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Intro to Testing ASP.NET APIs with k6 - When Unit Tests Meet Load Testing

Software organizations are always on the hunt for ways to satisfy and delight their customers. Application performance is certainly a part of that, and ensuring good performance requires testing. Load testing is one of the types of performance testing and, in this post, you'll see how to use k6 to perform load testing on a .NET RESTful API.

By the end of the post, you’ll have a more solid knowledge of load testing, why it’s important and how it’s performed. You’ll also have practical experience performing load testing with k6, and you’ll understand some of the unique ways in which this tool sets itself apart from competitors. With all of that being said, let’s get started!

Load Testing: The What, Why, and How

Before getting our hands dirty, let’s start with some fundamentals. What is load testing?

Defining Load Testing

Load testing is one of the many types of web performance testing. It consists of verifying how an application behaves under a heavy load—hence the name. With the help of load testing, you can assess the response of your app to increasing demand and then improve it accordingly.

Why Should You Care About Load Testing?

Why is load testing important? Well, as we’ve explained, when it comes to online services, it’s essential to provide a great user experience to your customers, since the competition is only a few clicks away. And performance is a crucial part of the overall user experience of a website.

But why use load testing specifically? In short, you should adopt load testing when you're concerned about the availability and scalability of your website. Even though the frontend is responsible for relevant performance issues, you should focus on the backend to ensure your performance and reliability goals when scaling your site. And load testing is exactly the tool you need to verify how your website reacts when put under load.

How Do You Perform Load Testing

How is load testing actually performed? As you’ll see, that can vary, even because load testing tools can come in different categories. However, the gist of it is that load testing simulates multiple users sending multiple requests to the app, concurrently. You choose the number of users and requests based on the estimated load you expect your app to be able to handle.

Rolling Up Your Sleeves: Let's Load Test An ASP.NET API

After our brief introduction to load testing, it’s time to learn how to do it in practice.

Obtaining The Requirements

The application we’ll be testing throughout this tutorial is a RESTful API written in ASP.Net Core. To be able to run the application, you’ll need the .NET SDK version 5.0 or later.

Presenting the Sample Application

For this tutorial, our sample application will be the sample API developed using ASP.Net Core, demonstrated in this tutorial.

Download or clone the complete application from this GitHub repository.

As soon as you get the code, cd into the project’s directory and run the following command:

dotnet run
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Now, open your browser and go to localhost:<port>/api/TodoItems to see the resources that are returned.

Let's Start Testing

Here’s where the fun begins: we’ve gotten ourselves an API, so let’s test it.

One thing to keep in mind when doing performance testing is that, as we’ve mentioned, it comes in different types. More specifically, we can split load testing tools into two main camps: scriptable and non-scriptable ones.

Scriptable tools are those that allow you to use a full-fledged scripting language to write your tests. Such tools might offer a steeper learning curve and, in some situations, consume slightly more resources. However, the upside is that they enable users to write flexible and more complex tests.

On the other end of the spectrum, you have non-scriptable tools. They tend to be simpler and consume fewer resources. But on the other hand, they're way less powerful when it comes to the capabilities they offer.

This post is about k6, which is a powerful scriptable tool. But we won’t jump into k6 right away. Instead, we’ll start by testing our API with a non-scriptable tool. That way, you’ll be able to appreciate the differences between those two categories of tools.

Testing a GET Endpoint With a Non-Scriptable Tool

As I’ve mentioned earlier, it’s possible to divide load testing tools into two broad groups: scriptable and non-scriptable ones. So, to appreciate the qualities of the former group, you need at least a little bit of experience with the latter. That’s why before we get to k6 we’ll take a brief detour to show you how testing an endpoint with a non-scriptable tool looks like.

The non-scriptable tool we’ll use is wrk. Feel free to follow along with this part of the tutorial but, since it’s a detour, I won’t provide detailed instructions for it. With that out of the way, let’s begin.

After installing wrk, I’ll run my sample project again by executing:

dotnet run
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Then, I’ll use wrk to test the GET endpoint for the TodoItem resource:

./wrk -t12 -c400 -d30s https://localhost:5001/api/TodoItems/
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This is exactly the same "basic usage" example taken from the project's README. It runs a 30 seconds test that uses 12 threads keeping 400 connections open. What do the results look like? See in the following image:

Alt Text

Let's unpack that. After a short text summarizing the executed test (30 seconds, 12 threads, 400 connections) we see statistics regarding the latency and requests per second. For each of those we see, among other info, the average value (1), the standard deviation (2), the maximum value (3).

