Specify Replicas in Deployments or Statefulsets?

In this lesson, we will explore different strategies regarding where to define the replicas, in our Deployments or StatefulSets.

Knowing that HorizontalPodAutoscaler (HPA) manages auto-scaling of our applications, the question might arise regarding replicas. Should we define them in our Deployments and StatefulSets, or should we rely solely on HPA to manage them? Instead of answering that question directly, we’ll explore different combinations and, based on results, define the strategy.

HPA modifies the Deployment #

First, let’s see how many Pods we have in our cluster right now.

kubectl -n go-demo-5 get pods

The output is as follows.

NAME    READY STATUS  RESTARTS AGE
api-... 1/1   Running 0        27m
api-... 1/1   Running 2        31m
db-0    2/2   Running 0        20m
db-1    2/2   Running 0        20m
db-2    2/2   Running 0        21m

We can see that there are two replicas of the api Deployment, and three replicas of the db StatefulSets.

Let’s say that we want to roll out a new release of our go-demo-5 application. The definition we’ll use is as follows.

cat scaling/go-demo-5-replicas-10.yml

The output, limited to the relevant parts, is as follows.

...
apiVersion: apps/v1
kind: Deployment
metadata:
  name: api
  namespace: go-demo-5
spec:
  replicas: 10
...

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: api
  namespace: go-demo-5
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: api
  minReplicas: 2
  maxReplicas: 5
  metrics:
  - type: Resource
    resource:
      name: cpu
      targetAverageUtilization: 80
  - type: Resource
    resource:
      name: memory
      targetAverageUtilization: 80

The important thing to note is that our api Deployment has 10 replicas and that we have the HPA. Everything else is the same as it was before.

What will happen if we apply that definition?

kubectl apply \
  -f scaling/go-demo-5-replicas-10.yml

kubectl -n go-demo-5 get pods

We applied the new definition and retrieved all the Pods from the go-demo-5 Namespace. The output of the latter command is as follows.

NAME    READY STATUS            RESTARTS AGE
api-... 1/1   Running           0        9s
api-... 0/1   ContainerCreating 0        9s
api-... 0/1   ContainerCreating 0        9s
api-... 1/1   Running           2        41m
api-... 1/1   Running           0        22s
api-... 0/1   ContainerCreating 0        9s
api-... 0/1   ContainerCreating 0        9s
api-... 1/1   Running           0        9s
api-... 1/1   Running           0        9s
api-... 1/1   Running           0        9s
db-0    2/2   Running           0        31m
db-1    2/2   Running           0        31m
db-2    2/2   Running           0        31m

Kubernetes complied with our desire to have ten replicas of the api and created eight Pods (we had two before). At the first look, it seems that HPA does not have any effect. Let’s retrieve the Pods one more time.

kubectl -n go-demo-5 get pods

The output is as follows.

NAME    READY STATUS  RESTARTS AGE
api-... 1/1   Running 0        30s
api-... 1/1   Running 2        42m
api-... 1/1   Running 0        43s
api-... 1/1   Running 0        30s
api-... 1/1   Running 0        30s
db-0    2/2   Running 0        31m
db-1    2/2   Running 0        32m
db-2    2/2   Running 0        32m

Our Deployment de-scaled from ten to five replicas. HPA detected that there are more replicas than the maximum threshold and acted accordingly. But what did it do? Did it simply remove five replicas? That could not be the case since that would only have a temporary effect. If HPA removes or adds Pods, Deployment would also remove or add Pods, and the two would be fighting with each other. The number of Pods would be fluctuating indefinitely. Instead, HPA modified the Deployment.

Let’s describe the api.

kubectl -n go-demo-5 \
  describe deployment api

The output, limited to the relevant parts, is as follows.

...
Replicas: 5 desired | 5 updated | 5 total | 5 available | 0 unavailable
...
Events:
... Message
... -------
...
... Scaled up replica set api-5bbfd85577 to 10
... Scaled down replica set api-5bbfd85577 to 5

The number of replicas is set to 5 desired. HPA modified our Deployment. We can observe that better through the event messages. The second to last states that the number of replicas was scaled up to 10, while the last message indicates that it scaled down to 5. The former is the result of us executing a rolling update by applying the new Deployment, while the latter was produced by HPA modifying the Deployment by changing its number of replicas.

So far, we observed that HPA modifies our Deployments. No matter how many replicas we defined in a Deployment (or a StatefulSets), HPA will change it to fit its own thresholds and calculations. In other words, when we update a Deployment, the number of replicas will be temporarily changed to whatever we have defined, only to be modified again by HPA a few moments later. That behavior is unacceptable.

If HPA changed the number of replicas, there is usually a good reason for that. Resetting that number to whatever is set in a Deployment (or a StatefulSet) can produce serious side-effects.

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