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Scaling Applications in Kubernetes using the HorizontalPodAutoscaler

Published on February 1, 2024
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By Cristian Marius Tiutiu and Bikram Gupta

Scaling Applications in Kubernetes using the HorizontalPodAutoscaler

Introduction

In a real-world scenario, you want to have resiliency for your applications. On the other hand, you want to be more efficient on resources and cut down costs as well. You need something that can scale your application on demand.

Most of the time you created Kubernetes Deployments for your applications using a fixed ReplicaSet number. But, in production environments which are variably put under load by external requests, it is not always that simple. It is inefficient to change the replica set value every time your web application is under load and starts to misbehave. Main reason(s) can be: your application receives more requests each afternoon (a specific period of the day), or when holidays are coming and many users visit your shopping site. Then, when the crowded period is over, you must go back and scale down your web deployments to avoid waste of resources and reduce infrastructure costs.

This is where Horizontal Pod Autoscaling kicks in, and provides a solution for more efficient resource usage, as well as reducing costs. It is a closed loop system that automatically grows or shrinks resources (application Pods), based on current needs. You create a HorizontalPodAutoscaler (or HPA) resource for each application deployment that needs autoscaling and let it take care of the rest for you automatically.

Horizontal pod scaling deals with adjusting replicas for application Pods, whereas vertical pod scaling deals with resource requests and limits for containers within Pods.

How Horizontal Pod Autoscaling Works

At a high level, HPA does the following:

  1. Keeps an eye on resource requests metrics coming from your application workloads (Pods), by querying the metrics server.
  2. Compare the target threshold value that you set in the HPA definition with the average resource utilization observed for your application workloads (CPU and memory).
  3. If the target threshold is reached, then HPA will scale up your application deployment to meet higher demands. Otherwise, if below the threshold, it will scale down the deployment. To see what logic HPA uses to scale your application deployment, please visit the algorithm details page from the official documentation.

Under the hood, a HorizontalPodAutoscaler is just another CRD (Custom Resource Definition) that drives a Kubernetes control loop implemented via a dedicated controller within the Control Plane of your cluster. Basically, you create a HorizontalPodAutoscaler YAML manifest targeting your application Deployment, and then use kubectl to apply the HPA resource in your cluster. Please bear in mind that you cannot target objects that cannot be scaled, such as a DaemonSet for example.

In order to do its magic, HPA needs a metrics server available in your cluster to scrape required metrics, such as CPU and memory utilization. One popular option is the Kubernetes Metrics Server.

The metrics server acts as an extension for the Kubernetes API server, thus offering more features. In a nutshell, the Kubernetes Metrics Server works by collecting resource metrics from Kubelets and exposing them via the Kubernetes API Server to be consumed by Horizontal Pod Autoscaler. Metrics API can also be accessed by kubectl top, making it easier to debug autoscaling pipelines.

Please make sure to read and understand the metrics server’s main purpose by visiting the use cases section from the main documentation (this is important).

For custom metrics (anything other than CPU or memory), you can use Prometheus via a special adapter named prometheus-adapter. It means, you can scale applications using other metrics (such as the number of HTTP requests), rather than CPU and/or memory utilization.

This tutorial will show you how to:

  • Deploy Kubernetes Metrics Server to your DOKS cluster.
  • Understand main concepts and how to create HPAs for your applications.
  • Test each HPA setup using two scenarios: constant and variable application load.
  • Configure and use the Prometheus Adapter to scale applications using custom metrics.

For more in-depth explanations, please visit the official documentation page for the Horizontal Pod Autoscaler.

Metrics Server and HPA Overview Diagram

The below diagram shows a high-level overview of how HPA works in conjunction with metrics-server:

K8S Metrics Server and HPA Overview

Table of Contents

Prerequisites

To complete this tutorial, you will need:

  1. A Git client to clone the Starter Kit repository.
  2. Helm, for managing Kubernetes Metrics Server releases and upgrades.
  3. Kubectl, for Kubernetes interaction. Kubectl must be configured and ready to use. Please follow these instructions to connect to your cluster with kubectl.

In the next step, you will learn how to deploy the Kubernetes Metrics Server, using the Helm package manager.

