Keeping geospatial web services (WMS, WFS etc) up and running is challenging. You may already have general HTTP-monitoring.

But how would you know when a single layer is failing, or degrading in performance?

This requires Geospatial Service-aware Monitoring.

The Open Source tool GeoHealthCheck (GHC) does exactly that: detailed healthchecks on many (QoS) aspects of (OGC) geospatial web services. But you may want to be relieved of deploying and maintaining a GHC service instance yourself. Hence, geoqos.com offers Hosted GeoHealthCheck, in the Cloud, on a subscription basis.