Suppose a deployment needs to scale on GPU utilization rather than CPU or memory. A Kubernetes Horizontal Pod Autoscaler (HPA) can already access CPU and memory through its built-in metrics endpoint, but it needs a registered custom or external metrics API service to access other metrics. Prometheus Adapter can expose Prometheus metrics through that API for autoscaling. section 6.2.2
The trade-off is configuration and flexibility. Prometheus Adapter requires metrics to be configured in a ConfigMap. KEDA can use metrics already exposed through the Prometheus API and can scale a service to or from zero on suitable signals, such as traffic or queue metrics. Scaling from zero does not work with CPU, memory, or GPU resource metrics. Only one adapter API service can be registered at a time for each metrics API, which complicates switching providers. section 6.2.2
