.NET API Asynchronous Programming
Welcome to this comprehensive course on asynchronous programming in .NET Web APIs. In modern web development, building scalable and responsive APIs is crucial for handling high traffic and providing excellent user experiences. This course will guide you through everything you need to know to implement efficient async patterns in your ASP.NET Core applications.
Introduction
- What is asynchronous programming?
- Why async matters specifically for Web APIs (.NET / ASP.NET Core)
- Throughput, scalability, and user experience
Async Basics in .NET
- The Task-based Asynchronous Pattern (TAP)
- Task, Task<T>, and ValueTask
- async / await explained with simple examples
- CPU-bound vs I/O-bound operations
Asynchronous ASP.NET Core API Controllers
- Async action method signatures (Task<IActionResult>, etc.)
- Request pipeline & where async fits
- Real-life example: simple CRUD endpoint made fully async
Async Data Access
- Entity Framework Core async methods (ToListAsync, SingleOrDefaultAsync, etc.)
- Async with Dapper / ADO.NET (ExecuteReaderAsync, ExecuteNonQueryAsync)
- Real-life example: “Get Products” API hitting a database asynchronously
Calling External APIs Asynchronously
- Using HttpClient vs IHttpClientFactory
- Timeouts, retries, and transient failures
- Real-life example: API that calls a payment gateway or 3rd-party service
Cancellation & Timeouts
- Understanding CancellationToken
- Propagating HttpContext.RequestAborted to deeper layers
- Handling timeouts gracefully
- Real-life example: cancelling long-running report generation
Error Handling in Async APIs
- try / catch with async / await
- Avoiding swallowed exceptions
- Global exception handling middleware for async APIs
- Returning proper problem details / error models
Async Streams & Streaming Responses
- IAsyncEnumerable<T> and await foreach
- When streaming responses makes sense (large data, live feeds)
- Real-life example: streaming logs / data chunks to the client
Background Work & Async Processing
- IHostedService / BackgroundService
- Queued background tasks (e.g., using channels / queues)
- Real-life example: send email/SMS or generate PDF in background after API returns
Common Pitfalls & Anti-Patterns
- Sync-over-async (blocking on async with .Result / .Wait())
- Deadlocks in ASP.NET & ConfigureAwait(false) discussion
- Thread pool starvation
- Fire-and-forget inside controllers and why it’s risky
Resilience, Idempotency & Retries in Async APIs
- Idempotent API design (for retries safely)
- Using Polly (or similar) for retries, circuit breakers, bulkheads
- Real-life example: idempotent "Create Order" API with retry-safe logic
Observability for Async APIs
- Logging async operations (correlation IDs, scopes)
- Metrics (timers for async calls, success/failure rates)
- Distributed tracing for async call chains
Testing Asynchronous APIs
- Unit testing async methods correctly (Assert.ThrowsAsync, etc.)
- Integration testing ASP.NET Core async controllers (WebApplicationFactory)
- Mocking async dependencies (repositories, HTTP clients)
Performance Tips & Best Practices
- Minimizing allocations (e.g., when ValueTask helps)
- Avoiding unnecessary async
- Connection management (DB and HTTP)
- Checklist for a high-throughput async API
End-to-End Real-Life Example
- A mini domain: e.g., "Leave Management" or "E-commerce Orders"
- Endpoints: create, list, approve, cancel, etc. – all async
- With repository layer, external service calls, background jobs, cancellation, logging
Summary & Final Best-Practices Checklist
- Do’s and Don’ts
- How to gradually async-ify a legacy sync API
- Links to further reading (official docs)
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