I have a class called 'TestService' which I use in a Windows Service to receive message from ServiceBus queue.
My question is how to make ServiceBusClient and ServiceBusProcessor dispose properly in Dispose() method as ServiceBusClient and ServiceBusProcessor use DisposeAsync()?
public class TestService : BackgroundService
{
private ServiceBusClient _client;
private ServiceBusProcessor _processor;
public TestService ()
{
}
protected async override Task ExecuteAsync(CancellationToken stoppingToken)
{
_client = new ServiceBusClient("myConnection");
_processor = _client.CreateProcessor("myQueue", new ServiceBusProcessorOptions());
_processor.ProcessMessageAsync += MessageHandler;
await _processor.StartProcessingAsync();
}
public override async Task StopAsync(CancellationToken cancellationToken)
{
await _processor.StopProcessingAsync();
await base.StopAsync(cancellationToken);
}
public override void Dispose()
{
_processor.DisposeAsync(); //my question is here
_client.DisposeAsync(); //and here
base.Dispose();
}
private static async Task MessageHandler(ProcessMessageEventArgs args)
{
string body = args.Message.Body.ToString();
await args.CompleteMessageAsync(args.Message);
}
}
You can do it in this way
public override void Dispose()
{
_processor.DisposeAsync().GetAwaiter().GetResult();
_client.DisposeAsync().GetAwaiter().GetResult();
/// [...]
}
But you should better think of implementing it with the IAsyncDisposable pattern described here or in the comment to this very similar question here
public async ValueTask DisposeAsync(CancellationToken token)
{
await _processor.DisposeAsync(token);
await _client.DisposeAsync(token);
/// [...]
}
Example from Microsoft Documentation
Related
Good day, I'm trying to create a general purpose task queue service that executes in the background using BackgroundService. I avoided using a Delegate function Func<T1,T2, OuT> as input to EnqueueTask(Task<ResponseHelper> newTask) method because I wanted a generic solution, so I opted to pass a Task<ResponseHelper> instead. But this solution does not allow me to re-enqueue a failed task inside ExecuteAsync(CancellationToken stoppingToken) because the task returns an instance of ResponseHelper which is not a copy of the failed task. Kindly help me correct the code I have to return a dequeued task from DequeueTaskAsync(CancellationToken cancellationToken) function instead of an instance of ResponseHelper.
public interface ITaskQueueHelper
{
void EnqueueTask(Task<ResponseHelper> newTask);
Task<ResponseHelper> DequeueTaskAsync(CancellationToken cancellationToken);
}
public class TaskQueueHelper : ITaskQueueHelper
{
private readonly SemaphoreSlim signal;
private readonly ConcurrentQueue<Task<ResponseHelper>> taskQueue;
public TaskQueueHelper()
{
signal = new SemaphoreSlim(0);
taskQueue = new ConcurrentQueue<Task<ResponseHelper>>();
}
public void EnqueueTask(Task<ResponseHelper> newTask)
{
if (newTask == null)
{
throw new ArgumentNullException(nameof(newTask));
}
taskQueue.Enqueue(newTask);
signal.Release();
}
public async Task<ResponseHelper> DequeueTaskAsync(CancellationToken cancellationToken)
{
await signal.WaitAsync(cancellationToken);
taskQueue.TryDequeue(out var currentTask);
/*I need to return currentTask here, instead of an instance of ResponseHelper*/
return await currentTask;
}
}
public class TaskQueueService : BackgroundService
{
private readonly ITaskQueueHelper taskQueue;
private readonly ILogger<TaskQueueService> logger;
public TaskQueueService(
ITaskQueueHelper _taskQueue,
ILogger<TaskQueueService> _logger)
{
logger = _logger;
taskQueue = _taskQueue;
}
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
while (!stoppingToken.IsCancellationRequested)
{
ResponseHelper response = await taskQueue.DequeueTaskAsync(stoppingToken);
try
{
if (!response.Status.Equals(ResultCode.Success))
{
/*I need to re-enqueue a failed task here*/
//taskQueue.EnqueueTask(currentTask);
}
}
catch (Exception e)
{
logger.LogError(e, $"Error occurred executing {nameof(TaskQueueService)}");
}
}
}
}
for retrying you could do:
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
while (!stoppingToken.IsCancellationRequested)
{
ResponseHelper response = await taskQueue.DequeueTaskAsync(stoppingToken);
try
{
if (!response.Status.Equals(ResultCode.Success))
{
// Retry the task.
response = await taskQueue.DequeueTaskAsync(stoppingToken);
}
}
catch (Exception e)
{
logger.LogError(e, $"Error occurred executing {nameof(TaskQueueService)}");
}
}
}
I'm trying to use an IHostedService as a fire-and-forget email sender in an ASP.NET Core application. It looks like the best way to do this is using the BufferBlock class; the trouble is, ReceiveAsync never finishes, even when I post new items into the BufferBlock.