Step 1 - Installing Kubernetes Metrics Server

Kubernetes metrics server can be installed in two ways:

  1. Via a single kubectl command in high availability mode using:
kubectl apply -f https://github.com/kubernetes-sigs/metrics-server/releases/latest/download/high-availability.yaml
  1. Via Helm, by deploying metrics-server chart to your cluster.

This tutorial (and all tutorials from the Starter Kit) uses the Helm installation method because it’s more flexible, and you can fine-tune the release parameters later on if needed. High availability is a must, and can be set easily via the replica field from the metrics-server Helm chart.

Please bear in mind that the metrics server deployment requires some special permissions. For more information about all prerequisites, please read the official requirements page.

Steps to deploy metrics server via Helm:

  1. First, clone the Starter Kit repository and change the directory to your local copy.
git clone https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers.git
cd Kubernetes-Starter-Kit-Developers
  1. Next, add the metrics-server Helm repo and list the available charts:
helm repo add metrics-server https://kubernetes-sigs.github.io/metrics-server
helm repo update metrics-server
helm search repo metrics-server

The output looks similar to the following:

Output
NAME CHART VERSION APP VERSION DESCRIPTION metrics-server/metrics-server 3.8.2 0.6.1 Metrics Server is a scalable, efficient source ...

Note: The chart of interest is metrics-server/metrics-server, which will install Kubernetes metrics server on the cluster. Please visit the metrics-server chart page, for more details. 3. Then, open and inspect the metrics-server Helm values file provided in the Starter Kit repository, using an editor of your choice (preferably with YAML lint support). For example, you can use VS Code:

code 07-scaling-application-workloads/assets/manifests/metrics-server-values-v3.8.2.yaml
  1. Finally, install the Kubernetes Metrics Server using Helm (a dedicated metrics-server namespace will be created as well):
HELM_CHART_VERSION="3.8.2
helm install metrics-server metrics-server/metrics-server --version "$HELM_CHART_VERSION" \
--namespace metrics-server \
--create-namespace \
--f "07-scaling-application-workloads/assets/manifests/metrics-server-values-v${HELM_CHART_VERSION}.yaml"

Note: A specific version for the metrics-server Helm chart is used. In this case 3.8.2 was picked, which maps to the 0.6.1 release of the metrics-server application (see the output from Step 2.). It’s good practice in general, to lock on a specific version. This helps to have predictable results, and allows versioning control via Git.

Observations and results:

You can verify metrics-server deployment status via:

helm ls -n metrics-server

The output looks similar to (notice that the STATUS column value is deployed):

Output
NAME NAMESPACE REVISION UPDATED STATUS CHART APP VERSION metrics-server metrics-server 1 2022-02-24 14:58:23.785875 +0200 EET deployed metrics-server-3.8.2 0.6.1

Next, check all Kubernetes resources status from the metrics-server namespace:

kubectl get all -n metrics-server

The output looks similar to (notice the deployment and replicaset resources, which should be healthy and counted as 2):

Output
NAME READY STATUS RESTARTS AGE pod/metrics-server-694d47d564-9sp5h 1/1 Running 0 8m54s pod/metrics-server-694d47d564-cc4m2 1/1 Running 0 8m54s NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE service/metrics-server ClusterIP 10.245.92.63 <none> 443/TCP 8m54s NAME READY UP-TO-DATE AVAILABLE AGE deployment.apps/metrics-server 2/2 2 2 8m55s NAME DESIRED CURRENT READY AGE replicaset.apps/metrics-server-694d47d564 2 2 2 8m55s

Finally, check if the kubectl top command works (similar to Linux top command - prints current resource usage, such as CPU and memory). Below command displays current resource usage for all Pods in the kube-system namespace:

kubectl top pods -n kube-system

The output looks similar to (CPU usage is expressed in millicores, and memory usage in Mebibytes):

Output
NAME CPU(cores) MEMORY(bytes) cilium-operator-5db58f5b89-7kptj 2m 35Mi cilium-r2n9t 4m 150Mi cilium-xlqkp 9m 180Mi coredns-85d9ccbb46-7xnkg 1m 21Mi coredns-85d9ccbb46-gxg6d 1m 20Mi csi-do-node-p6lq4 1m 19Mi csi-do-node-rxd6m 1m 21Mi do-node-agent-2z2bc 0m 15Mi do-node-agent-ppds8 0m 21Mi kube-proxy-l9ddv 1m 25Mi kube-proxy-t6c29 1m 30Mi

If the output looks like above, then you configured metrics server correctly. In the next step, you will learn how to configure and test your first HorizontalPodAutoscaling resource.

