TL;DR
ReadyAPI includes LoadUI Pro for load testing, but the cost is bundled into an expensive per-user license and the tooling is outdated for modern API workflows. For REST and GraphQL API testing, k6 and Gatling are more capable, free alternatives. Use Apidog for functional API testing and pair it with k6 for efficient, modern load testing.
đź’ˇApidog is a free, all-in-one API development platform for API design, functional testing, mocking, and documentation. Pair it with k6 for a complete modern testing stack. Try Apidog free, no credit card required.
Introduction
Load testing is essential for any API serving real users. You need to verify how your system behaves under concurrent load—like 100 users querying a search endpoint or 500 parallel writes from background jobs. Catching scalability bottlenecks in production is both risky and expensive.
ReadyAPI offers built-in load testing via LoadUI Pro, appealing to teams already using ReadyAPI for functional tests. You can reuse existing test definitions and manage everything through a single interface.
However, LoadUI Pro brings trade-offs. For some teams it fits, but for modern API development, open source tools like k6 and Gatling offer more flexibility, lower cost, and better alignment with developer workflows. This article compares LoadUI Pro, k6, and Gatling, and shows how Apidog integrates with a modern load testing stack.
What LoadUI Pro Actually Does
LoadUI Pro is ReadyAPI’s load testing module. It extends ReadyAPI’s functional tests, letting you run them at scale with configurable virtual users, ramp-up profiles, and test durations.
Key LoadUI Pro features:
- Convert functional tests to load tests: Take any ReadyAPI test case and run it under load—no rewriting needed.
- Load profiles: Simulate different types of load (virtual user scaling, bursts, ramp-ups, custom profiles) via the GUI.
- Live metrics: Monitor response times, error rates, throughput, and virtual user counts in real time. Reports are generated post-run.
- Assertions under load: Define assertions (e.g., 95th percentile response time < 2000ms) and fail tests if thresholds are breached.
Limitations:
- Single-machine execution: Distributed load requires extra infrastructure or isn’t supported.
- GUI-driven: Test definitions are stored in project files, not code—making code review and version control difficult.
- Groovy scripting: Extending test logic requires Groovy, which most developers don’t use.
- JVM overhead: ReadyAPI’s threading model is less efficient than modern tools like k6.
k6: The Modern Open Source Alternative
k6 is an open source load testing tool from Grafana Labs (AGPL-3.0). It’s become the leading choice for REST API load testing.
Why use k6:
- Tests as code: Write load tests in JavaScript. Store them in your git repo, review them like application code, and edit them easily.
import http from 'k6/http';
import { check, sleep } from 'k6';
export const options = {
vus: 50,
duration: '30s',
};
export default function () {
const res = http.get('https://api.example.com/users');
check(res, {
'status is 200': (r) => r.status === 200,
'response time under 500ms': (r) => r.timings.duration < 500,
});
sleep(1);
}
This script runs 50 virtual users for 30 seconds, each making a GET request and asserting status and response time.
- Performance: k6 is written in Go. A single machine can simulate thousands of virtual users with minimal overhead.
- Thresholds: Define pass/fail criteria that integrate with CI/CD pipelines.
export const options = {
thresholds: {
http_req_duration: ['p(95)<500'],
http_req_failed: ['rate<0.01'],
},
};
If 95th percentile response time exceeds 500ms or >1% of requests fail, k6 exits with an error code—ideal for automated pipelines.
- Distributed load: Use k6 Cloud for distributed testing (paid). Local runs use the same scripts.
Pricing: The open source CLI is free. k6 Cloud plans start at ~$49/month and scale with usage.
Gatling: Performance Testing for Java Teams
Gatling is an open source load testing tool popular with Java and Scala teams. It offers a Scala-based simulation DSL and Java API.
Strengths:
- Expressive scenarios: Model complex, stateful user flows (login, browse, cart, checkout) naturally in code.
- HTML reports: Gatling produces detailed dashboards out of the box.
- Traffic recorder: Capture browser/API traffic and auto-generate simulations.
Gatling Enterprise: Paid product for distributed execution, CI/CD integration, and collaboration. For local testing, open source suffices.