Here is the HostedService base class:
public abstract class HostedService
{
private Task _executingTask;
private CancellationTokenSource _cts;
public Task StartAsync(CancellationToken cancellationToken)
{
_cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
_executingTask = ExecuteAsync(_cts.Token);
return _executingTask.IsCompleted ? _executingTask : Task.CompletedTask;
}
public async Task StopAsync(CancellationToken cancellationToken)
{
if (_executingTask == null)
{
return;
}
_cts.Cancel();
await Task.WhenAny(_executingTask, Task.Delay(-1, cancellationToken));
cancellationToken.ThrowIfCancellationRequested();
}
protected abstract Task ExecuteAsync(CancellationToken cancellationToken);
}
from which my EmailService is derived as follows:
public sealed class EmailService : HostedService, IEmailService
{
private readonly ISendEmail _emailClient;
private readonly BufferBlock<MailMessage> _emailQueue;
public EmailService(ISendEmail emailClient)
{
_emailClient = emailClient;
_emailQueue = new BufferBlock<MailMessage>();
}
public void EnqueueEmail(MailMessage email)
{
var accepted = _emailQueue.Post(email);
}
protected override async Task ExecuteAsync(CancellationToken cancellationToken)
{
while (!cancellationToken.IsCancellationRequested)
{
var nextEmail = await _emailQueue.ReceiveAsync(cancellationToken).ConfigureAwait(false);
await _emailClient.SendMailAsync(nextEmail);
}
}
}
The IEmailService interface is just a simple fire-and-forget method:
public interface IEmailService : IHostedService
{
void EnqueueEmail(MailMessage email);
}
So this should all be enough. Within my controllers, I should be able to inject an IEmailService, and then enqueue messages as required. The problem is that when I run the following test:
[Theory]
[InlineData(1)]
[InlineData(2)]
[InlineData(3)]
[InlineData(10)]
public async Task Emails_are_sent_after_they_are_enqueued(int emailCount)
{
for (var i = 0; i < emailCount; ++i)
{
_emailService.EnqueueEmail(new MailMessage());
}
await _testEmailClient.WaitForEmailsToBeSentAsync(emailCount);
}
the ReceiveAsync method never completes. I have tried using ConfigureAwait(false), but this seems to have no effect.
Within my tests, the HostedService is started by the ASP.NET Core pipeline, and ExecuteAsync is entered. I would expect ReceiveAsync to complete when an item is available in the BufferBlock, but there must be some threading subtlety I'm missing.
The problem was that my IoC container was wiring up multiple instances of IEmailService, and the instance that was calling ReceiveAsync was different to the instance that was calling Post. This is because EmailService was an instance both of IEmailService and IHostedService.
The answer was to ditch IEmailService altogether. To use EmailService in production code, I can inject an IEnumerable<IHostedService> instance, then pull my EmailService from that collection using OfType<EmailService>().First().
How to write to the database on a timer in the background. For example, check mail and add new letters to the database. In the example, I simplified the code just before writing to the database.
The class names from the example in Microsoft.
The recording class itself:
namespace EmailNews.Services
{
internal interface IScopedProcessingService
{
void DoWork();
}
internal class ScopedProcessingService : IScopedProcessingService
{
private readonly ApplicationDbContext _context;
public ScopedProcessingService(ApplicationDbContext context)
{
_context = context;
}
public void DoWork()
{
Mail mail = new Mail();
mail.Date = DateTime.Now;
mail.Note = "lala";
mail.Tema = "lala";
mail.Email = "lala";
_context.Add(mail);
_context.SaveChangesAsync();
}
}
}
Timer class:
namespace EmailNews.Services
{
#region snippet1
internal class TimedHostedService : IHostedService, IDisposable
{
private readonly ILogger _logger;
private Timer _timer;
public TimedHostedService(IServiceProvider services, ILogger<TimedHostedService> logger)
{
Services = services;
_logger = logger;
}
public IServiceProvider Services { get; }
public Task StartAsync(CancellationToken cancellationToken)
{
_logger.LogInformation("Timed Background Service is starting.");
_timer = new Timer(DoWork, null, TimeSpan.Zero,
TimeSpan.FromMinutes(1));
return Task.CompletedTask;
}
private void DoWork(object state)
{
using (var scope = Services.CreateScope())
{
var scopedProcessingService =
scope.ServiceProvider
.GetRequiredService<IScopedProcessingService>();
scopedProcessingService.DoWork();
}
}
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();
}
}
#endregion
}
Startup:
services.AddHostedService<TimedHostedService>();
services.AddScoped<IScopedProcessingService, ScopedProcessingService>();
It seems everything is done as in the example, but nothing is added to the database, which is not so?
