rudironsoni dotnet-background-services
Patterns for long-running background work in .NET applications. Covers BackgroundService, IHostedService, hosted
service lifecycle, and graceful shutdown handling.
Scope
- BackgroundService and IHostedService patterns
- Hosted service lifecycle and startup ordering
- Graceful shutdown and cancellation handling
- Periodic work, polling, and queue-draining loops
Out of scope
- DI registration mechanics and service lifetimes -- see [skill:dotnet-csharp-dependency-injection]
- Async/await patterns and cancellation token propagation -- see [skill:dotnet-csharp-async-patterns]
- Project scaffolding -- see [skill:dotnet-scaffold-project]
- Testing strategies for background services -- see [skill:dotnet-testing-strategy] and [skill:dotnet-integration-testing]
- Channel fundamentals and drain patterns -- see [skill:dotnet-channels]
Cross-references: [skill:dotnet-csharp-async-patterns] for async patterns in background workers, [skill:dotnet-csharp-dependency-injection] for hosted service registration and scope management, [skill:dotnet-channels] for Channel patterns used in background work queues.
BackgroundService vs IHostedService
| Feature | BackgroundService |
IHostedService |
|---|---|---|
| Purpose | Long-running loop or continuous work | Startup/shutdown hooks |
| Methods | Override ExecuteAsync |
Implement StartAsync + StopAsync |
| Lifetime | Runs until cancellation or host shutdown | StartAsync runs at startup, StopAsync at shutdown |
| Use when | Polling queues, processing streams, periodic jobs | Database migrations, cache warming, resource cleanup |
BackgroundService Patterns
Basic Polling Worker
public sealed class OrderProcessorWorker(
IServiceScopeFactory scopeFactory,
ILogger<OrderProcessorWorker> logger) : BackgroundService
{
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
logger.LogInformation("Order processor started");
while (!stoppingToken.IsCancellationRequested)
{
try
{
using var scope = scopeFactory.CreateScope();
var processor = scope.ServiceProvider
.GetRequiredService<IOrderProcessor>();
var processed = await processor.ProcessPendingAsync(stoppingToken);
if (processed == 0)
{
// No work available -- back off to avoid tight polling
await Task.Delay(TimeSpan.FromSeconds(5), stoppingToken);
}
}
catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
{
// Expected during shutdown -- do not log as error
break;
}
catch (Exception ex)
{
logger.LogError(ex, "Error processing orders");
// Back off on error to prevent tight failure loops
await Task.Delay(TimeSpan.FromSeconds(30), stoppingToken);
}
}
logger.LogInformation("Order processor stopped");
}
}
// Registration
builder.Services.AddHostedService<OrderProcessorWorker>();
```text
### Critical Rules for BackgroundService
1. **Always create scopes** -- `BackgroundService` is registered as a singleton. Inject `IServiceScopeFactory`, not
scoped services directly.
2. **Always handle exceptions** -- by default, unhandled exceptions in `ExecuteAsync` stop the host (configurable via
`HostOptions.BackgroundServiceExceptionBehavior`). Wrap the loop body in try/catch.
3. **Always respect the stopping token** -- check `stoppingToken.IsCancellationRequested` and pass the token to all
async calls.
4. **Back off on empty/error** -- avoid tight polling loops that waste CPU. Use `Task.Delay` with the stopping token.
---
## IHostedService Patterns
### Startup Hook (Cache Warming, Migrations)
```csharp
public sealed class CacheWarmupService(
IServiceScopeFactory scopeFactory,
ILogger<CacheWarmupService> logger) : IHostedService
{
public async Task StartAsync(CancellationToken cancellationToken)
{
logger.LogInformation("Warming caches");
using var scope = scopeFactory.CreateScope();
var cache = scope.ServiceProvider.GetRequiredService<IProductCache>();
await cache.WarmAsync(cancellationToken);
logger.LogInformation("Cache warmup complete");
}
public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
}
```text
### Startup + Shutdown (Resource Lifecycle)
```csharp
public sealed class MessageBusService(
ILogger<MessageBusService> logger) : IHostedService
{
private IConnection? _connection;
public async Task StartAsync(CancellationToken cancellationToken)
{
logger.LogInformation("Connecting to message bus");
_connection = await CreateConnectionAsync(cancellationToken);
}
public async Task StopAsync(CancellationToken cancellationToken)
{
logger.LogInformation("Disconnecting from message bus");
if (_connection is not null)
{
await _connection.CloseAsync(cancellationToken);
_connection = null;
}
}
private static Task<IConnection> CreateConnectionAsync(
CancellationToken ct)
{
// Connection setup logic
throw new NotImplementedException();
}
}
```text
---
## Hosted Service Lifecycle
Understanding the startup and shutdown sequence is critical for correct behavior.
### Startup Sequence
1. `IHostedService.StartAsync` is called for each registered service **in registration order**
2. `BackgroundService.ExecuteAsync` is called after `StartAsync` completes (it runs concurrently -- the host does not
wait for it to finish)
3. The host is ready to serve requests after all `StartAsync` calls complete
**Important:** `ExecuteAsync` must not block before yielding to the caller. The first `await` in `ExecuteAsync` is where
control returns to the host. If you have synchronous setup before the first `await`, keep it short or move it to
`StartAsync` via an override.
```csharp
public sealed class MyWorker : BackgroundService
{
// StartAsync runs to completion before the host is ready.
