K6 Interview Preparation Guide [2025]

Introduction to K6 Performance Testing

1. What is K6 and its primary purpose?

• K6 is an open-source load testing tool.
• Designed for developers and QA engineers.
• Uses JavaScript for writing test scripts.
• Simulates real user traffic for APIs.
• Focuses on performance and scalability testing.
• Integrates with CI/CD pipelines seamlessly.
• Provides metrics for system reliability evaluation.

2. Why choose K6 for load testing?

K6 stands out for its developer-friendly approach, allowing JavaScript scripting that familiarizes teams with web development. It excels in scalability, handling millions of virtual users while consuming minimal resources, making it ideal for continuous performance validation in CI/CD workflows. Its open-source nature fosters community contributions, ensuring regular updates and extensions for diverse testing needs.

3. When should you use K6 in testing?

• For API and website performance validation.
• During CI/CD pipeline integration for automation.
• When simulating high concurrent user loads.
• For stress testing system breaking points.
• In soak tests for long-duration stability.
• For spike testing sudden traffic surges.
• When needing JavaScript-based script reusability.

4. Where does K6 fit in DevOps?

K6 integrates into DevOps pipelines for automated performance testing, running scripts in CI/CD to catch regressions early. It supports local, cloud, and distributed execution, ensuring scalability.

This placement enables shift-left testing, aligning with agile practices for faster feedback loops.

5. Who typically uses K6 in teams?

• QA engineers for comprehensive load scenarios.
• DevOps professionals for pipeline automation.
• Backend developers for API reliability checks.
• Frontend engineers for user experience simulation.
• SREs for production monitoring and alerts.
• Product managers for performance benchmarking.
• Security teams for vulnerability load impacts.

6. Which scripting language does K6 use?

K6 uses JavaScript (ES6+) for scripting, leveraging its familiarity to simulate realistic user behaviors like HTTP requests and data parameterization.

7. How does K6 simulate virtual users?

• Virtual users execute JavaScript functions repeatedly.
• Controls concurrency with VU options.
• Supports scenarios for phased load increases.
• Measures response times and error rates.
• Integrates think times with sleep functions.
• Handles data parameterization for realism.
• Outputs metrics for performance analysis.

Discover how DevOps automation enhances K6 scripting.

8. What are K6 executors?

• Executors define load test execution patterns.
• Ramping-VU for gradual user increases.
• Constant-VUs for steady load simulation.
• Per-VU-Iterations for fixed iterations per user.
• Shared-Iterations for distributed task completion.
• Supports custom scenarios for complex tests.
• Configures duration and VU limits dynamically.

9. Why use thresholds in K6 tests?

Thresholds set pass/fail criteria for metrics like response time or error rates, automating test outcomes in CI/CD. They ensure reliability by failing builds on performance regressions, providing immediate feedback.

This practice prevents issues from reaching production.

10. When integrate K6 with CI/CD?

• For automated performance checks in pipelines.
• During build stages for early detection.
• With Jenkins or GitHub Actions triggers.
• For regression testing on code changes.
• In release gates for quality assurance.
• For continuous monitoring of deployments.
• When scaling tests with cloud resources.

11. What metrics does K6 collect?

• HTTP response times for endpoint performance.
• Throughput rates for request handling capacity.
• Error rates for failure analysis.
• Virtual user iteration counts.
• Custom metrics for application-specific data.
• Network transfer volumes for bandwidth usage.
• Duration and VU statistics for load patterns.

12. Why is K6 developer-friendly?

K6's JavaScript scripting allows developers to reuse code libraries, making tests familiar and maintainable. Its CLI simplicity and integrations reduce setup time, fostering collaboration between dev and QA teams.

13. When perform smoke testing with K6?

• For quick validation of basic functionality.
• Post-deployment to confirm system health.
• With low VU counts for minimal load.
• Before full load tests for baseline.
• In CI/CD for rapid feedback loops.
• For checking connectivity and responses.
• When identifying early configuration issues.

