One of the more complicated aspects of testing is modeling your desired load profile. These profiles define the volume and velocity of requests coming into the system being tested. k6 provides a set of executors to shape the execution patterns for your tests.
☝️ An important point to keep in mind is that each executor controls the number of VUs and/or the number--or even rate--of iterations per testing cycle.
We've provided example exercises to walk through the details when working with each of the available executors.
Shared Iterations is the most basic of the executors. As can be inferred from the name, the primary focus will be the number of iterations for your test; this is the number of times your test function will be run.
| Option | Description | Default |
|---|---|---|
iterations |
How many times should your test execute? | 1 |
maxDuration |
Forcibly stop your test if not finished within this timeframe | "10m" |
vus |
Number of virtual users to run concurrently | 1 |
As noted previously, the primary objective is to perform your test iterations number of times, over a time period not to exceed maxDuration. At any time during the test scenario, there should be vus iteration(s) happening unless the total desired iterations have been reached. In this scenario, it is possible for some VUs to perform more work than others.
Experiment with Shared Iterations for yourself!
Per VU Iterations is a slight evolution on the Shared Iterations executor. With this executor, we're still focused on the number of iterations, however, this time we want each virtual user to execute the same number of iterations.
| Option | Description | Default |
|---|---|---|
iterations |
For each VU, how many times should your test execute? | 1 |
maxDuration |
Forcibly stop your test if not finished within this timeframe | "10m" |
vus |
Number of virtual users to run concurrently | 1 |
In this case, we're looking for each of the vus VU(s) to perform your test iterations number of times over a time period not to exceed maxDuration. This means that test iterations are fairly distributed; no single VU performs more than another.
Experiment with Per VU Iterations for yourself!
Constant VUs focuses on continually performing your test over a specified amount of time. This allows each virtual user to perform as many requests as it can within the allowed timeframe.
| Option | Description | Default |
|---|---|---|
duration |
Overall scenario duration | - (required) |
vus |
Number of virtual users to run concurrently | 1 |
As noted above, the primary objective is for each of the vus VU(s) to perform as many test iterations as possible for the required duration time-period, e.g. "30s", "1h", etc.
Experiment with Constant VUs for yourself!
Ramping VUs is an evolution of the Constant VUs executor which introduces stages. This allows k6 to transition the number of desired VUs from one stage to another. Each stage defines its own timeframe for which all VUs will continually perform your test.
| Option | Description | Default |
|---|---|---|
stages |
Consists of a time duration and target for the number of desired VUs |
- (required) |
gracefulRampDown |
Grace period for a test iteration to finish before shutting down a VU when ramping down | "30s" |
startVUs |
Number of virtual users at the beginning of test | 1 |
As a time-based scenario, the total duration is equal to the sum of duration timeframe(s) from each stage. The first stage will begin with startVUs VU(s) and ramp up (or down) linearly over the configured duration to the target number of VUs specified within the stage. The next stage, if configured, will then ramp up or down from that point to the desired target VU(s) over the specified duration timeframe. This pattern continues for each remaining stage. As with Constant VUs, each running VU will continually perform test iterations until the scenario ends.
Experiment with Ramping VUs for yourself!
With the Constant Arrival Rate, we now start to focus on the rate at which your test iterations are performed over a prescribed period of time. k6 will dynamically adjust the number of VUs to achieve the desired rate.
| Option | Description | Default |
|---|---|---|
duration |
Overall scenario duration | - (required) |
preAllocatedVUs |
Number of virtual users at the beginning of test | - (required) |
rate |
Desired iterations per timeUnit to be achieved and maintained |
- (required) |
maxVUs |
Maximum number of virtual users allowed to scale | - (no scaling) |
timeUnit |
Duration to which the desired rate applies |
"1s" |
Our primary focus will be to achieve and maintain an iteration rate of rate per timeUnit over the desired duration. The number of VUs to achieve the desired rate will be managed by k6, and will be anywhere from preAllocatedVUs to maxVUs. Note that there is time and resource overhead associated with VU creation, so defining a reasonable preAllocatedVUs will allow for more testing time at the desired rate.
Experiment with Constant Arrival Rate for yourself!
