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A blog post is in progress.
A Channel by merriam-webster refers to among other meaings
the bed where a natural stream of water runs
I will start this post by adopting the water channel analogy to explore how it relates with C# System.Threading.Channels.
Analogy
Channel is the path where information flows from one point to another, like a water channel. If you observe the water channel, it is made up of multiple things. The first thing you will observe is water; second, water is produced by source(s), you can think of glaciers, Rain, etc., and thirdly the water ends up at the destination(s), you can think of the Sea, River, etc.
In .NET terminology, the source(s) are referred to as Producer(s), and the destination(s) are referred to as Consumer(s). And the water is referred to as Message(s) or the .NET object.
For a moment, think about the water channel that how it runs. You will observe that one or multiple consumers and producers are concurrently at work in the water channel meaning produced by rains, melting of galciers, and consumed by many in different forms.
Edge Case of flooding and Channel Types
The flood can happen in the water channel for many reasons, such as:
- The glaciers are melting at a high pace.
- The river is at capacity.
In .NET, you can map the analogies to Backpressure, which means the consumer is not consuming the messages at the same pace that the producer(s) are producing.
The Backpressure is a technique to mitigate the flooding problem by configuring behavior on the producer to produce in a different way.
That brings us to mainly two types of channels in the .NET
- Bounded Channel
The Bounded Channel has a defined capacity based on your application needs. It allows you to configure the behavior of what to do in case flooding happens.
- UnBounded Channel
The UnBounded Channel has no defined capacity, but the limitation of memory availability of the process will apply.
Channel is made up of Channel Reader and Channel Writer. Channel class acts as a Facade to synchronize the access to the shared data structure of type T.
Channel class offers two factory methods to create the typed channel. You can configure the channel behavior using options classes.
The below code snippet shows factory methods which are creating both types of channels.
Channel Factory Methods to create bounded or unbounded channels
Channel.CreateBounded<int>(new BoundedChannelOptions(capacity: 4)
{
FullMode = BoundedChannelFullMode.Wait,
SingleReader = true,
SingleWriter = true,
AllowSynchronousContinuations = false
});
Channel.CreateUnbounded<int>(new UnboundedChannelOptions()
{
SingleReader = true,
SingleWriter = false,
AllowSynchronousContinuations = false
});
Bounded Channel Mode when its full and backpressure
The bounded channel:
- Always have a capacity.
- Allows you to configure the mode of writer when the capacity is full.
- Allows you to register a callback function when an item is dropped as a result of
BoundedChannelFullMode mode.
In the below code snippet, I have implemented BackgroundBoundedProcessingQueue to observe the different modes of the channel writer behavior in a unit test.
The unit test ShouldDropItemsWhenChannelIsAtCapacity asserts the dropped items count in DropNewest, DropWrite, and DropOldest modes, and you will notice that there are items in the dropped list. But in Wait mode, the writer wait until the operation is cancelled or the channel capacity becomes available.
I encourage you to paste the whole snippet code in xunit project and observe the logs of the unit test.
A unit test which shows how the bounded channel handles the writes when its full
using System.Threading.Channels;
using Xunit.Abstractions;
namespace ChannelTests;
public class BackgroundBoundedProcessingQueue
{
private readonly Channel<int> _channel;
private readonly ITestOutputHelper _logger;
public readonly List<int> DroppedItems = new();
public BackgroundBoundedProcessingQueue(ITestOutputHelper logger, int capacity,
BoundedChannelFullMode boundedChannelFullMode)
{
_logger = logger;
_channel = Channel.CreateBounded<int>(new BoundedChannelOptions(capacity)
{
FullMode = boundedChannelFullMode
},
droppedItem =>
{
_logger.WriteLine($"{droppedItem} has been dropped.");
DroppedItems.Add(droppedItem);
});
}
public async ValueTask QueueItem(int item, CancellationToken cancellationToken)
{
await _channel.Writer.WriteAsync(item, cancellationToken);
_logger.WriteLine($"{item} has been written to channel");
}
public ValueTask<int> DequeueItem(CancellationToken cancellationToken)
{
return _channel.Reader.ReadAsync(cancellationToken);
}
public async Task ProcessItems(CancellationToken cancellationToken)
{
var items = _channel.Reader.ReadAllAsync(cancellationToken);
await foreach (var item in items.WithCancellation(cancellationToken))
_logger.WriteLine($"{item} has been processed.");
}
}
public class BoundedChannelTests
{
private readonly ITestOutputHelper _testOutputHelper;
public BoundedChannelTests(ITestOutputHelper testOutputHelper)
{
_testOutputHelper = testOutputHelper;
}
[Theory]
[InlineData(BoundedChannelFullMode.DropNewest)]
[InlineData(BoundedChannelFullMode.DropWrite)]
[InlineData(BoundedChannelFullMode.DropOldest)]
[InlineData(BoundedChannelFullMode.Wait)]
public async Task ShouldDropItemsWhenChannelIsAtCapacity(BoundedChannelFullMode dropMode)
{
var tokenSource = new CancellationTokenSource(2000);
_testOutputHelper.WriteLine($"Bounded Channel is operating in mode {dropMode}");
BackgroundBoundedProcessingQueue queue = new(_testOutputHelper, 3, dropMode);
if (dropMode == BoundedChannelFullMode.Wait)
{
await Assert.ThrowsAsync<OperationCanceledException>(async () =>
{
foreach (var i in Enumerable.Range(1, 5))
{
//On fifth item it will wait until token is cancelled after 2000 ms
if (i == 5)
_testOutputHelper.WriteLine("5th item will never be written");
await queue.QueueItem(i, tokenSource.Token);
if (i == 3)
{
//Dequeuing the item so 4th item can be written
_testOutputHelper.WriteLine("Dequeuing the item so 4th item can be written");
await queue.DequeueItem(tokenSource.Token);
}
}
});
}
else
{
foreach (var i in Enumerable.Range(1, 5))
await queue.QueueItem(i, tokenSource.Token);
Assert.True(queue.DroppedItems.Count == 2);
}
}
}
UnBounded Channel has no capacity - A unit test to write for x amount of time to the unbounded channel
The Unbounded Channel:
- Has no capacity but the available memory to the process.