14. Where run K6 tests in distributed mode?

K6 runs distributed tests across multiple machines or cloud instances for high-scale simulations, using orchestration tools like Kubernetes.

This setup handles massive VU counts effectively.

15. Who benefits from K6 cloud service?

• Teams needing scalable test infrastructure.
• Developers for on-demand execution.
• QA for result visualization dashboards.
• SREs for production-like load simulation.
• Product teams for trend analysis.
• Enterprises for compliance reporting.
• Remote teams for collaborative testing.

16. Which protocols does K6 support?

K6 supports HTTP/1.1, HTTP/2, WebSocket, gRPC, and browser APIs for comprehensive protocol testing.

17. How parameterize data in K6 scripts?

• Use CSV files for external data sources.
• Leverage JavaScript arrays for inline values.
• Implement random selection functions.
• Support environment variables for dynamic inputs.
• Handle JSON parsing for complex payloads.
• Ensure thread-safe data access.
• Validate data integrity during execution.

Explore DevOps governance with K6.

18. What is a K6 scenario?

• Scenarios define test execution phases.
• Combine executors for load patterns.
• Support multiple phases in one test.
• Configure VU and duration per phase.
• Enable graceful load transitions.
• Integrate with thresholds for validation.
• Allow parallel scenario execution.

19. Why modularize K6 scripts?

Modular scripts improve maintainability by separating concerns like setup, requests, and checks. This allows reuse across tests, easier debugging, and collaboration in team environments.

It follows JavaScript best practices for scalability.

20. When use K6 browser module?

• For end-to-end user experience testing.
• When simulating browser interactions.
• For frontend performance validation.
• In testing single-page applications.
• For capturing real browser metrics.
• During UI load impact analysis.
• For cross-browser compatibility checks.

K6 Scripting and Executors

21. What is the default function in K6?

• Entry point for virtual user execution.
• Contains core test logic and requests.
• Runs repeatedly for each VU iteration.
• Supports sleep for think time simulation.
• Integrates checks for pass/fail.
• Handles HTTP and other protocol calls.
• Exports for script modularity.

22. Why use options in K6 scripts?

Options configure test parameters like VUs, duration, and scenarios declaratively. They enable reproducible tests, integration with CI, and flexible load profiles without code changes.

23. When use ramping-vus executor?

• For gradual load increase to baseline.
• In stress testing for peak identification.
• When simulating user ramp-up events.
• For avoiding sudden system overloads.
• During warm-up phases of tests.
• For controlled concurrency growth.
• In production-like traffic patterns.

24. Where define K6 thresholds?

Thresholds are defined in the options object, specifying metrics like http_req_duration and criteria for test success.

They automate failure detection in runs.

25. Who writes custom metrics in K6?

• Developers for application-specific tracking.
• QA for business logic validation.
• SREs for infrastructure performance.
• Testers for user journey checkpoints.
• Product teams for KPI monitoring.
• Security for response anomaly detection.
• Teams for shared metric libraries.

26. Which checks are available in K6?

K6 checks validate responses like status codes, body content, and timings, providing pass/fail rates for test reliability.

27. How handle errors in K6 scripts?

• Use try-catch for exception handling.
• Implement custom error logging.
• Set thresholds for error rate limits.
• Continue execution on non-critical failures.
• Group errors for metric aggregation.
• Integrate with alerting systems.
• Analyze post-run for root causes.

Integrate with SRE practices for error handling.

28. What is sleep in K6 scripting?

• Simulates user think time between actions.
• Prevents overwhelming the system.
• Configures realistic pacing for VUs.
• Supports random delays for variation.
• Integrates in default function loops.
• Measures impact on throughput.
• Adjusts for different user personas.

29. Why use groups in K6?

Groups organize script sections logically, enabling metric grouping for analysis, like separating login and checkout phases.

This improves readability and reporting.

