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What is the difference between active and passive health checks in NGINX?

Both mechanisms exist to keep traffic away from unhealthy backend servers, but they detect failure differently.

Passive Health ChecksActive Health Checks
Available in open-source NGINX via max_fails / fail_timeout.Requires NGINX Plus's health_check directive.
Detects failure only from real client traffic failing.Proactively sends dedicated probe requests on a schedule.
A server must fail live requests before being marked down.Can catch and remove a failing server before real users hit it.

Open-source NGINX only supports passive checks, which means at least a handful of real requests have to fail before a bad server is taken out of rotation. NGINX Plus's active checks close that gap by continuously probing servers independent of client traffic, catching problems earlier at the cost of a commercial license.

Which type of health check requires real client traffic to fail before detecting a problem?
What is required to use active health checks in NGINX?

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