<CancellationToken, Task>> DequeueAsync( CancellationToken cancellationToken) { await _signal.WaitAsync(cancellationToken); _workItems.TryDequeue(out var workItem); return workItem; } }
在 QueueHostedService中，队列中的后台任务出队列并且作为BackroundService执行。BackgroundService是一个实现了IHostedService接口的类。
public class QueuedHostedService : BackgroundService { private readonly ILogger _logger; public QueuedHostedService(IBackgroundTaskQueue taskQueue, ILoggerFactory loggerFactory) { TaskQueue = taskQueue; _logger = loggerFactory.CreateLogger<QueuedHostedService>(); } public IBackgroundTaskQueue TaskQueue { get; } protected async override Task ExecuteAsync( CancellationToken cancellationToken) { _logger.LogInformation("Queued Hosted Service is starting."); while (!cancellationToken.IsCancellationRequested) { var workItem = await TaskQueue.DequeueAsync(cancellationToken); try { await workItem(cancellationToken); } catch (Exception ex) { _logger.LogError(ex, $"Error occurred executing {nameof(workItem)}."); } } _logger.LogInformation("Queued Hosted Service is stopping."); } }
服务注册在Startup.ConfigureService方法中。IHostedService的实现用AddHostedService扩展方法注册：
services.AddHostedService<QueuedHostedService>();
services.AddSingleton<IBackgroundTaskQueue, BackgroundTaskQueue>();
在Index page model类中：
- IBackgroundTaskQueue被注入到构造函数并且指定给Queue
- 一个 IServiceScopeFactory被注入并且指定给_serviceScopeFactory. 这个工厂用来创建IServiceScope实例， IServiceScope实例是用来在scope内创建 services的。一个scope被创建时为了用应用的AppDbContext（a scoped service）来写数据库记录在 IBackgroundTaskQueue 中（a singleton service）.
public class IndexModel : PageModel { private readonly AppDbContext _db; private readonly ILogger _logger; private readonly IServiceScopeFactory _serviceScopeFactory; public IndexModel(AppDbContext db, IBackgroundTaskQueue queue, ILogger<IndexModel> logger, IServiceScopeFactory serviceScopeFactory) { _db = db; _logger = logger; Queue = queue; _serviceScopeFactory = serviceScopeFactory; } public IBackgroundTaskQueue Queue { get; }
当 Index page 上的Add Task按钮被选中时，OnPostAddTask方法被执行。QueueBackgroundWorkItem被调用来使work item入队。
public IActionResult OnPostAddTaskAsync() { Queue.QueueBackgroundWorkItem(async token => { var guid = Guid.NewGuid().ToString(); using (var scope = _serviceScopeFactory.CreateScope()) { var scopedServices = scope.ServiceProvider; var db = scopedServices.GetRequiredService<AppDbContext>(); for (int delayLoop = 1; delayLoop < 4; delayLoop++) { try { db.Messages.Add( new Message() { Text = $"Queued Background Task {guid} has " + $"written a step. {delayLoop}/3" }); await db.SaveChangesAsync(); } catch (Exception ex) { _logger.LogError(ex, "An error occurred writing to the " + $"database. Error: {ex.Message}"); } await Task.Delay(TimeSpan.FromSeconds(5), token); } } _logger.LogInformation( $"Queued Background Task {guid} is complete. 3/3"); }); return RedirectToPage(); }
四. 总结
注意上面的方法都有一个共同点：即直接或间接实现 IHostedService 方法
IHostedService interface
Hosted servcies实现IHostService接口. 这个接口定义了两个方法，为被主机管理的对象：
- StartAsync - StartAsync包含启动后台任务的逻辑。
- StopAsync - 当host 执行关闭时触发。StopAsync包含终止后台任务的逻辑。实现IDisposable 和finalizers 来释放任意unmanaged resources.
你可以把这种用法的后台任务加到任意应用，例如web api , mvc , 控制台等，因为后台服务在应用启动时，就被加载了。它是被以服务的方式加到了管道上了
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