dotnet core系列之Background tasks with hosted services (後臺任務)
阿新 • • 發佈：2019-06-19
這篇簡單講asp.net core 中的後臺任務
用到的包：
Microsoft.AspNetCore.App metapackage
或者加入
Microsoft.Extensions.Hosting
一. Timed background tasks（定時後臺任務）
使用到System.Threading.Timer類。定時器觸發任務的DoWork方法。定時器在StopAsync上停止，並且釋放是在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();
}
}服務是在Startup.ConfigureServices上使用AddHostedService擴充套件方法註冊：
services.AddHostedService<TimedHostedService>();
二. Consuming a scoped service in a background task 在後臺任務中執行scoped service
使用IHostService中的scoped services, 建立一個scope. 對於一個hosted service預設沒有scope被建立。
這個scoped 後臺任務服務包含後臺任務邏輯。下面的例子中，一個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.");
}
}這個hosted service 建立了一個scope解析了scoped後臺任務服務來呼叫它的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;
}
}
服務註冊在Startup.ConfigureServices中。IHostedService的實現用AddHostedService擴充套件方法註冊：
services.AddHostedService<ConsumeScopedServiceHostedService>(); services.AddScoped<IScopedProcessingService, ScopedProcessingService>();
三. Queued background tasks 排隊的後臺任務
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，佇列中的後臺任務出佇列並且作為BackroundService執行。BackgroundService是一個實現了IHostedService介面的類。
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.");
}
}
服務註冊在Startup.ConfigureService方法中。IHostedService的實現用AddHostedService擴充套件方法註冊：
services.AddHostedService<QueuedHostedService>(); services.AddSingleton<IBackgroundTaskQueue, BackgroundTaskQueue>();
在Index page model類中：
- IBackgroundTaskQueue被注入到建構函式並且指定給Queue
- 一個 IServiceScopeFactory被注入並且指定給_serviceScopeFactory. 這個工廠用來建立IServiceScope例項， IServiceScope例項是用來在scope內建立 services的。一個scope被建立時為了用應用的AppDbContext（a scoped service）來寫資料庫記錄在 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; }
當 Index page 上的Add Task按鈕被選中時，OnPostAddTask方法被執行。QueueBackgroundWorkItem被呼叫來使work item入隊。
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. 總結
注意上面的方法都有一個共同點：即直接或間接實現 IHostedService 方法
IHostedService interface
Hosted servcies實現IHostService介面. 這個介面定義了兩個方法，為被主機管理的物件：
- StartAsync - StartAsync包含啟動後臺任務的邏輯。
- StopAsync - 當host 執行關閉時觸發。StopAsync包含終止後臺任務的邏輯。實現IDisposable 和finalizers 來釋放任意unmanaged resources.
參考網址：
https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/hosted-services?view=aspnetcore-2.2&tabs=visual-studio
&n