API Single‑Interface Load Testing: Background, Influencing Factors, Tools, and Process

Background : Single‑interface load testing is carried out during development to quickly gauge an API’s handling capacity, discover performance bottlenecks early, and eliminate risk before production deployment. It is also useful for reproducing real‑world concurrency issues when a service encounters performance problems in production.

Factors Influencing Results : High‑concurrency outcomes are easily affected by external conditions such as the local file‑handle limit, DNS resolution speed, network quality, and server‑side connection limits. Exceeding the OS handle limit (commonly 1024) causes connection failures, so selecting an appropriate concurrency level is crucial; more concurrency does not always mean better results.

Practical guidance suggests testing with incremental concurrency levels (e.g., 10, 100, 500, 1000). If the failure rate stays below 1 %, the load can be increased further. Healthy scaling is indicated only when the per‑second request count continues to rise without a rise in error rates.

Load‑Testing Tools :

JMeter – an Apache‑based open‑source tool supporting HTTP, HTTPS, WebSocket, gRPC, etc.; powerful but resource‑intensive and somewhat complex for single‑API tests.

ab – Apache’s command‑line utility for simple HTTP load testing; works with various servers (Apache, Nginx, Tomcat, IIS) but is cumbersome for complex request payloads.

Apipost – from version 7.0.12 offers a one‑click concurrent testing feature built on a custom Go engine (open‑sourced at https://github.com/Apipost-Team/runnerGo). It handles high‑concurrency HTTP/HTTPS tests well, though it currently lacks WebSocket and gRPC support.

This article focuses on HTTP/HTTPS testing using Apipost for convenience.

Testing Process :

Locate the load‑testing module under the API Debug section (see screenshot).

Enter the target URL, then specify the desired concurrency and iteration count in the "Load Test" tab.

Click the "Start Test" button; the interface shows a "Testing..." status (see screenshot).

After completion, the results are displayed, concluding the test (see screenshot).

Result Calculation : The final metrics are derived as shown in the accompanying diagram.