API resilience is more than uptime
An API can stay online while still failing users. Latency can climb, queues can back up, authentication can slow down, and expensive endpoints can starve the rest of the service. API load testing helps reveal those problems before a traffic spike or Layer 7 attack does.
For DDoS resilience, the test should measure both origin health and defensive controls. A WAF rule that keeps the server alive but blocks legitimate clients is not a clean success.
Build realistic request mixes
Do not test only one endpoint. A realistic API plan includes read-heavy calls, write-heavy calls, authenticated requests, cacheable routes, uncached routes, slow database paths, and health checks. The mix should match production traffic closely enough to expose real bottlenecks.
Use test accounts and controlled data. Keep the source ranges, duration, and concurrency inside the approved scope. Increase load in stages so the team can identify the threshold where behavior changes.
Measure the right signals
Track request latency, error rate, status code mix, queue depth, database connections, cache hit ratio, WAF decisions, rate-limit responses, CPU, memory, and autoscaling behavior. Correlate everything by timestamp so you can see cause and effect.
If the WAF starts challenging users before the origin is saturated, the tuning may be too aggressive. If origin CPU spikes while the WAF passes everything, rules may be too permissive.
Turn results into safeguards
After the test, refine caching, rate limits, WAF rules, authentication cost, database indexes, and autoscaling thresholds. Document the result and rerun the same profile. API load testing becomes valuable when it produces a repeatable improvement cycle.