Batch horizontal pod autoscaling - kubernetes

Question

Looking at HPA (pretty new to this), usecase I'm dealing with is to apply the same HPA rules to all deployment (in a specific namespace).
so I'd ideally want to implement something like this :
apiVersion: autoscaling/v1
kind: HorizontalPodAutoscaler
metadata:
name: generalHpa
spec:
scaleTargetRef:
apiVersion: apps/v1
kind: Deployment
name: [deploymentObject1, deploymentObject2, deploymentObject3,...]
minReplicas: 1
maxReplicas: 10
targetCPUUtilizationPercentage: 50
I was hoping to handle this via label/selector, whereas all deployment objects are marked with a specific label (e.g. enableHpa) and somehow use selector/macthLabels inside HorizontalPodAutoscaler to apply it to all those objects.
But it looks like name is required, and need to be targeted to a specific deployment object.
Any suggestion on how to handle this case and avoid creating hpas one by one for every single deployment by name?

Answer 1

There are two ways of setting up a new HorizontalPodAutoscaler object:
Declarative approach described here:
Creating the autoscaler declaratively
Instead of using kubectl autoscale command to create a HorizontalPodAutoscaler imperatively we can use the following file to create it declaratively:
application/hpa/php-apache.yaml
apiVersion: autoscaling/v1
kind: HorizontalPodAutoscaler
metadata:
name: php-apache
spec:
scaleTargetRef:
apiVersion: apps/v1
kind: Deployment
name: php-apache
minReplicas: 1
maxReplicas: 10
targetCPUUtilizationPercentage: 50
We will create the autoscaler by executing the following command:
kubectl create -f https://k8s.io/examples/application/hpa/php-apache.yaml
Imperative approach i.e. by invoking kubectl autoscale command:
kubectl autoscale deployment nginx-deployment --cpu-percent=50 --min=1 --max=5
The first approach doesn't leave much room for further interpretation. The syntax is strictly specified and you cannot do much about it. As you can see both kind and name of our scaling target should be specified and although your pseudo code may seem like an interesting proposal, it have no chances to work. According to the specification name field is a map/dictionary and a list simply cannot be used in this context.
When it comes to the imperative approach, actually you can automate it by using a fairly simple bash one-liner and make your life a bit easier. If you have... let's say 50 different deployments and you want to autoscale all of them, it can save you a lot of time.
For the sake of simplicity I've created only 3 different deployments:
$ kubectl get deployments
NAME READY UP-TO-DATE AVAILABLE AGE
nginx-deployment-1 3/3 3 3 4m3s
nginx-deployment-2 3/3 3 3 3m58s
nginx-deployment-3 3/3 3 3 3m54s
In order not to create hpa one by one manually, I used the following one-liner bash script:
$ for i in $(kubectl get deployments -o jsonpath='{.items[*].metadata.name}');do kubectl autoscale deployment $i --cpu-percent=50 --min=1 --max=3; done
the result of which is:
horizontalpodautoscaler.autoscaling/nginx-deployment-1 autoscaled
horizontalpodautoscaler.autoscaling/nginx-deployment-2 autoscaled
horizontalpodautoscaler.autoscaling/nginx-deployment-3 autoscaled
Command:
kubectl get deployments -o jsonpath='{.items[*].metadata.name}'
returns only the names of your deployments, so they can be easily iterated through a for loop. Notice that we still have 1-to-1 relation here. To one Deployment corresponds exactly one HorizontalPodAutoscaler object. If you additionally need to deal with different namespaces, the script can be further expanded.
Going back to your specific requirement, the question arises as to the legitimacy of such a solution. Although it may seem quite tempting to manage all your Deployments by one single HorizontalPodAutoscaler object (less work in the very beginning), if you take a closer look at all potential downsides of such approach, you would probably change your mind quickly. First of all, such solution isn't very scalable. In fact it is not scalable at all. Just imagine that for some reason you want to change the targetCPUUtilizationPercentage for a single Deployment object. Well... you have a problem. It is managed by one global autoscaler and you need to quickly redesign your environment and create a separate hpa. So 1-to-1 relation between HorizontalPodAutoscaler and Deployment/ReplicationController/ReplicaSet makes a perfect sense. What you usually need is more granular level of control rather than possibility to manage everything by one huge general object.