We also see the number of requests that were performed, in how much time, and the total volume of data that was read (5). Finally, we see the total amount of requests per second and the volume of data transferred.

Testing a Get Endpoint with k6

Now it’s time to repeat the previous test, this time using k6.


Let’s start by installing the tool. If you use macOS, install k6 is a breeze using homebrew:

brew install k6
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Users of Debian, Ubuntu or other Debian-based distros should run the following commands:

sudo apt-key adv --keyserver hkp:// --recv-keys 379CE192D401AB61
echo "deb stable main" | sudo tee -a /etc/apt/sources.list
sudo apt-get update
sudo apt-get install k6
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If you’re on Windows, you can download the.msi installer. Users of other platforms, take a look at the k6 installation guide.

After the installation is complete, regardless of OS, you should be able to run k6 version to see the current version number and verify the installation was a success.

Writing and Running Your First Test Script

To perform k6 tests, you’ll need to create a test script. Test scripts in k6 are written in JavaScript, which is a perfect choice, considering most web developers will already be familiar with the language—as opposed to something like Python, Ruby, or a custom DSL.

Using your favorite editor, create a file called script.js. I’ll use Visual Studio Code:

code script.js
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Then, add the following code to the new file:

import http from 'k6/http';

export default function () {
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Of course, make sure that you’ve started the API. Also, change the port number if necessary.

After saving the test script, you can test it by running:

k6 run script.js
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The results should look something like this:

          /\      |‾‾| /‾‾/   /‾‾/   
     /\  /  \     |  |/  /   /  /    
    /  \/    \    |     (   /   ‾‾\  
   /          \   |  |\  \ |  (‾)  | 
  / __________ \  |__| \__\ \_____/ .io

  execution: local
     script: script.js
     output: -

  scenarios: (100.00%) 1 scenario, 1 max VUs, 10m30s max duration (incl. graceful stop):
           * default: 1 iterations for each of 1 VUs (maxDuration: 10m0s, gracefulStop: 30s)

running (00m00.1s), 0/1 VUs, 1 complete and 0 interrupted iterations
default ✓ [ 100% ] 1 VUs  00m00.1s/10m0s  1/1 iters, 1 per VU

     data_received..................: 1.8 kB 33 kB/s
     data_sent......................: 816 B  15 kB/s
     http_req_blocked...............: avg=22ms    min=5ms     med=22ms    max=39ms    p(90)=35.6ms   p(95)=37.3ms 
     http_req_connecting............: avg=499.9µs min=0s      med=499.9µs max=999.8µs p(90)=899.82µs p(95)=949.8µs
     http_req_duration..............: avg=3.99ms  min=999.7µs med=3.99ms  max=6.99ms  p(90)=6.39ms   p(95)=6.69ms 
       { expected_response:true }...: avg=3.99ms  min=999.7µs med=3.99ms  max=6.99ms  p(90)=6.39ms   p(95)=6.69ms 
     http_req_failed................: 0.00%  ✓ 0 ✗ 2
     http_req_receiving.............: avg=0s      min=0s      med=0s      max=0s      p(90)=0s       p(95)=0s     
     http_req_sending...............: avg=0s      min=0s      med=0s      max=0s      p(90)=0s       p(95)=0s     
     http_req_tls_handshaking.......: avg=19ms    min=0s      med=19ms    max=38ms    p(90)=34.2ms   p(95)=36.1ms 
     http_req_waiting...............: avg=3.99ms  min=999.7µs med=3.99ms  max=6.99ms  p(90)=6.39ms   p(95)=6.69ms 
     http_reqs......................: 2      37.030248/s
     iteration_duration.............: avg=54ms    min=54ms    med=54ms    max=54ms    p(90)=54ms     p(95)=54ms   
     iterations.....................: 1      18.515124/s
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Improving The Test

You’ve just written and run your first k6 test script. Congrats! However, we need to beef it up a little bit so it compares with the test we’ve done using wrk. Our k6 test is, at the moment, a simple request. It sends a get request to the specified URL and then waits for 1 second.