30. When use constant-arrival-rate executor?

• For steady request rate simulation.
• In API throughput capacity testing.
• When VUs vary but rate constant.
• For production traffic modeling.
• During endurance test phases.
• For precise load control.
• In distributed test setups.

31. What are trends in K6 metrics?

• Trends track custom metric percentiles.
• Monitor application-specific performance.
• Support alerting on deviations.
• Integrate with external dashboards.
• Handle time-series data efficiently.
• Enable historical comparison.
• Visualize in Grafana for insights.

32. Why parameterize requests in K6?

Parameterization uses variables for dynamic data like user IDs, preventing caching issues and simulating diverse user behaviors in tests.

33. When use per-vu-iterations executor?

• For fixed iterations per virtual user.
• In functional load combined testing.
• When ensuring all scenarios covered.
• For regression testing with loads.
• During script validation phases.
• For consistent test coverage.
• In small-scale performance checks.

34. Where output K6 results?

K6 outputs results to stdout, JSON, CSV, or InfluxDB for analysis, supporting real-time visualization.

Integrate with Grafana for dashboards.

35. Who customizes K6 extensions?

• Developers for protocol support.
• QA for custom assertions.
• SREs for monitoring integrations.
• Testers for specialized metrics.
• Teams for shared libraries.
• Security for encryption modules.
• Product for business metrics.

36. Which http methods does K6 support?

K6 supports GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS via the http module for comprehensive API testing.

37. How use environment variables in K6?

• Access via __ENV variable in scripts.
• Set for endpoints or credentials.
• Secure sensitive data externally.
• Enable CI/CD parameterization.
• Support different test environments.
• Validate presence in setup.
• Log for debugging purposes.

Learn policy tools for secure env vars.

38. What is setup in K6 scripts?

• Runs once before test execution.
• Prepares shared data or resources.
• Returns values for default function.
• Handles global configurations.
• Supports teardown for cleanup.
• Integrates with external services.
• Enables efficient test initialization.

39. Why use teardown in K6?

Teardown cleans up resources post-test, like closing connections or deleting test data, ensuring no side effects in shared environments.

40. When use shared-iterations executor?

• For distributing iterations across VUs.
• In large-scale functional testing.
• When total iterations predefined.
• For even workload distribution.
• During endurance scenario simulation.
• For resource-efficient execution.
• In parallel test phases.

Advanced K6 Features and Integrations

41. What is K6 browser module?

• Simulates browser interactions for frontend testing.
• Uses Chrome DevTools Protocol.
• Captures real browser metrics.
• Supports navigation and actions.
• Integrates with existing scripts.
• Handles JavaScript execution.
• Measures Core Web Vitals.

42. Why integrate K6 with Grafana?

Grafana visualizes K6 metrics in dashboards, enabling trend analysis, alerting, and team collaboration for performance insights.

43. When use WebSocket in K6?

• For testing real-time applications.
• In chat or streaming service validation.
• When simulating persistent connections.
• For message throughput measurement.
• During latency analysis for events.
• For connection stability checks.
• In full-stack performance testing.

44. Where use gRPC in K6?

gRPC testing validates microservices communication, measuring latency and throughput for high-performance APIs.

Ideal for gRPC-based architectures.

45. Who extends K6 with custom modules?

• Developers for protocol extensions.
• QA for assertion libraries.
• SREs for custom metrics.
• Testers for scenario helpers.
• Teams for reusable components.
• Security for encryption tools.
• Product for KPI integrations.

46. Which output formats does K6 support?

K6 supports JSON, CSV, InfluxDB, Prometheus for flexible result export and analysis.

47. How distribute K6 tests?

• Use cloud service for massive scale.
• Run on multiple local machines.
• Integrate with Kubernetes clusters.
• Leverage Docker for containerized execution.
• Configure load generators centrally.
• Monitor distributed runs in real-time.
• Aggregate results from all nodes.

Explore multi-cloud for distributed K6.