This is a rather interesting question, that boils down to "How do you correctly handle an async timer callback?"
The immediate problem is that SaveChangesAsync isn't getting awaited. The DbContext almost certainly gets disposed before SaveChangesAsync has a chance to run. To await it, DoWork must become an async Task method (never async void) :
internal interface IScheduledTask
{
Task DoWorkAsync();
}
internal class MailTask : IScheduledTask
{
private readonly ApplicationDbContext _context;
public MailTask(ApplicationDbContext context)
{
_context = context;
}
public async Task DoWorkAsync()
{
var mail = new Mail
{ Date = DateTime.Now,
Note = "lala",
Tema = "lala",
Email = "lala" };
_context.Add(mail);
await _context.SaveChangesAsync();
}
}
The problem now is how to call DoWorkAsync from the timer callback. If we just call it without awaiting, we'll get the same problem we had in the first place. A timer callback can't handle methods that return Task. We can't make it async void either, because this would result in the same problem - the method will return before any async operation has a chance to finish.
David Fowler explains how to properly handle asynchronous timer callbacks in the Timer Callbacks section of his Async Guidance
article :
private readonly Timer _timer;
private readonly HttpClient _client;
public Pinger(HttpClient client)
{
_client = new HttpClient();
_timer = new Timer(Heartbeat, null, 1000, 1000);
}
public void Heartbeat(object state)
{
// Discard the result
_ = DoAsyncPing();
}
private async Task DoAsyncPing()
{
await _client.GetAsync("http://mybackend/api/ping");
}
The actual method should be async Task but the returned task only has to be assigned, not awaited, in order for it to work properly.
Applying this to the question leads to something like this :
public Task StartAsync(CancellationToken cancellationToken)
{
...
_timer = new Timer(HeartBeat, null, TimeSpan.Zero,
TimeSpan.FromMinutes(1));
return Task.CompletedTask;
}
private void Heartbeat(object state)
{
_ = DoWorkAsync();
}
private async Task DoWorkAsync()
{
using (var scope = Services.CreateScope())
{
var schedTask = scope.ServiceProvider
.GetRequiredService<IScheduledTask>();
await schedTask.DoWorkAsync();
}
}
David Fowler explains why async void is ALWAY BAD in ASP.NET Core - it's not only that async actions won't be awaited, exceptions will crash the application.
He also explains why we can't use Timer(async state=>DoWorkAsync(state)) - that's an async void delegate.
Slender answered my original question about what happens to fire and forget, after the HTTP Response is sent, but Now I'm left with the question how to properly queue background tasks
EDIT
As we all know Async void is generally bad, except for in the case when it comes to event handlers, I would like to execute some background logic without have to have the client wait. My original Idea was to use Fire and Forget
Say I have an event:
public event EventHandler LongRunningTask;
And then someone subscribes a fire and forget task:
LongRunningTask += async(s, e) => { await LongNetworkOperation;};
the web api method is call:
[HttpGet]
public async IActionResult GetTask()
{
LongRunningTask?.Invoke(this, EventArgs.Empty);
return Ok();
}
But If I do this my long running task isn't guaranteed to finish, How can I handle running background task without affect the time the time it take to make my request (e.g I don't want to wait for the task to finish first)?
.NET Core 2.1 has an IHostedService, which will safely run tasks in the background. I've found an example in the documentation for QueuedHostedService which I've modified to use the BackgroundService.
public class QueuedHostedService : BackgroundService
{
private Task _backgroundTask;
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 stoppingToken)
{
while (false == stoppingToken.IsCancellationRequested)
{
var workItem = await TaskQueue.DequeueAsync(stoppingToken);
try
{
await workItem(stoppingToken);
}
catch (Exception ex)
{
this._logger.LogError(ex, $"Error occurred executing {nameof(workItem)}.");
}
}
}
}
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;
}
}
Now we can safely queue up tasks in the background without affecting the time it takes to respond to a request.
Just wanted to add some additional notes to #johnny5 answer. Right now you can use https://devblogs.microsoft.com/dotnet/an-introduction-to-system-threading-channels/ instead of ConcurrentQueue with Semaphore.
