DOTNET: Threading Design Example threading anti-patternsm, proper async/await & concurrency behavior

We can do this with one self-contained minimal API that has:

• A “bad” endpoint showing threading anti-patterns (blocking, .Result, Thread.Sleep)
• A “good” endpoint using proper async/await
• A /stats endpoint to see concurrency behavior
• Then we’ll use dotnet-counters + simple load to see the impact.

1️⃣ Create the project

dotnet new web -n ThreadingDemo
cd ThreadingDemo
Code language: JavaScript (javascript)

Replace all contents of Program.cs with this:

This single file gives you:

• /bad → blocked threads / ThreadPool pressure
• /good → healthy async behavior
• /stats → concurrency snapshot

2️⃣ Run the app

From the ThreadingDemo folder:

dotnet run

It will listen on:

• http://localhost:5000 (default)

Test quickly:

curl http://localhost:5000/bad
curl http://localhost:5000/good
curl http://localhost:5000/stats
Code language: JavaScript (javascript)

3️⃣ Experience the problem: BAD threading under load

The /bad endpoint:

• Uses Thread.Sleep() → blocks the thread
• Uses Task.Delay(100).Result → sync-over-async, blocks the thread
• Under concurrent load, the ThreadPool threads get stuck, new requests wait ⇒ latency goes up, throughput drops.

🧪 Simulate load on /bad

Option A – PowerShell (parallel-ish)

# Fire 50 requests, roughly in parallel
1..50 | ForEach-Object {
    Start-Job { curl "http://localhost:5000/bad" }
}
Code language: PHP (php)

Or more aggressively:

1..200 | ForEach-Object {
    Start-Job { curl "http://localhost:5000/bad" }
}
Code language: JavaScript (javascript)

Then check:

curl "http://localhost:5000/stats"
Code language: JavaScript (javascript)

You’ll see:

• maxBad grow
• Responses from /bad will be slow (hundreds or thousands of ms)

4️⃣ Compare with GOOD endpoint under same load

Do the same with /good:

1..200 | ForEach-Object {
    Start-Job { curl "http://localhost:5000/good" }
}
curl "http://localhost:5000/stats"
Code language: JavaScript (javascript)

You should observe:

• GOOD handled in ... ms is more stable
• maxGood may be higher (more concurrency successfully handled)
• Latency is smoother because threads are not blocked — they’re freed while awaiting.

5️⃣ Debug / Observe Threading Issues with Tools

Now let’s add tools on top, so you can show this in training.

🔹 Step 1: Find the process ID

In a new terminal:

dotnet-counters ps

Look for ThreadingDemo / dotnet with the project path.

Note the PID (e.g., 12345).

🔹 Step 2: Monitor runtime with dotnet-counters

Run:

dotnet-counters monitor --process-id <PID> System.Runtime Microsoft.AspNetCore.Hosting
Code language: CSS (css)

Watch these metrics while hitting /bad vs /good:

Key ones:

• ThreadPool Thread Count
• ThreadPool Queue Length
• CPU Usage
• Requests / sec (from hosting)
• gc-heap-size

What you should see

When hammering /bad:

• ThreadPool Thread Count goes up
• ThreadPool Queue Length might stay elevated
• CPU can be high due to lots of blocking and context switching
• Requests/sec typically lower than expected

When hammering /good:

• ThreadPool threads are reused efficiently
• Queue Length often stays low
• CPU usage is better for same number of requests
• Requests/sec improves, latency is lower

🔹 Step 3: Visual Studio Diagnostic Tools (optional)

If you run from Visual Studio:

1. Start the app with Debug → Start Debugging.
2. Open Debug → Windows → Parallel Stacks / Parallel Tasks.
3. Watch the number of running threads and tasks as you hammer /bad and /good.

You’ll see:

• For /bad: more stuck threads, longer lifetimes
• For /good: tasks start/complete quickly, threads not held hostage

6️⃣ How to explain this behavior conceptually

In /bad:

• Thread.Sleep → the worker thread is doing nothing but cannot process other requests.
• Task.Delay(...).Result → the operation is asynchronous internally, but you force it to be synchronous, so:
  • The thread blocks until delay finishes
  • Under load: too many blocked threads → ThreadPool grows → context switching overhead → queue length and latency grow.

In /good:

• await Task.Delay(...) → the thread returns to the pool while waiting
• When the delay completes, the continuation resumes (on a thread pool worker)
• The same set of threads can handle many more in-flight requests.

So it’s mostly a programming practice issue, not a “.NET design flaw”:

• ❌ Bad code: blocking, sync-over-async, Thread.Sleep on server
• ✅ Good code: async all the way, no .Result, no .Wait()

7️⃣ Quick summary for your training slide

You can summarize this demo as:

• /bad: Blocking calls (Thread.Sleep, .Result) → ThreadPool starvation → high latency, low throughput
• /good: Proper async/await → threads freed → better scalability and responsiveness
• Tools: dotnet-counters + /stats endpoint give direct visibility into concurrency & runtime behavior.

Rajesh Kumar

I’m a DevOps/SRE/DevSecOps/Cloud Expert passionate about sharing knowledge and experiences. I have worked at Cotocus. I share tech blog at DevOps School, travel stories at Holiday Landmark, stock market tips at Stocks Mantra, health and fitness guidance at My Medic Plus, product reviews at TrueReviewNow , and SEO strategies at Wizbrand.

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