- Spock

Adoption of HorizontalPodAutoscaler (HPA) usually goes through three phases.

The first phase is discovery. The first time we find out what it does, we usually end up utterly amazed. "Look at this. It scales our applications automatically. I don't need to worry about the number of replicas anymore."

The second phase is the usage. Once we start using HPA, we quickly realize that scaling applications based memory and CPU is not enough. Some apps do increase their memory and CPU usage with the increase in load, while many others don't. Or, to be more precise, not proportionally. HPA, for some applications, works well. For many others, it does not work at all, or it is not enough. Sooner or later, we'll need to extend HPA thresholds beyond those based on memory and CPU. This phase is characterized by disappointment. "It seemed like a good idea, but we cannot use it with most of our applications. We need to fall back to alerts based on metrics and manual changes to the number of replicas."

The third phase is re-discovery. Once we go through the HPA v2 documentation (still in beta at the time of this writing) we can see that it allows us to extend it to almost any type of metrics and expressions. We can hook HPAs to Prometheus, or nearly any other tool, though adapters. Once we master that, there is almost no limit to the conditions we can set as triggers of automatic scaling of our applications. The only restriction is our ability to transform data into Kubernetes' custom metrics.

Our next goal is to extend HorizontalPodAutoscaler definitions to include conditions based on data stored in Prometheus.