- It allows you to configure the number of readers and writers using
UnboundedChannelOptions.
I encourage to try to run the code snippet to observe the unbounded channel capacity. The below code snippet is a unit test, you can run it by copying the whole thing in xunit project.
using System.Threading.Channels;
using Xunit.Abstractions;
namespace ChannelTests;
public class BackgroundUnBoundedProcessingQueue
{
private readonly Channel<int> _channel;
private readonly ITestOutputHelper _logger;
public BackgroundUnBoundedProcessingQueue(ITestOutputHelper logger)
{
_logger = logger;
_channel = Channel.CreateUnbounded<int>(new UnboundedChannelOptions()
{
SingleReader = true,
SingleWriter = false,
AllowSynchronousContinuations = false
});
}
public async ValueTask QueueItem(int item, CancellationToken cancellationToken)
{
await _channel.Writer.WriteAsync(item, cancellationToken);
}
public ChannelReader<int> Reader => _channel.Reader;
}
public class UnBoundedChannelTests
{
private readonly ITestOutputHelper _testOutputHelper;
public UnBoundedChannelTests(ITestOutputHelper testOutputHelper)
{
_testOutputHelper = testOutputHelper;
}
[Fact]
public async Task ShouldAlwaysAcceptItems()
{
var source = new CancellationTokenSource(100);
BackgroundUnBoundedProcessingQueue queue = new(_testOutputHelper);
await Assert.ThrowsAsync<TaskCanceledException>(async () =>
{
for (var i = 0; i < int.MaxValue; i++)
{
await queue.QueueItem(i, source.Token);
}
});
var totalItems = 0;
while (queue.Reader.TryRead(out var x))
{
totalItems++;
}
_testOutputHelper.WriteLine($"Total items on the channel are {totalItems}");
Assert.True(true);
}
}
The case of Decoupling
The sea (consumer) does not care about how the water is produced. It just consumes it metaphorically. Neither the sea knows about the production source, the glacier metaphorically.
Similarly, the producer and consumer in the .NET channel do not know about each other yet operate on the shared resource, the messages.
Many canals of the River and .NET Concurrency
Without going into the chemistry of water molecules, the producers put water into the river (Channel), a distinct function performed concurrently. Similarly, the river is divided into canals (consumers) that consume the water concurrently.
The important thing to notice here, the same water drop is not shared among any involved parties; this complexity is hidden behind the .NET Channel abstraction.
Background processing the work using Channels and Hosted Service
A web application API is a multithreading process with which many users interact concurrently. If an API logic involves an operation to perform a long-running or directly unrelated task, that will not impact the response returned to the user. You would like to offload or perform the task in the background. This leads us to find a mechanism that enables offloading. Channel is the answer.
If you have worked with .NET Framework, such background tasks were usually queued using HostingEnvironment.QueueBackgroundItem method or queue on the Thread Pool using ThreadPool.QueueUserWorkItem.
Both these approaches are hard to test and does not provide you any control on when these will be processed.
You can consider the below scenarios if you have yet to work with framework applications.
There are some commonly known requirements in traditional applications, such as:
- Send out an email.
- Send an event to the analytics service.
- Sync the data to a third-party service.
In the below example, I have defined a simple API end point, and a hosted service. Suppose, you have an API call which should complete immediately but queue the work which can eventually complete at later time.
To achieve this, I have implemented the IBackgroundTaskQueue to create an abstraction over the UnBoundedChannel, now this abstraction is injected into API endpoint which uses it to queue the work.
And the background service is continuously trying to read any available items to process. And as soon as it receives an item, it will start processing.