When running a test with k6, you can pass parameters to the test. Run the following command:

k6 --vus 400 --duration 30s run script.js
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You can see we’ve used two parameters here. The --duration option defines that the test should run for 30 seconds.

The --vus option stands for virtual users, which are used to send the requests to the system under test.To understand the concept of VUs, think of them like parallel infinite loops. If you go back to our script, you'll see there is a default function. Scripts in k6 need to have that default function as a requirement. That's the entry point for the test script. The code inside such a function is what the VUs execute; it gets executed over and over again, for the whole duration of the tests.

Besides the default function, you can have additional code. However, code outside default is run only once per VU. You use that code for initialization duties, such as importing different modules or loading something from your filesystem.

That being said, let’s see what the result of the command’s execution looks like:

          /\      |‾‾| /‾‾/   /‾‾/   
     /\  /  \     |  |/  /   /  /    
    /  \/    \    |     (   /   ‾‾\  
   /          \   |  |\  \ |  (‾)  | 
  / __________ \  |__| \__\ \_____/ .io

  execution: local
     script: script.js
     output: -

  scenarios: (100.00%) 1 scenario, 400 max VUs, 1m0s max duration (incl. graceful stop):
           * default: 400 looping VUs for 30s (gracefulStop: 30s)

running (0m00.9s), 400/400 VUs, 8 complete and 0 interrupted iterations
default   [   3% ] 400 VUs  00.9s/30s


running (0m30.0s), 000/400 VUs, 202254 complete and 0 interrupted iterations
default ✓ [ 100% ] 400 VUs  30s

     data_received..................: 60 MB  2.0 MB/s
     data_sent......................: 36 MB  1.2 MB/s
     http_req_blocked...............: avg=1.66ms   min=0s    med=0s      max=2.68s p(90)=0s      p(95)=0s     
     http_req_connecting............: avg=213.12µs min=0s    med=0s      max=1.32s p(90)=0s      p(95)=0s     
     http_req_duration..............: avg=27.65ms  min=0s    med=24ms    max=1.96s p(90)=40ms    p(95)=54.99ms
       { expected_response:true }...: avg=27.65ms  min=0s    med=24ms    max=1.96s p(90)=40ms    p(95)=54.99ms
     http_req_failed................: 0.00%  ✓ 0     ✗ 404508
     http_req_receiving.............: avg=703.91µs min=0s    med=0s      max=1.35s p(90)=0s      p(95)=999.1µs
     http_req_sending...............: avg=167.24µs min=0s    med=0s      max=1.39s p(90)=0s      p(95)=0s     
     http_req_tls_handshaking.......: avg=1.3ms    min=0s    med=0s      max=2.65s p(90)=0s      p(95)=0s     
     http_req_waiting...............: avg=26.77ms  min=0s    med=24ms    max=1.46s p(90)=39.99ms p(95)=51ms   
     http_reqs......................: 404508 13474.167948/s
     iteration_duration.............: avg=59.23ms  min=998µs med=49.99ms max=2.84s p(90)=78.99ms p(95)=98.99ms
     iterations.....................: 202254 6737.083974/s
     vus............................: 380    min=380 max=400 
     vus_max........................: 400    min=400 max=400 
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What you see above is what the results look like for me after running the latest version of the command. As you can see, there’s a lot going on here. Let’s walk through some of the main pieces of information:

  • data_received: that’s the total amount of data received (109 MB) at a rate of 3.4 MB per second;
  • http_req_duration: information about the duration of the HTTP requests performed, including the average, median, minimum and maximum values;
  • http_req_failed: the rate of requests that failed;
  • http_req_waiting: time spent waiting for the server’s response.

To learn more about the metrics displayed in k6’s results, you can refer to the documentation.

Testing With a Higher Load

We’ll now perform a different test, sending a PUT request instead of a GET one. We’ll also use an additional option of k6 that allows us to simulate an increase in the load on our application.

