SLA (Service Level Agreement)
// Definition
A formal, often contractual commitment about service performance — e.g. "99.9% uptime" or "p95 response under 500ms" — with consequences (credits, penalties) if missed. The SLA is the externally-promised number; it's what the business has told customers it will deliver.
// Why it matters
The SLA is the pass/fail line QA's performance tests should assert against — "is p95 under the SLA threshold?" turns a vague "feels slow" into a contractual fact. QA cares because shipping below SLA has real financial and reputational cost, so the SLA is often the single most important performance assertion in the suite.
// How to test
SLA assertions live in load-test tooling (k6/JMeter), not Cypress: • express the SLA as a hard threshold (p95 < 500ms, error rate < 0.1%) • fail the load-test run if the threshold is breached • track against the SLA over time, not just one run — trends predict breaches • distinguish SLA (external promise) from SLO (internal, usually stricter target)
// Common mistakes
- Testing against an average when the SLA is specified as a percentile
- No automated threshold, so an SLA breach ships unnoticed
- Conflating SLA (contractual) with SLO (internal goal) — they're different bars
// Related terms
SLO (Service Level Objective)
An internal target for service performance — usually stricter than the SLA, giving an early-warning buffer before the contractual line is crossed. If the SLA promises 99.9% uptime, the SLO might be 99.95%, so the team reacts before customers are affected. SLOs are paired with "error budgets" — how much you're allowed to miss.
Percentile (p95, p99)
A statistic that reports the value below which a given percentage of measurements fall. p95 means 95% of requests were faster than this number — and 5% were slower. Performance teams report tail percentiles because averages hide the slow long tail.
Response Time
Total time from request initiation to response completion. Includes latency plus transfer and processing time. The end-user-perceived performance metric.