NGINX Load balancing feature

Nginx Passive health checks are effective only when the upstream group is defined to have multiple members. When only one upstream server is defined, it is never marked unavailable and users see an outage when it’s unhealthy.

Nginx Active health checks – which are exclusive to NGINX Plus – are special requests that are regularly sent to application endpoints to make sure they are responding correctly. They are separate from and in addition to passive health checks.

Example: NGINX Plus might send a periodic HTTP request to the application’s web server to ensure it responds with a valid response code and the correct content. Active health checks enable continuous monitoring of the health of specific application components and processes. It constitutes a direct measurement of application availability, although that depends on how representative the specified health check is of overall application health.

Refer article : https://www.nginx.com/blog/active-or-passive-health-checks-which-is-right-for-you/