Step 2 - Getting to Know HPAs

So far, each time you created a new Kubernetes deployment for your application, the ReplicaSet field was configured to a fixed value. This may suffice for simple cases, or development environments. In this step you will learn how to define a HPA CRD, and then focus on the first practical example.

In an HPA-based setup, the HorizontalPodAutoscaler takes control over the application deployment replica set field value. Typical HorizontalPodAutoscaler CRD looks like below:

apiVersion: autoscaling/v2beta2
kind: HorizontalPodAutoscaler
metadata:
  name: my-app-hpa
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: my-app-deployment
  minReplicas: 1
  maxReplicas: 3
  metrics:
    - type: Resource
      resource:
        name: cpu
        target:
          type: Utilization
          averageUtilization: 50

Explanations for the above configuration:

  • spec.scaleTargetRef: Reference to the scaled resource.
  • spec.minReplicas: The lower limit for the number of replicas to which the autoscaler can scale down.
  • spec.maxReplicas: The upper limit for the number of pods that can be set by the autoscaler.
  • spec.metrics.type: Type of metric to use to calculate the desired replica count. The above example uses using Resource type, which tells the HPA to scale the deployment based on average CPU (or memory) utilization (average utilization is set to a threshold value of 50).

Next, you have two options to create an HPA for your application deployment:

  1. Use the kubectl autoscale command on an existing deployment.
  2. Create an HPA YAML manifest, and then use kubectl to apply changes to your cluster.

The first option is for performing a quick test (or spin), and you don’t want to mess with YAML stuff yet. Please go ahead and try the first example provided in the Starter Kit:

  1. First, please clone the Starter Kit repository (if not already), and change the directory to your local copy:
git clone https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers.git
cd Kubernetes-Starter-Kit-Developers
  1. Then, create the myapp-test deployment (main purpose is to create some CPU load, by printing a message indefinitely):
kubectl apply -f https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers/blob/main/07-scaling-application-workloads/assets/manifests/hpa/metrics-server/myapp-test.yaml
  1. Finally, create a HorizontalPodAutoscaler targeting the myapp-test deployment:
kubectl autoscale deployment myapp-test --cpu-percent=50 --min=1 --max=3

The above action translates to: Please create an HPA resource for me that automatically scales my myapp-test deployment to a maximum of 3 replicas (and no less than 1 replica) whenever average CPU utilization hits 50%. You can check if the HPA resource was created by running:

kubectl get hpa

The output looks similar to (the TARGETS column shows a value of 50% which is the average CPU utilization that the HPA needs to maintain, whereas 240% is the current usage):

Output
NAME REFERENCE TARGETS MINPODS MAXPODS REPLICAS AGE myapp-test Deployment/myapp-test 240%/50% 1 3 3 52s

The TARGETS column value will display <unknown>/50% for a while (around 15 seconds). This is normal, and it has to do with HPA fetching the specific metric and computing the average value over time. By default, HPA checks metrics every 15 seconds.

You can also observe the events a HPA generates under the hood via:

kubectl describe hpa myapp-test

The output looks similar to (notice in the Events list how the HPA is increasing the replica count automatically):

Output
Name: myapp-test Namespace: default Labels: <none> Annotations: <none> CreationTimestamp: Mon, 28 Feb 2022 10:10:50 +0200 Reference: Deployment/myapp-test Metrics: ( current / target ) resource cpu on pods (as a percentage of request): 240% (48m) / 50% Min replicas: 1 Max replicas: 3 Deployment pods: 3 current / 3 desired ... Events: Type Reason Age From Message ---- ------ ---- ---- ------- Normal SuccessfulRescale 17s horizontal-pod-autoscaler New size: 2; reason: cpu resource utilization (percentage of request) above target Normal SuccessfulRescale 37s horizontal-pod-autoscaler New size: 3; reason: cpu resource utilization (percentage of request) above target

In a real-world scenario, you will want to use a dedicated YAML manifest to define each HPA. This way, you can track the changes by having the manifest committed in a Git repository, as well as come back to it later and perform changes.