Ramping Arrival Rate is an evolution of the Constant Arrival Rate executor which introduces stages. This is probably the best candidate for modeling real-world testing scenarios, allowing k6 to transition the desired iteration rate from one stage to another. Each stage defines its own timeframe for which to achieve the desired rate.
| Option | Description | Default |
|---|---|---|
preAllocatedVUs |
Number of virtual users at the beginning of test | - (required) |
stages |
Consists of a time duration and target for the desired iterations per timeUnit |
- (required) |
maxVUs |
Maximum number of virtual users allowed to scale | - (no scaling) |
startRate |
Desired iterations per timeUnit to be achieved and maintained |
0 |
timeUnit |
Duration to which the desired rate applies |
"1s" |
Similar to the Constant Arrival Rate, the main focus is achieving a target iteration rate. The primary difference being the desired rate is achieved within each defined stage. The overall duration will be equal to the sum of duration timeframe(s) from each stage. The first stage will begin with an iteration rate of startRate per timeUnit performed by preAllocatedVUs virtual user(s). The iteration rate will ramp up (or down) linearly over the configured duration to the target rate specified for the stage. The next stage, if configured, will then ramp up or down from that point to the desired target rate over the specified duration timeframe. This pattern continues for each remaining stage. Spikes, valleys, and plateaus can be simulated with these stages.
Experiment with Ramping Arrival Rate for yourself!
Externally Controlled is a completely different executor in that it does not alter the number of virtual users beyond starting the test and setting limits on VUs and duration. This is expected to be provided by external processes using either the k6 REST API or the k6 CLI.
| Option | Description | Default |
|---|---|---|
duration |
Overall scenario duration | - (required) |
maxVUs |
Maximum number of virtual users allowed to be utilized | 0 |
vus |
Number of virtual users to run concurrently | 0 |
The primary objective for this executor is to define the duration timeframe of the test. If nothing else is provided, k6 will essentially be in a state of waiting for commands. If none are given, k6 will simply exit once the duration has been reached. If vus are specified with a non-zero value, the executor will run similarly to the Constant VUs until acted upon by an external actor. The maxVUs setting will put in place a limit to be enforced when receiving external requests to scale up.
Experiment with Externally Controlled for yourself!
See how well you understand the executors with the following quiz. Check your answers with the answer key toward the bottom of the page.
A system runs with approximately 80 concurrent users and encounters occasional spikes of 50 additional users. How would this best be modelled?
A: Use the ramping-arrival-rate with multiple stages. Start with 80 VUs with a stage to maintain for a short period, then another stage to target 130 users within a short duration, then another to bring back to 80 users.
B: Use the constant-vus having the 130 virtual users. Have this run with a 5-minute duration.
C: Use the ramping-vus with multiple stages. Start with 80 VUs with a stage to maintain for a short period, then another stage to target 130 users within a short duration, then another to bring back to 80 users.
We're looking to establish some baseline metrics to define an SLA for an existing service, what is the quickest way to achieve this?
A: Use the shared-iterations executor to run through 1,000 requests.
B: Use the constant-arrival-rate to see how many virtual users it takes to maintain a constant 50 requests per second (RPS).
C: Use the externally-controlled executor to start k6 in server mode to have your sweet Bash script ramp up virtual users.
Your SRE team has been seeing issues with your service hitting a garbage collection pause once an instance starts exceeding 30 RPS. You're not on Kubernetes yet, so scaling isn't an easy option. How can your developers simulate the load locally in order to test their json marshaling code?
A: Use the constant-vus to simulte 30 virtual users performing requests as quickly as possible.
B: Use the constant-arrival-rate to maintain a constant 30 requests per second (RPS).
C: Use the per-vu-iterations executor to have 30 virutal users run 1,000 requests each.
🚀 Want more Check out the k6 Office Hours session where we talked in depth about Executors in k6!
- C.
ramping-vusallows you to model spikes in concurrent virtual users. - A. With
shared-iterationswe can easily run through a fixed number of test iterations, with or without concurrency. We're after simple service-levels to establish a baseline, so probably won't need anything overly complex. - B. With the
constant-arrival-rateexecutor, you can have your script achieve and maintain the targeted request rate to monitor your memory heap.