An exmple of background processing in the hosted service using the UnBounded Channel
using System.Threading.Channels;
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddSingleton<IBackgroundTaskQueue, LongRunningTaskBackgroundQueue>();
builder.Services.AddHostedService<LongRunningTaskBackgroundService>();
var app = builder.Build();
app.UseHttpsRedirection();
app.MapGet("dowork", async Task<IResult>(IBackgroundTaskQueue backgroundTaskQueue) =>
{
Console.WriteLine($"I was written to queue {DateTime.Now}");
await backgroundTaskQueue.QueueBackgroundWorkItemAsync(_ =>
{
return Task.Run(async () =>
{
await Task.Delay(1000);
Console.WriteLine($"I ran {DateTime.Now}");
});
});
return Results.Ok();
});
app.Run();
public interface IBackgroundTaskQueue
{
ValueTask QueueBackgroundWorkItemAsync(
Func<CancellationToken, Task> workItem);
ValueTask<Func<CancellationToken, Task>> DequeueAsync(
CancellationToken cancellationToken);
}
public sealed class LongRunningTaskBackgroundQueue : IBackgroundTaskQueue
{
private readonly Channel<Func<CancellationToken, Task>> _queue;
public LongRunningTaskBackgroundQueue()
{
UnboundedChannelOptions options = new()
{
SingleReader = true,
SingleWriter = false
};
_queue = Channel.CreateUnbounded<Func<CancellationToken, Task>>(options);
}
public async ValueTask QueueBackgroundWorkItemAsync(
Func<CancellationToken, Task> workItem)
{
await _queue.Writer.WriteAsync(workItem);
}
public async ValueTask<Func<CancellationToken, Task>> DequeueAsync(
CancellationToken cancellationToken)
{
var workItem =
await _queue.Reader.ReadAsync(cancellationToken);
return workItem;
}
}
public sealed class LongRunningTaskBackgroundService : BackgroundService
{
private readonly ILogger<LongRunningTaskBackgroundQueue> _logger;
private readonly IBackgroundTaskQueue _taskQueue;
public LongRunningTaskBackgroundService(
IBackgroundTaskQueue taskQueue,
ILogger<LongRunningTaskBackgroundQueue> logger)
{
(_taskQueue, _logger) = (taskQueue, logger);
}
public override async Task StartAsync(CancellationToken cancellationToken)
{
var serviceName = nameof(LongRunningTaskBackgroundService);
_logger.LogInformation("{ServiceName} is starting", serviceName);
await base.StartAsync(cancellationToken);
}
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
var serviceName = nameof(LongRunningTaskBackgroundService);
_logger.LogInformation("{ServiceName} is running", serviceName);
await ProcessTaskQueueAsync(stoppingToken);
}
private async Task ProcessTaskQueueAsync(CancellationToken stoppingToken)
{
await Task.Yield();
while (!stoppingToken.IsCancellationRequested)
try
{
await Task.Delay(5000, stoppingToken);
var task = await _taskQueue.DequeueAsync(stoppingToken);
await task(stoppingToken);
}
catch (OperationCanceledException operationCanceledException)
{
_logger.LogInformation(operationCanceledException,
"Operation was cancelled because host is shutting down");
}
catch (AggregateException aggregateException)
{
_logger.LogError(aggregateException.Flatten(), "Aggregate exception occurred");
}
catch (Exception ex)
{
_logger.LogError(ex, "Error occurred executing task work item");
}
}
public override async Task StopAsync(CancellationToken cancellationToken)
{
var serviceName = nameof(LongRunningTaskBackgroundService);
_logger.LogInformation("{ServiceName} is stopping", serviceName);
await base.StopAsync(cancellationToken);
}
}
It was the most thrilling improvement I have felt when compared to .NET Framework. I asked myself, how would I test the above, and I came up with the below test.
The below test:
- Is starting the web server.
- Is starting the hosted service which continuously tries to read from the channel.
- Verifies that work item is written to channel, and then read by hosted service, and it starts the work.
That is beautiful .NET
using Microsoft.AspNetCore.Mvc.Testing;
using Microsoft.Extensions.DependencyInjection;
using Xunit.Abstractions;
namespace ChannelTests;
public class BackgroundProcessingTest : IClassFixture<WebApplicationFactory<Program>>
{
private readonly WebApplicationFactory<Program> _applicationFactory;
private readonly ITestOutputHelper _testOutputHelper;
public BackgroundProcessingTest(ITestOutputHelper testOutputHelper,
WebApplicationFactory<Program> applicationFactory)
{
_testOutputHelper = testOutputHelper;
_applicationFactory = applicationFactory;
}
[Fact]
public async Task ShouldProcessTaskInBackgroundWhenWrittenToQueue()
{
var backgroundTaskQueue = _applicationFactory.Services.GetRequiredService<IBackgroundTaskQueue>();
var expectedCount = 2;
var countdownEvent = new CountdownEvent(expectedCount);
for (var i = 0; i < 2; i++)
await backgroundTaskQueue.QueueBackgroundWorkItemAsync(token =>
{
countdownEvent.Signal();
_testOutputHelper.WriteLine(
$"I was called by the background service, should be called {countdownEvent.CurrentCount} times more.");
return Task.CompletedTask;
});
var countReached = countdownEvent.Wait(TimeSpan.FromSeconds(12));
Assert.True(countReached);
}
}
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