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Load testing key concepts

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Harness Load Testing uses virtual users, load profiles, and engine-specific test definitions to validate application performance. These concepts help you select an engine, configure infrastructure, and interpret how Harness runs a test.

What you will learn from this topic​

  • How virtual users and load profiles shape a test.
  • How the Python, JavaScript, and Java engines differ.
  • Where each engine defines pass or fail thresholds.
  • How infrastructure, services, and test definition modes affect a load test.

Virtual users​

A virtual user (VU) simulates a real user who interacts with your application. Each virtual user independently executes the scenario you define: it sends HTTP requests, waits for responses, and repeats the sequence for the duration of the test.

The Number of Users setting controls how many virtual users run concurrently at peak load. Higher concurrency sends more simultaneous requests to your system.

Load profile​

The load profile describes how virtual users enter a test. Harness Load Testing uses a ramp-up phase followed by a steady-state phase:

Users
  |         ___________________
  |        /
  |       /
  |______/
  |________________________________ Time
     ^          ^
  Ramp-Up    Steady State
  • Ramp-up phase: Harness adds virtual users linearly from zero to the target number over the Ramp-Up Duration. This gradual increase lets you observe the system response as load rises.
  • Steady-state phase: The configured number of virtual users continues for the rest of the Test Duration. Steady-state duration equals Test Duration - Ramp-Up Duration.

For example, a 600-second test with a 120-second ramp-up runs at peak load for 480 seconds.

Load test engines​

When you create a load test, you select a Load Test Engine. Each engine provides a language surface over a load tool:

Load Test EngineBased onScript languageInfrastructure
PythonLocustPythonLinux VM and Kubernetes
JavaScriptk6JavaScriptKubernetes
JavaJMeterJava (.jmx plans)Kubernetes
  • Python: Models user behavior as Python classes. Use it for Python-based tests, a simple ramp-up model, or Linux VM infrastructure.
  • JavaScript: Runs k6 scripts. Use it for script-defined pass or fail thresholds and precise load shapes such as spike and soak tests.
  • Java: Runs existing JMeter .jmx test plans. Use it to reuse JMeter plans, override properties at run time, and distribute load across workers.

Only Python runs on a Linux VM. When you set Target Type to Linux VM, the JavaScript and Java options are disabled and marked Coming Soon.

Thresholds​

A threshold is a pass or fail rule for a metric, such as "the 95th-percentile request duration must stay under 5000 ms." A threshold breach marks the run as failed, so you can use the test as a continuous integration gate.

The threshold location depends on the engine:

  • Java: Add criteria in the Thresholds section of the Load Test Studio. Go to Java to configure thresholds.
  • JavaScript: Declare thresholds in options.thresholds inside the script. Harness reports the outcome in the results. Go to JavaScript to review the syntax.

Host URL​

When you upload a script, the Host URL is the base URL of the application under test. The Python engine prepends it to relative paths in your script. For example, with Host URL https://api.example.com, a request to /users becomes https://api.example.com/users. The JavaScript engine receives the same value as __ENV.HOST_URL.

Load test infrastructure​

Load tests run on infrastructure managed by the Harness chaos agent. Harness supports two target types:

Target TypeHow it worksBest forLoad Test Engines
KubernetesThe agent orchestrates a master pod and optional worker pods inside a Kubernetes cluster. Requires v1.85.3+. Load testing is enabled by default, so no additional configuration is needed.Scalable distributed tests, cloud-native environments, and high-concurrency workloads.Python, JavaScript, Java
Linux VMThe agent on a Linux host runs the Locust process locally and streams results back to Harness.Simple setups, on-premises hosts, and direct network access to internal services.Python only

When you create a load test, select a target type first. The Load Test Infrastructure dropdown then lists only infrastructure that matches that type. Go to Infrastructure to configure infrastructure.

Services​

Every load test targets one or more onboarded resilience testing services. The Services section appears on the Overview tab after you select infrastructure, and the picker lists only services onboarded against that infrastructure. You cannot save a load test without at least one service.

This association lets one service report its chaos, load, and disaster recovery results together. Onboard a workload before you create a load test.

Load Test Studio​

You build each load test in the Load Test Studio, a two-step flow:

  • Overview: Configure metadata, infrastructure, services, and the engine.
  • Test Configuration: Define the workload.

The Advanced Options and Variables panels apply to both steps. Use Advanced Options to configure pod cleanup and resource requirements. Use Variables to define reusable values.

Use the VISUAL and YAML toggle to switch between the form and the underlying definition. The YAML shows the exact test configuration:

kind: LoadTest
apiVersion: v1alpha1
metadata:
  name: checkout-load
  serviceReferences:
    - paymentservice-boutique-prod
spec:
  identity: checkoutload
  toolType: Locust
  targetType: kubernetes
  cleanupPolicy: delete
  toolConfig:
    locust:
      mode: script
      variables:
        - name: hostvar
          type: String
          value: example.com

serviceReferences records the target services. cleanupPolicy reflects the Clean-up Load Resources toggle. Variables sit under the engine in toolConfig, so they apply to that engine's inputs.

Test definition modes​

Each engine offers two ways to define a workload. You supply the test as a script, plan, or container image. Harness does not generate the test.

ModePythonJavaScriptJavaDescription
Upload a script or plan.py.js.jmx, .xml, or .zipUpload your script or test plan. Harness runs it as uploaded.
Use a custom imageSupportedSupportedSupportedUse a prebuilt container image that contains the engine binary and test.

A Java .zip file can bundle the JMeter plan with CSV data files and dependencies. The upload limit is 1 MB.

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