Time-based scaling with Kubernetes CronJob: How to avoid deployments overriding minReplicas - kubernetes

Question

I have a HorizontalPodAutoscalar to scale my pods based on CPU. The minReplicas here is set to 5:
apiVersion: autoscaling/v2beta2
kind: HorizontalPodAutoscaler
metadata:
name: myapp-web
spec:
scaleTargetRef:
apiVersion: apps/v1
kind: Deployment
name: myapp-web
minReplicas: 5
maxReplicas: 10
metrics:
- type: Resource
resource:
name: cpu
target:
type: Utilization
averageUtilization: 50
I've then added Cron jobs to scale up/down my horizontal pod autoscaler based on time of day:
kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
namespace: production
name: cron-runner
rules:
- apiGroups: ["autoscaling"]
resources: ["horizontalpodautoscalers"]
verbs: ["patch", "get"]
---
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1beta1
metadata:
name: cron-runner
namespace: production
subjects:
- kind: ServiceAccount
name: sa-cron-runner
namespace: production
roleRef:
kind: Role
name: cron-runner
apiGroup: rbac.authorization.k8s.io
---
apiVersion: v1
kind: ServiceAccount
metadata:
name: sa-cron-runner
namespace: production
---
apiVersion: batch/v1beta1
kind: CronJob
metadata:
name: django-scale-up-job
namespace: production
spec:
schedule: "56 11 * * 1-6"
successfulJobsHistoryLimit: 0 # Remove after successful completion
failedJobsHistoryLimit: 1 # Retain failed so that we see it
concurrencyPolicy: Forbid
jobTemplate:
spec:
template:
spec:
serviceAccountName: sa-cron-runner
containers:
- name: django-scale-up-job
image: bitnami/kubectl:latest
command:
- /bin/sh
- -c
- kubectl patch hpa myapp-web --patch '{"spec":{"minReplicas":8}}'
restartPolicy: OnFailure
----
apiVersion: batch/v1beta1
kind: CronJob
metadata:
name: django-scale-down-job
namespace: production
spec:
schedule: "30 20 * * 1-6"
concurrencyPolicy: Forbid
successfulJobsHistoryLimit: 0 # Remove after successful completion
failedJobsHistoryLimit: 1 # Retain failed so that we see it
jobTemplate:
spec:
template:
spec:
serviceAccountName: sa-cron-runner
containers:
- name: django-scale-down-job
image: bitnami/kubectl:latest
command:
- /bin/sh
- -c
- kubectl patch hpa myapp-web --patch '{"spec":{"minReplicas":5}}'
restartPolicy: OnFailure
This works really well, except that now when I deploy it overwrites this minReplicas value with the minReplicas in the HorizontalPodAutoscaler spec (in my case, this is set to 5)
I'm deploying my HPA using kubectl apply -f ~/autoscale.yaml
Is there a way of handling this situation? Do I need to create some kind of shared logic so that my deployment scripts can work out what the minReplicas value should be? Or is there a simpler way of handling this?

Answer 1

I think you could also consider the following two options:
Use helm to manage the life-cycle of your application with lookup function:
The main idea behind this solution is to query the state of specific cluster resource (here HPA) before trying to create/recreate it with helm install/upgrade commands.
Helm.sh: Docs: Chart template guide: Functions and pipelines: Using the lookup function
I mean to check the current minReplicas value each time before you upgrade your application stack.
Manage the HPA resource separately to application manifest files
Here you can handover this task to a dedicated HPA operator, which can coexist with your CronJobs that adjust minReplicas according specific schedule:
Banzaicloud.com: Blog: K8S HPA Operator