The code will be something like this:
public class HostedService: BackgroundService
{
private readonly ILogger _logger;
private readonly ChannelReader<Stream> _channel;
public HostedService(
ILogger logger,
ChannelReader<Stream> channel)
{
_logger = logger;
_channel = channel;
}
protected override async Task ExecuteAsync(CancellationToken cancellationToken)
{
await foreach (var item in _channel.ReadAllAsync(cancellationToken))
{
try
{
// do your work with data
}
catch (Exception e)
{
_logger.Error(e, "An unhandled exception occured");
}
}
}
}
[ApiController]
[Route("api/data/upload")]
public class UploadController : ControllerBase
{
private readonly ChannelWriter<Stream> _channel;
public UploadController (
ChannelWriter<Stream> channel)
{
_channel = channel;
}
public async Task<IActionResult> Upload([FromForm] FileInfo fileInfo)
{
var ms = new MemoryStream();
await fileInfo.FormFile.CopyToAsync(ms);
await _channel.WriteAsync(ms);
return Ok();
}
}
I have a background service that runs every 30 seconds in my ASP .NET Core WebApi application. Its registered to the service container in ConfigureServices in Startup.cs by the line
services.AddSingleton<IHostedService, SimpleService>();
Then I have this class and its upper class to execute a given method every 30 seconds:
The general background service class:
public abstract class BackgroundService : IHostedService, IDisposable
{
private Task currentTask;
private readonly CancellationTokenSource stopCts = new CancellationTokenSource();
public virtual Task StartAsync(CancellationToken cancellationToken)
{
currentTask = ExecuteAsync(stopCts.Token);
if (currentTask.IsCompleted)
return currentTask;
return Task.CompletedTask;
}
public virtual async Task StopAsync(CancellationToken cancellationToken)
{
if (currentTask == null)
return;
try
{
stopCts.Cancel();
}
finally
{
await Task.WhenAny(currentTask, Task.Delay(Timeout.Infinite, cancellationToken));
}
}
protected virtual async Task ExecuteAsync(CancellationToken cancellationToken)
{
DateTime nextExecution = DateTime.Now;
do
{
DateTime currentTime = DateTime.Now;
if (nextExecution <= currentTime)
{
nextExecution = currentTime.Add(TimeSpan.FromSeconds(30));
await Process(cancellationToken);
}
else
{
await Task.Delay(nextExecution - currentTime, cancellationToken);
}
}
while (!cancellationToken.IsCancellationRequested);
}
protected abstract Task Process(CancellationToken cancellationToken);
public void Dispose()
{
stopCts.Cancel();
}
}
The one to use when a specific scope is needed:
public abstract class ScopedBackgroundService : BackgroundService
{
private readonly IServiceScopeFactory serviceScopeFactory;
public ScopedBackgroundService(IServiceScopeFactory serviceScopeFactory)
{
this.serviceScopeFactory = serviceScopeFactory;
}
protected override async Task Process(CancellationToken cancellationToken)
{
using (var scope = serviceScopeFactory.CreateScope())
{
await ProcessInScope(scope.ServiceProvider, cancellationToken);
}
}
public abstract Task ProcessInScope(IServiceProvider serviceProvider, CancellationToken cancellationToken);
}
When implementing the service itself I use this class (which is also registered in the ConfigureServices method):
public class SimpleService : ScopedBackgroundService
{
private ISimpleBusiness simpleBusiness;
public SimpleService(IServiceScopeFactory serviceScopeFactory) : base(serviceScopeFactory)
{
}
public override async Task ProcessInScope(IServiceProvider serviceProvider, CancellationToken cancellationToken)
{
this.simpleBusiness = serviceProvider.GetService<ISimpleBusiness>();
foreach (var b in simpleBusiness.GetAll())
{
await this.simpleBusiness.Check(b);
}
}
}
This works good. But I have the problem that after each request the used memory increased. The garbage collector also doesnt get triggered. After a while and under certain circumstances I cant detect/find the memory increases rapidly in about 20 ms by up to 150 MB.
The SimpleBusiness class (implementing the interface ISimpleBusiness) uses a db context in simpleDA obtained via constructor injection:
public class SimpleBusiness : ISimpleBusiness
{
private ISimpleDA simpleDA;
private IHostingEnvironment hostingEnvironment;
...
public SimpleBusiness(ISimpleDA simpleDA, IHostingEnvironment environment, ...)
{
this.simpleDA = simpleDA;
this.hostingEnvironment = environment;
...
}
...
I guess this dbcontext and its attached objects dont get disposed/removed from memory. How can I pass the db context in a request-lifetime scope to the constructor when calling ProcessInScope in ScopedBackgroundService for the following created instances of all objects used in the underlying methods without changing the current data access and business classes/interfaces?
EDIT:
This is the memory and cpu usage over about 26 minutes.
The usage of about 26 minutes runtime