Comparison to k6: Gatling fits Java/Scala shops. k6 fits JavaScript-heavy teams. Both are more developer-friendly than LoadUI Pro’s GUI/Groovy model.
LoadUI Pro vs k6: Direct Comparison
| Capability | LoadUI Pro | k6 |
|---|---|---|
| Price | Bundled in ReadyAPI (~$749+/user/year) | Free (open source) |
| Test definitions | ReadyAPI GUI/project file | JavaScript code files |
| Version control | Limited (project XML) | Full (code files) |
| Scripting language | Groovy | JavaScript |
| Protocol support | REST, SOAP, HTTP | REST, WebSocket, gRPC (beta) |
| Distributed load | Limited | Via k6 Cloud |
| CI/CD integration | Testrunner command | k6 CLI |
| Virtual user efficiency | Moderate (JVM) | High (Go runtime) |
| Reuse functional tests | Yes (key strength) | Separate test files |
| Community | Smaller | Large, active |
Key takeaway: LoadUI Pro’s main advantage is reusing ReadyAPI functional test cases as load tests. If you have an extensive ReadyAPI suite, this matters. For new projects or teams migrating to modern stacks, k6’s code-first, git-friendly approach is superior.
How Apidog + k6 Replaces ReadyAPI + LoadUI Pro
Replacing the ReadyAPI + LoadUI Pro stack requires two focused tools:
1. Apidog for Functional Testing
- Apidog covers API design, REST/GraphQL/gRPC/WebSocket testing, mocking, and documentation.
- Test scripts use JavaScript.
- Integrate with CI/CD via the Apidog CLI.
- Suitable replacement for ReadyAPI’s functional test features (except for SOAP/WS-Security specifics).
2. k6 for Load Testing
- Write load tests in JavaScript.
- Run locally or via k6 Cloud for distributed scenarios.
- Integrate via the k6 CLI.
Unified workflow: Both tools work off your OpenAPI spec. Apidog imports specs for functional tests; k6 scripts hit the same endpoints for load tests. API changes are reflected in both test suites.
Example CI/CD pipeline:
stages:
- functional-tests
- load-tests
functional-tests:
stage: functional-tests
script:
- apidog run collection.json --environment staging
only:
- merge_requests
load-tests:
stage: load-tests
script:
- k6 run load-tests/api-load.js --env BASE_URL=$STAGING_URL
only:
- main
- Run functional tests for every merge request.
- Run load tests on merges to main.
- Get fast feedback for regressions and periodic load validation before production.
Cost comparison:
- ReadyAPI + LoadUI Pro (10 users): $7,490–$20,000/year.
- Apidog Basic (10 users) + k6 open source: ~$1,080/year.
- Apidog Basic (10 users) + k6 Cloud (basic): ~$1,668/year.
- The modern stack is far more cost-effective.
FAQ
Does k6 support SOAP load testing?
k6 can POST XML for SOAP, but lacks WSDL import and SOAP-specific tools. For modern REST APIs, use k6. For SOAP, LoadUI Pro is stronger.
Can I convert ReadyAPI load tests to k6?
No automated converter exists. Rewrite scenarios as k6 scripts—usually a few hours per scenario for experienced devs. The k6 scripting model is simpler than Groovy.
How many virtual users can k6 run on a laptop?
k6 can handle 1,000–10,000 virtual users on a modern laptop, depending on test and request rate. ReadyAPI/LoadUI Pro typically maxes out at a few hundred users due to JVM overhead.
Does Gatling support gRPC load testing?
Gatling has experimental gRPC support (v3.10+). k6’s gRPC support is more mature.
Can I run k6 tests without the cloud?
Yes. The open source k6 CLI runs tests locally. k6 Cloud is only required for distributed execution and advanced result storage.
Does LoadUI Pro count against ReadyAPI’s per-user license?
LoadUI Pro is bundled with specific ReadyAPI editions, applied per user. Test execution can often run on a CI agent without a user license, but confirm with your SmartBear contract.
ReadyAPI’s LoadUI Pro works for teams deeply invested in that ecosystem. For modern teams, k6 offers a developer-friendly, scalable, and cost-effective load testing workflow. Pair it with Apidog for a complete, modern API testing stack—at a fraction of the cost.
Top comments (0)