How to create the following test scenario in k6?

Question

My default function is just an http post request.

I would like to test if my app is able to deal with the following scenario:

5 requests, per second, for 2 seconds
15 requests, per second, for 10 seconds
20 requests, per second, for 5 seconds

I'm not sure whether to use ramping-arrival-rate, or ramping-vus

Here's an example of what I used for the first option.

export const options = {
  scenarios: {
    contacts: {
      executor: 'ramping-arrival-rate',
      timeUnit: '1s',
      preAllocatedVUs: 10,
      maxVUs: 200,
      stages: [
        { target: 5, duration: '2s' },
        { target: 15, duration: '10s' },
        { target: 20, duration: '5s' },
        { target: 0, duration: '10s' },
      ],
    },
  },
  thresholds: {
    http_req_duration: ['p(95)<60000'], //units in miliseconds 60000ms = 1m 
    http_req_failed: ['rate<0.01'], // http errors should be less than 1%
    checks: ["rate>0.99"]
  },
};

The strange thing is that I'm not exactly sure if I'm doing something wrong, in light of the results I get:

http_reqs......................: 4       0.070166/s
     iteration_duration.............: avg=41.13s   min=24.94s   med=42.3s    max=54.98s   p(90)=53.5s    p(95)=54.24s  
     iterations.....................: 4       0.070166/s
     vus............................: 126     min=10     max=126
     vus_max........................: 126     min=10     max=126

Why did I only have 4 http requests? I was expecting to have 5 * 2 + 15 * 10 + 20 * 5 = 260

knittl · Accepted Answer · 2023-08-02T06:39:29.927

"ramping-arrival-rate" should be okay, it defines how often your test function is started per time unit (e.g. per second).

Note however that there seems to be a small misunderstanding how stages work.

{ target: 5, duration: '2s' },
{ target: 15, duration: '10s' },

Does not mean "run 5 rps for 2 seconds, then run 15 rps for 10 seconds", but rather "go from 5 rps to 15 rps over the duration of 10 seconds". So after 1 second, you will have 6 rps, after 2 seconds, you will have 7 rps and so on. In other words: the rate will be interpolated linearly.

This gives you a request rate similar to the following diagram:

    /
  /
/

If you want your rate to "jump" immediately to the next value, you need to sprinkle 0-length stages between your regular stages, e.g.

{ target: 5, duration: '0s' },
{ target: 5, duration: '2s' },
{ target: 15, duration: '0s' },
{ target: 15, duration: '10s' },

This gives you a request rate similar to the following diagram:

   __
 _|
|

But even with your current setup, you should get more than 4 iterations. I see that your P(95) is very high (1 minute!) and 40 seconds average response time, so perhaps most of the requests are still running and have not finished when your test terminates? You can set the gracefulStop property in your scenario to a high-enough value, e.g. '2m'.

To further debug, I'd suggest to add console.log statements to check if your function is actually called with the desired rate.

You might also be interested in the official k6 example about instant load increase.

Thanks for your answer. I'll try to implement it during the week... And then I'll accept your answer. I have 1 question, though. What type of statements would you advise me to log on the console? The response, or something else? — An old man in the sea., Jun 04 '22 at 19:14
@Anoldmaninthesea. log _before_ executing your request. Simply something, can be an "x" character. Or you could log the curren iteration or stage. Or VU id. Anything helps. The important part is to write the log message before, not after the request (you want to see if the request is executed, not if it has completed. Only completed requests are included in the summary stats). — knittl, Jun 05 '22 at 07:46

How to create the following test scenario in k6?

1 Answers1