48. What are K6 extensions?

• Custom Go modules for new protocols.
• Enhance core functionality dynamically.
• Support non-HTTP testing needs.
• Integrate external data sources.
• Enable advanced metric collection.
• Compile for specific environments.
• Share via community repositories.

49. Why use tags in K6?

Tags categorize metrics for filtering and analysis, like per-endpoint or user-group, improving post-test insights.

They enable targeted reporting.

50. When use constant-throughput-rate executor?

• For maintaining steady request rates.
• In API capacity planning tests.
• When VUs adjust to rate needs.
• For production traffic emulation.
• During endurance testing phases.
• For precise load control.
• In distributed high-volume scenarios.

51. What is VU in K6 context?

• Virtual User simulates one user thread.
• Executes script iterations independently.
• Consumes resources like real users.
• Configurable for concurrency levels.
• Supports shared state carefully.
• Measured in test options.
• Scales for load simulation.

52. Why handle correlation in K6?

Correlation extracts dynamic values like tokens from responses, using them in subsequent requests for realistic session simulation.

53. When use setup function?

• For preparing shared test data.
• In initializing global resources.
• Before VU execution starts.
• For external API calls.
• To return data for default.
• In complex scenario setups.
• For efficient initialization.

54. Where visualize K6 results?

Visualize in Grafana Cloud or local tools like InfluxDB/Grafana for dashboards and trends.

Supports real-time and historical views.

55. Who uses K6 for API testing?

• Backend developers for endpoint validation.
• QA for load scenario creation.
• SREs for reliability checks.
• DevOps for pipeline integration.
• Product for SLA monitoring.
• Security for rate limit testing.
• Teams for collaborative scripts.

56. Which HTTP module functions in K6?

http.get, http.post, http.put for requests, with batch for parallel calls.

57. How use checks in K6?

• Validate response status codes.
• Check body content matches.
• Assert timing thresholds.
• Group for scenario-specific.
• Report pass/fail rates.
• Integrate with thresholds.
• Debug with detailed logs.

Align with automation pitfalls in checks.

58. What is iteration in K6?

• Single execution of default function.
• Counts per VU for coverage.
• Configures in executor options.
• Measures throughput indirectly.
• Supports data-driven loops.
• Tracks for completion rates.
• Analyzes for performance trends.

59. Why use trends for custom metrics?

Trends aggregate custom data over time, enabling percentile analysis for application-specific performance tracking beyond built-in metrics.

60. When use spike testing in K6?

• For sudden traffic surge simulation.
• In e-commerce peak event testing.
• To identify breaking points quickly.
• With ramping executors for spikes.
• For resilience validation.
• In short-duration high-VU runs.
• For failover mechanism checks.

K6 Best Practices and Troubleshooting

61. What are K6 scripting best practices?

• Modularize code for reusability and maintenance.
• Use environment variables for configuration.
• Implement proper error handling mechanisms.
• Set meaningful thresholds for automation.
• Optimize sleep for realistic pacing.
• Tag metrics for targeted analysis.
• Validate data parameterization thoroughly.

62. Why set thresholds in production tests?

Thresholds automate pass/fail decisions, integrating with CI/CD to block deployments on regressions, ensuring consistent performance standards.

63. When debug K6 script failures?

• For unexpected error rate spikes.
• During high response time anomalies.
• When VU iterations incomplete.
• In CI/CD pipeline failures.
• For custom metric deviations.
• After distributed test inconsistencies.
• Before scaling to cloud.

64. Where log K6 execution details?

Log to console or files with --log-output, capturing errors and metrics for post-analysis.

Integrate with ELK for advanced logging.

65. Who troubleshoots K6 in teams?

• QA leads for script validation.
• DevOps for pipeline issues.
• SREs for resource bottlenecks.
• Developers for code errors.
• Platform for env problems.
• Security for auth failures.
• Teams for collaborative debugging.

66. Which common errors in K6?

Common errors include script syntax issues, missing modules, or invalid options; debug with verbose logging and envtest-like isolation.