For that, I’ll create a new test script and name it script2.js. Here’s what its code looks like:

import http from 'k6/http';

const url = 'https://localhost:5001/api/TodoItems/';

export let options = {
  stages: [
    { duration: '15s', target: 100 },
    { duration: '30s', target: 500 },
    { duration: '15s', target: 0 },

export default function () {

  const headers = { 'Accept': '*/*', 'Content-Type': 'application/json', 'Host': 'localhost:5001' };
  const data = {
      name: 'Send e-mail',
      isComplete: false      

  http.put(url, JSON.stringify(data), { headers: headers });
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As you can see, in the script above we’re sending a PUT request to our API, trying to add a new TodoItem resource. Before that, though, we create an options variable in which we define three stages for our test. In the first one, which lasts for 15 seconds, k6 will increase the number of VUs from 0 to 100. Then, it will ramp up to 500 for another 30seconds, before ramping down to 0 in the last 15 seconds.

Here’s what the results look like:

          /\      |‾‾| /‾‾/   /‾‾/   
     /\  /  \     |  |/  /   /  /    
    /  \/    \    |     (   /   ‾‾\  
   /          \   |  |\  \ |  (‾)  | 
  / __________ \  |__| \__\ \_____/ .io

  execution: local
     script: script2.js
     output: -

  scenarios: (100.00%) 1 scenario, 500 max VUs, 1m30s max duration (incl. graceful stop):
           * default: Up to 500 looping VUs for 1m0s over 3 stages (gracefulRampDown: 30s, gracefulStop: 30s)

running (0m00.8s), 006/500 VUs, 6743 complete and 0 interrupted iterations
default   [   1% ] 006/500 VUs  0m00.8s/1m00.0s


running (1m00.0s), 000/500 VUs, 671850 complete and 0 interrupted iterations
default ✓ [ 100% ] 000/500 VUs  1m0s

     data_received..............: 32 MB   537 kB/s
     data_sent..................: 84 MB   1.4 MB/s
     http_req_blocked...........: avg=73.54µs min=0s med=0s      max=530.01ms p(90)=0s      p(95)=0s     
     http_req_connecting........: avg=16.56µs min=0s med=0s      max=212ms    p(90)=0s      p(95)=0s     
     http_req_duration..........: avg=19.56ms min=0s med=14ms    max=307.99ms p(90)=41.99ms p(95)=55ms   
     http_req_failed............: 100.00% ✓ 671850 ✗ 0    
     http_req_receiving.........: avg=34.6µs  min=0s med=0s      max=229.99ms p(90)=0s      p(95)=0s     
     http_req_sending...........: avg=85.33µs min=0s med=0s      max=157ms    p(90)=0s      p(95)=999µs  
     http_req_tls_handshaking...: avg=56.37µs min=0s med=0s      max=364ms    p(90)=0s      p(95)=0s     
     http_req_waiting...........: avg=19.44ms min=0s med=14ms    max=272.99ms p(90)=41ms    p(95)=54.99ms
     http_reqs..................: 671850  11196.657731/s
     iteration_duration.........: avg=20.07ms min=0s med=14.99ms max=632.99ms p(90)=42.99ms p(95)=56.99ms
     iterations.................: 671850  11196.657731/s
     vus........................: 1       min=1    max=500
     vus_max....................: 500     min=500  max=500
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Here's Where a Scriptable Tool Shines: Testing a User Scenario

Up until now, we’ve been testing using fairly simple scripts. You might be wondering what’s all the fuss about scriptable tools after all. Well, here’s the thing: a non-scriptable tool might be enough if you only need to perform basic requests.

However, there are scenarios in which a non-scriptable tool can really make a difference. One of those is where you need to test a user scenario. In other words, you might need to verify a real usage workflow in your app. You simply can’t do that with a non-scriptable tool.

Up until now, we've been testing an endpoint in isolation. Often, when monitoring your services, you might have found a REST API underperforming at a certain load level and want to simulate this behavior again. A non-scriptable tool is often enough for this type of testing.

Such a type of verification, however, isn’t the most realistic. Why? Well, users don’t behave like that. They don’t do things in a completely isolated way. In the real world, your application continuously responds to a flow of real-user interactions, e.g., visit a page, log in, list items, purchase some items, and so on.

Testing real-world scenarios allow you to validate critical business logic or the most frequent user flows. If you want to mimic this type of interaction, a scriptable tool makes this job possible and more manageable.

