dotnet core系列之Background tasks with hosted services (後臺任務)

時間 2019-11-11

標籤 dotnet core 系列 background tasks hosted services 後臺任務简体版

原文原文鏈接

這篇簡單講asp.net core 中的後臺任務web

用到的包：數據庫

Microsoft.AspNetCore.App metapackage api

或者加入mvc

Microsoft.Extensions.Hostingapp

一. Timed background tasks（定時後臺任務）asp.net

使用到System.Threading.Timer類。定時器觸發任務的DoWork方法。定時器在StopAsync上中止，而且釋放是在Dispose上async

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擴展方法註冊：ide

services.AddHostedService<TimedHostedService>();

二. Consuming a scoped service in a background task 在後臺任務中運行scoped service函數

使用IHostService中的scoped services, 建立一個scope. 對於一個hosted service默認沒有scope被建立。oop

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後臺任務服務來調用它的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;
    }
}

服務註冊在Startup.ConfigureServices中。IHostedService的實現用AddHostedService擴展方法註冊：

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，隊列中的後臺任務出隊列而且做爲BackroundService執行。BackgroundService是一個實現了IHostedService接口的類。

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擴展方法註冊：

services.AddHostedService<QueuedHostedService>();
services.AddSingleton<IBackgroundTaskQueue, BackgroundTaskQueue>();

在Index page model類中：

IBackgroundTaskQueue被注入到構造函數而且指定給Queue
一個 IServiceScopeFactory被注入而且指定給_serviceScopeFactory. 這個工廠用來建立IServiceScope實例， IServiceScope實例是用來在scope內建立 services的。一個scope被建立時爲了用應用的AppDbContext（a scoped service）來寫數據庫記錄在 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被調用來使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();
}