67. How optimize K6 for high loads?

• Use distributed execution modes.
• Optimize script for low overhead.
• Leverage cloud service scaling.
• Monitor system resources closely.
• Adjust VU concurrency limits.
• Profile script performance.
• Use efficient data structures.

Avoid over-automation pitfalls in optimization.

68. What is soak testing with K6?

• Long-duration low-load stability test.
• Identifies memory leaks over time.
• Uses constant-vus executor.
• Monitors gradual degradation.
• For production endurance validation.
• Integrates with alerting.
• Analyzes trends for anomalies.

69. Why use CSV for data in K6?

CSV files enable large dataset parameterization, simulating diverse users without hardcoding, improving test realism and maintainability.

70. When run K6 in Docker?

• For containerized CI/CD pipelines.
• In Kubernetes for distributed tests.
• When isolating test environments.
• For reproducible execution setups.
• During cloud-agnostic deployments.
• For resource-limited machines.
• In team-shared testing.

71. What are K6 trends?

• Custom metric percentile tracking.
• Aggregate data over test duration.
• Support alerting on thresholds.
• Visualize in external tools.
• Handle application-specific KPIs.
• Enable historical comparisons.
• Integrate with Grafana dashboards.

72. Why idempotent K6 scripts?

Idempotent scripts ensure repeatable results, avoiding side effects like duplicate data, crucial for CI/CD reliability.

73. When use browser module?

• For frontend interaction simulation.
• In SPA performance testing.
• To capture Core Web Vitals.
• When API tests insufficient.
• For end-to-end user journeys.
• In cross-browser load scenarios.
• For UI rendering under load.

74. Where export K6 results?

Export to InfluxDB, Prometheus, or JSON for analysis, supporting long-term trend tracking.

Visualize with Grafana Cloud.

75. Who defines K6 test scenarios?

• QA for comprehensive coverage.
• DevOps for pipeline fit.
• SREs for reliability focus.
• Developers for code-aligned.
• Product for user-centric.
• Security for threat simulation.
• Teams for collaborative design.

76. Which K6 modules for HTTP?

http module for requests, check for assertions, crypto for auth.

77. How use tags for metrics?

• Tag requests for endpoint grouping.
• Filter results post-execution.
• Enable scenario-specific analysis.
• Support custom dimensions.
• Integrate with visualization tools.
• Track user personas separately.
• Analyze trends by category.

Enhance with DORA metrics.

78. What is teardown function?

• Cleans up after test completion.
• Releases shared resources safely.
• Deletes test data to avoid pollution.
• Closes external connections.
• Logs finalization actions.
• Supports graceful shutdown.
• Ensures environment reset.

79. Why use shared-iterations?

Shared-iterations distribute total tasks across VUs, ensuring even coverage for functional-load tests.

80. When stress test with K6?

• For finding system breaking points.
• In ramping-vus for overload simulation.
• To validate error handling.
• For resource exhaustion analysis.
• During capacity planning phases.
• For failover mechanism testing.
• In high-stakes production prep.

K6 Integrations and Real-World Use

81. What is K6 cloud service?

• Scalable execution for high VU counts.
• Provides dashboards for result visualization.
• Supports distributed test runs.
• Integrates with Git for script management.
• Enables team collaboration features.
• Offers alerting on test failures.
• Handles massive scale testing.

82. Why integrate K6 with Jenkins?

Jenkins integration automates performance tests in builds, failing pipelines on thresholds for quality gates.

83. When use WebSocket testing?

• For real-time app validation.
• In chat application load scenarios.
• To measure connection stability.
• For message throughput capacity.
• During latency under concurrent users.
• For event-driven system testing.
• In full-stack performance suites.

84. Where use gRPC with K6?

gRPC testing measures microservice efficiency, ideal for protobuf-based APIs in distributed systems.

Supports streaming and unary calls.