// Override only if you need guaranteed pre-ready initialization.
public override async Task StartAsync(CancellationToken cancellationToken)
{
// Initialization that MUST complete before host accepts requests
await InitializeAsync(cancellationToken);
await base.StartAsync(cancellationToken);
}
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
// This runs concurrently with the host
while (!stoppingToken.IsCancellationRequested)
{
await DoWorkAsync(stoppingToken);
}
}
private Task InitializeAsync(CancellationToken ct) => Task.CompletedTask;
private Task DoWorkAsync(CancellationToken ct) => Task.CompletedTask;
}
```text
### Shutdown Sequence
1. `IHostApplicationLifetime.ApplicationStopping` is triggered
2. The host calls `StopAsync` on each hosted service **in reverse registration order**
3. For `BackgroundService`, the stopping token is cancelled, then `StopAsync` waits for `ExecuteAsync` to complete
4. `IHostApplicationLifetime.ApplicationStopped` is triggered
---
## Channels Integration
See [skill:dotnet-channels] for comprehensive `Channel<T>` guidance including bounded/unbounded options,
`BoundedChannelFullMode`, backpressure strategies, `itemDropped` callbacks, multiple consumers, performance tuning, and
drain patterns.
The most common integration is a channel-backed background task queue consumed by a `BackgroundService`:
```csharp
// Channel-backed work queue -- register as singleton
public sealed class BackgroundTaskQueue
{
private readonly Channel<Func<IServiceProvider, CancellationToken, Task>> _queue
= Channel.CreateBounded<Func<IServiceProvider, CancellationToken, Task>>(
new BoundedChannelOptions(100) { FullMode = BoundedChannelFullMode.Wait });
public ChannelWriter<Func<IServiceProvider, CancellationToken, Task>> Writer => _queue.Writer;
public ChannelReader<Func<IServiceProvider, CancellationToken, Task>> Reader => _queue.Reader;
}
// Consumer worker
public sealed class QueueProcessorWorker(
BackgroundTaskQueue queue,
IServiceScopeFactory scopeFactory,
ILogger<QueueProcessorWorker> logger) : BackgroundService
{
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
while (await queue.Reader.WaitToReadAsync(stoppingToken))
{
while (queue.Reader.TryRead(out var workItem))
{
try
{
using var scope = scopeFactory.CreateScope();
await workItem(scope.ServiceProvider, stoppingToken);
}
catch (Exception ex)
{
logger.LogError(ex, "Error executing queued work item");
}
}
}
}
}
// Registration
builder.Services.AddSingleton<BackgroundTaskQueue>();
builder.Services.AddHostedService<QueueProcessorWorker>();
```text
---
## Graceful Shutdown
### Host Shutdown Timeout
By default, the host waits 30 seconds for services to stop. Configure this for long-running operations:
```csharp
builder.Services.Configure<HostOptions>(options =>
{
options.ShutdownTimeout = TimeSpan.FromSeconds(60);
});
```text
### Responding to Application Lifetime Events
```csharp
public sealed class LifecycleLogger(
IHostApplicationLifetime lifetime,
ILogger<LifecycleLogger> logger) : IHostedService
{
public Task StartAsync(CancellationToken cancellationToken)
{
lifetime.ApplicationStarted.Register(() =>
logger.LogInformation("Application started"));
lifetime.ApplicationStopping.Register(() =>
logger.LogInformation("Application stopping -- begin cleanup"));
lifetime.ApplicationStopped.Register(() =>
logger.LogInformation("Application stopped -- cleanup complete"));
return Task.CompletedTask;
}
public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
}
```text
---
## Periodic Work with PeriodicTimer
Use `PeriodicTimer` instead of `Task.Delay` for more accurate periodic execution:
```csharp
public sealed class HealthCheckReporter(
IServiceScopeFactory scopeFactory,
ILogger<HealthCheckReporter> logger) : BackgroundService
{
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
using var timer = new PeriodicTimer(TimeSpan.FromMinutes(1));
while (await timer.WaitForNextTickAsync(stoppingToken))
{
try
{
using var scope = scopeFactory.CreateScope();
var reporter = scope.ServiceProvider
.GetRequiredService<IHealthReporter>();
await reporter.ReportAsync(stoppingToken);
}
catch (Exception ex)
{
logger.LogError(ex, "Health check report failed");
}
}
}
}
```text
---
## Agent Gotchas
1. **Do not inject scoped services into BackgroundService constructors** -- they are singletons. Always use
`IServiceScopeFactory`.
2. **Do not use `Task.Run` for background work** -- use `BackgroundService` for proper lifecycle management and graceful
shutdown.
3. **Do not swallow `OperationCanceledException`** -- let it propagate or re-check the stopping token. Swallowing it
prevents graceful shutdown.
4. **Do not use `Thread.Sleep`** -- use `await Task.Delay(duration, stoppingToken)` or `PeriodicTimer`.
5. **Do not forget to register** -- `AddHostedService<T>()` is required; merely implementing the interface does nothing.
---
## References
- [Background tasks with hosted services](https://learn.microsoft.com/en-us/aspnet/core/fundamentals/host/hosted-services)
- [BackgroundService](https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.hosting.backgroundservice)
- [IHostedService interface](https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.hosting.ihostedservice)
- [Generic host shutdown](https://learn.microsoft.com/en-us/dotnet/core/extensions/generic-host#host-shutdown)
- [PeriodicTimer](https://learn.microsoft.com/en-us/dotnet/api/system.threading.periodictimer)