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09-Workload-modeling-with-scenarios.md

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Workload modeling with scenarios

In Workload modeling, we mentioned that one of the challenges is that user flows can be complex. A single test may need to cover multiple end-to-end flows simultaneously. For example, in a storefront application, we may have several members browsing products while many others (hopefully!) are adding items to their cart. To help organize these various flows, the k6 team has created scenarios.

With scenarios, script authors may create distinct flows having different execution patterns and environment settings. Scenarios can be executed in parallel or staggered using time offsets with the startTime option.

By breaking down each of your flows into a different scenario, your overarching script can be more streamlined with logic that is easier to follow. From our storefront example, one scenario would be users populating a shopping cart and placing orders. Another scenario would represent other customers who may be searching for the availability of certain products. Each workflow could be complex; keeping them separate makes script maintenance easier.

Defining scenarios

Each scenario includes the same elements of a workload model in addition to the executor, startTime, and optionally env variables and tags exclusive to the scenario. Check the complete list of scenario options in the documentation.

Save the following script as playground.js.

import { sleep } from 'k6';

export const options = {
    scenarios: {
        // Scenario 1: Users browsing through our product catalog
        users_browsing_products: {
            executor: 'shared-iterations', // name of the executor to use
            env: { WORKFLOW: 'browsing' }, // environment variable for the workflow

            // executor-specific configuration
            vus: 3,
            iterations: 10,
            maxDuration: '10s',
        },
        // Scenario 2: Users adding items to their cart and checking out
        users_buying_products: {
            executor: 'per-vu-iterations', // name of the executor to use
            exec: 'usersBuyingProducts', // js function for the purchasing workflow
            env: { WORKFLOW: 'buying' }, // environment variable for the workflow
            startTime: '2s', // delay start of purchasing workflow
            
            // executor-specific configuration
            vus: 3,
            iterations: 1,
            maxDuration: '10s',
        },
    },
};

export default function () {
  // Model your workflow for searching through products
  console.log(`[VU: ${__VU}, iteration: ${__ITER}, workflow: ${__ENV.WORKFLOW}] Just looking!!!`);
  sleep(1);
}

export function usersBuyingProducts() {
  // Model your workflow for adding items to cart and checking out
  console.log(`[VU: ${__VU}, iteration: ${__ITER}, workflow: ${__ENV.WORKFLOW}] CHA-CHING!!!`);
  sleep(1);
}

Running the script as k6 run playground.js will produce results similar to the following:

$ k6 run playground.js

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

  execution: local
     script: playground.js
     output: -

  scenarios: (100.00%) 2 scenarios, 6 max VUs, 42s max duration (incl. graceful stop):
           * users_browsing_products: 10 iterations shared among 3 VUs (maxDuration: 10s, gracefulStop: 30s)
           * users_buying_products: 1 iterations for each of 3 VUs (maxDuration: 10s, exec: usersBuyingProducts, startTime: 2s, gracefulStop: 30s)

INFO[0000] [VU: 4, iteration: 0, workflow: browsing] Just looking!!!  source=console
INFO[0000] [VU: 1, iteration: 0, workflow: browsing] Just looking!!!  source=console
...
INFO[0002] [VU: 1, iteration: 2, workflow: browsing] Just looking!!!  source=console
INFO[0002] [VU: 5, iteration: 0, workflow: buying] CHA-CHING!!!  source=console
INFO[0003] [VU: 4, iteration: 3, workflow: browsing] Just looking!!!  source=console

running (04.0s), 0/6 VUs, 13 complete and 0 interrupted iterations
users_browsing_products ✓ [======================================] 3 VUs  04.0s/10s  10/10 shared iters
users_buying_products   ✓ [======================================] 3 VUs  01.0s/10s  3/3 iters, 1 per VU

Viewing the results, you should notice that users are browsing products, then after an initial delay, we have another user making purchases.

Configuration options

Our example used a minimal number of configuration options for the scenarios, but the available options include the following:

Remember that additional options may be required depending upon the executor specified. See setting load profiles with executors for more about executors.

Option Description Default
executor Specific executor to "shape" traffic patterns - (required)
env Environment variables specific to the scenario {}
exec Name of the exported JS function to execute for the scenario "default"
gracefulStop Time period to allow an iteration to complete before being forcefully terminated "30s"
startTime Time offset within the test to begin the scenario "0s"
tags Tags specific to the scenario {}

Looking more closely at our example, we created two scenarios: users_browsing_products and users_buying_products.

The idea for the users_browsing_products scenario is to create some background activity for our test. We use the shared-iterations executor to simulate three users browsing the product catalog. We're limiting this activity to 10 iterations and ensure our script lasts no longer than 10 seconds. The env option sets the WORKFLOW environment variable to browsing, which can be used within our JavaScript to provide some execution context. Because we are not specifying the exec option, our scenario will execute whichever JavaScript function is the default.

users_buying_products is our second scenario to simulate three different users making a purchase. This scenario uses the per-vu-iterations to control the activity. The scenario specifies exec: 'usersBuyingProducts' to call our purchasing logic for each scenario iteration. As with the previous scenario, we inject the WORKFLOW environment variable to provide the script with some context for the scenario. This time, however, we specify startTime: '2s' to define a delay for the scenario to begin; this gives the users_browsing_products a "head start" to create our background activity before we start having users making purchases.

Test your knowledge

Question 1

True or False, you can think of scenarios as layers of activity within a more extensive test?

Question 2

True or False, you must specify the exec option for each scenario?

Question 3

True or False, the exec option determines traffic shape for a scenario?

Answers

  1. True. Each scenario acts independently yet can be affected by other scenarios. From our example, we defined user browsing activity in the background, then having users making purchases in the foreground.
  2. False. While explicitly configuring the exec option would be advisable, it is not required. Multiple scenarios configured without the exec option result in each executing the same default function, which may be confusing.
  3. False. The executor ultimately defines the pattern of the load profile for your scenario. The exec denotes which JavaScript function executes for each iteration of your scenario.