85. Who builds K6 extensions?

• Go developers for core enhancements.
• JS experts for scripting integrations.
• Community contributors for protocols.
• Enterprise teams for custom needs.
• SREs for monitoring modules.
• QA for assertion extensions.
• Product for metric tools.

86. Which CI tools integrate K6?

GitHub Actions, Jenkins, GitLab CI, CircleCI for automated runs.

87. How use K6 in Kubernetes?

• Deploy as jobs for distributed testing.
• Scale pods for VU distribution.
• Integrate with service meshes.
• Monitor cluster resource impact.
• Automate via Helm charts.
• Collect metrics from nodes.
• Validate app scalability.

Manage with multi-cloud workflows.

88. What are K6 trends use cases?

• Track business transaction times.
• Monitor custom application KPIs.
• Alert on percentile deviations.
• Visualize in Grafana dashboards.
• Compare across test runs.
• Integrate with APM tools.
• Analyze user journey metrics.

89. Why idempotent requests in K6?

Idempotent requests ensure safe retries, avoiding duplicate actions in load tests.

90. When use constant-vus executor?

• For steady load endurance testing.
• In soak tests for stability.
• When simulating constant users.
• For baseline performance measurement.
• During long-running scenarios.
• For resource utilization analysis.
• In production traffic modeling.

91. What is K6 Studio?

• Visual tool for script generation.
• Records user sessions for tests.
• Converts to JavaScript code.
• Supports browser interactions.
• Integrates with cloud service.
• Enables non-coders to start.
• Exports for advanced editing.

92. Why use K6 for API CRUD?

K6 scripts full CRUD operations, validating create, read, update, delete under load for backend robustness.

93. When test OAuth with K6?

• For authentication flow validation.
• In secure API endpoint testing.
• To measure token acquisition time.
• For session management under load.
• During integration with identity providers.
• For error handling in auth failures.
• In end-to-end security scenarios.

94. Where use K6 for WebSocket?

WebSocket testing validates real-time bidirectional communication, measuring message latency and connection drops.

Suitable for chat or live updates.

95. Who uses K6 for soak testing?

• SREs for long-term stability.
• DevOps for endurance validation.
• QA for memory leak detection.
• Platform for resource planning.
• Teams for SLA compliance.
• Security for sustained attack simulation.
• Product for usage pattern analysis.

96. Which best practices for data parameterization?

Use CSV/JSON for large datasets, randomize selections, and ensure unique values to mimic real users accurately.

97. How use K6 with Docker?

• Run tests in containerized environments.
• Mount scripts for easy updates.
• Scale with docker-compose.
• Integrate in Kubernetes jobs.
• Collect logs from containers.
• Ensure network access to targets.
• Monitor resource consumption.

Complement with environment parity.

98. What are K6 checks best practices?

• Define specific assertion criteria.
• Group by transaction phases.
• Combine with thresholds for automation.
• Log failures for debugging.
• Test edge cases thoroughly.
• Review rates in results.
• Iterate based on analysis.

99. Why use K6 for spike testing?

Spike testing simulates sudden loads to identify system resilience, using ramping executors for rapid VU increases.

100. When use K6 extensions?

• For custom protocol support.
• In non-HTTP testing needs.
• To integrate external APIs.
• For advanced metric collection.
• When core modules insufficient.
• For team-specific tools.
• In production-scale customizations.

101. How troubleshoot high error rates?

• Check script syntax and logic.
• Validate target endpoint availability.
• Monitor network and timeouts.
• Review data parameterization.
• Analyze VU concurrency levels.
• Use verbose logging output.
• Isolate with single VU runs.

102. What trends for K6 in 2025?

Trends include AI-assisted script generation, deeper browser module enhancements, and seamless multi-cloud integrations for global testing.

Focus on sustainability metrics grows.

103. Why K6 over JMeter?

K6's JavaScript scripting and low resource use make it faster for developers, unlike JMeter's GUI-heavy approach, ideal for CI/CD.