The following script is a simple example of how testing a user workflow could look like. Using k6, we hit the GET endpoint, retrieving the existing resources. We then get the id of the first object, and use that to send another get request, obtaining that resource. After that, we set the value of the completed attribute and send a PUT request to update the resource on the server. Finally, we use a check to verify whether the response has the expected status code.

import http from 'k6/http';
import { check,  sleep } from 'k6';

export let options = {
    vus: 30,
    duration: '40s'

export default function () {
  let url = 'https://localhost:5001/api/TodoItems';

  // getting all todo items
  let response = http.get(url);

  // parsing the response body
  let obj = JSON.parse(response.body);

  // retrieving the id from the first resource
  let idFirstItem = obj[0].id;

  // retrieving the resource with the selected id
  response = http.get(`${url}/${idFirstItem}`);
  let item = JSON.parse(response.body);

  // setting the item as complete
  item.complete = true;

  // updating the item
  const headers = { 'Content-Type': 'application/json' };
  let res = http.put(`${url}/${idFirstItem}`, JSON.stringify(item), { headers: headers });

  // checking the response has the 204 (no content) status code
  check(res, {
    'is status 204': (r) => r.status === 204,

  // random think time between 0 and 5 seconds
  sleep(Math.random() * 5);
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In the previous example, the test runs the same user flow continuously until the test duration ends. But with k6, and you can also simulate different user flows with varying load patterns to run simultaneously. This flexibility allows you to test your application more precisely.

For example, you could run 50 virtual users doing some actions and 100 virtual users doing something different for a different period while generating a constant request rate to one endpoint.

export const options = {

  scenarios: {
    scenario1: {
      executor: 'constant-vus',
      duration: "1m",
      vus: 50,
      exec: "userFlowA"

    scenario2: {
      executor: 'ramping-vus',
      stages: [
        { duration: '1m', target: 100 },
        { duration: '30s', target: 0 },
      exec: "userFlowB"

    scenario3: {
      executor: 'constant-arrival-rate',
      rate: 200,
      timeUnit: '1s',
      duration: '1m',
      preAllocatedVUs: 50,
      exec: "hammerEndpointA"
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To learn more about configuring advanced traffic patterns, check out the Scenarios API.

Here's Where k6 Shines: Setting Performance Goals

The ability to set performance goals for your application is an aspect that differentiates k6 from other load testing tools. With k6, you can verify the performance of your app against expected baselines in a way that’s not that different from assertions in unit testing.

How does that work in practice? There are two main options that k6 provides: checks and thresholds.

Checks allow you to set expectations in your script and verify those expectations automatically. We’ve used one check in our previous example. Even if the checks fail, the execution of the script doesn’t stop.

Thresholds, on the other hand, do interrupt the script’s execution. They’re criteria you can use to fail your load test script when the system under test doesn’t meet the expectations.

Here are some examples of thresholds you could use:

  • The system generates at the most 1% errors.
  • Response time for 99% of requests should be below 400ms.
  • Response time for a specific endpoint must always be below 300ms.

Since k6 allows you to define custom metrics, you can always define thresholds on them as well. Here’s an example of what thresholds look like in an actual script:

export let options = {
  thresholds: {
    http_req_failed: ['rate<0.01'],   // http errors should be less than 1% 
    http_req_duration: ['p(99)<400'], // 99% of requests should be below 400ms
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k6: When Unit Tests Meet Load Testing

In this post, you’ve learned how to get started with load testing using k6. We’ve covered some fundamentals on load testing: you’ve learned the definition of this technique, and the differences between scriptable and non-scriptable load testing tools.

k6 belongs to the latter group. It allows developers to author test scenarios in JavaScript—a language they’re likely to already know. Because of that, k6 is a powerful tool, enabling the performing not only of HTTP benchmarking, but also the verification of realistic usage scenarios.

Perhaps the greatest differentiator of k6 is that it bridges the gap between unit testing and load testing. Developers can use the same workflow they’ve been using for years: creating tests, adding verifications with pass/fail criteria, adding those to the CI/CD pipeline, and then be notified when the tests do fail.

Top comments (2)

sivakrishna739 profile image
Siva Krishna

This is incredible, Thanks! Here is how you can devise a good strategy for load testing -

solixzsys profile image

Thanks. Great topic great writing