Before moving to the next step, please delete the myapp-test deployment and corresponding HPA resource created earlier:

kubectl delete hpa myapp-test
kubectl delete deployment myapp-test

Next, you’re going to discover and test HPAs by looking at two different scenarios:

  1. The application creates a constant load by running some CPU-intensive tasks.
  2. External load is simulated via many successive HTTP calls for the application under test.

Step 3 - Scaling Applications Automatically via Metrics Server

The following HPA experiments are based on two scenarios:

  1. An application deployment that creates constant load by performing some CPU-intensive computations.
  2. A shell script simulates external load by performing fast successive HTTP calls for a web application.

Scenario 1 - Constant Load Test

In this scenario, you will create a sample application implemented using Python, which performs some CPU-intensive computations. The Python code is shown below:

import math

while True:
  x = 0.0001
  for i in range(1000000):
    x = x + math.sqrt(x)
    print(x)
  print("OK!")

The above code can be deployed via the constant-load-deployment-test manifest from the Starter Kit repository. The deployment will fetch a docker image hosting the required Python runtime and then attach a ConfigMap to the application Pod hosting the sample Python script shown earlier.

First, please clone the Starter Kit repository, and change directory to your local copy:

git clone https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers.git
cd Kubernetes-Starter-Kit-Developers

Then, create the sample deployment for the first scenario via kubectl (a separate namespace is being created as well, for better observation):

kubectl create ns hpa-constant-load
kubectl apply -f https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers/blob/06eec522f859bba957297d3068341df089468e97/07-scaling-application-workloads/assets/manifests/hpa/metrics-server/constant-load-deployment-test.yaml -n hpa-constant-load

Note: The sample deployment provided in this repository configures resource request limits for the sample application Pods. This is important because HPA logic relies on having resource request limits set for your Pods, and it won’t work otherwise. In general, it is advised to set resource request limits for all your application Pods to avoid things running out of control in your cluster.

Verify that the deployment was created successfully and that it’s up and running:

kubectl get deployments -n hpa-constant-load

The output looks similar to (notice that only one application Pod is running):

Output
NAME READY UP-TO-DATE AVAILABLE AGE constant-load-deployment-test 1/1 1 1 8s

Next, create the constant-load-hpa-test resource in your cluster, via kubectl:

kubectl apply -f https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers/blob/main/07-scaling-application-workloads/assets/manifests/hpa/metrics-server/constant-load-hpa-test.yaml

The above command will create a HPA resource, targeting the sample deployment created earlier. You can check the constant-load-test HPA state via:

kubectl get hpa constant-load-test -n hpa-constant-load

The output looks similar to (notice the REFERENCE column targeting constant-load-deployment-test, as well as TARGETS column showing current CPU resource requests versus threshold value):

Output
NAME REFERENCE TARGETS MINPODS MAXPODS REPLICAS AGE constant-load-test Deployment/constant-load-deployment-test 255%/50% 1 3 3 49s

You can also notice in the above output that the REPLICAS column value increased from 1 to 3 for the sample application deployment, as stated in the HPA CRD spec. The process took a short time to complete because the application used in this example created a load in no time. Going further, you can also inspect the HPA events and see the actions taken, via: kubectl describe hpa -n hpa-constant-load.

Scenario 2 - External Load Test

A more interesting and realistic scenario to test and study is where an external load is created for the application under test. For this experiment, you’re going to use a different namespace and set of manifests to observe the final behavior in complete isolation from the previous scenario.

The application under test is the quote of the moment sample server. On each HTTP request it sends a different quote as a response. You create load on the service by sending HTTP requests very fast (each 1ms roughly). There’s a helper script provided in the Starter Kit repository, which helps you achieve the results.

First, please create the quote deployment and service using kubectl (also a dedicated hpa-external-load namespace is created beforehand). Please make sure to change the directory to Kubernetes-Starter-Kit-Developers first:

kubectl create ns hpa-external-load
kubectl apply -f 07-scaling-application-workloads/assets/manifests/hpa/metrics-server/quote_deployment.yaml -n hpa-external-load

Now, verify if the quote application deployment and services are healthy:

kubectl get all -n hpa-external-load

The output looks similar to:

Output
NAME READY STATUS RESTARTS AGE pod/quote-dffd65947-s56c9 1/1 Running 0 3m5s NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE service/quote ClusterIP 10.245.170.194 <none> 80/TCP 3m5s NAME READY UP-TO-DATE AVAILABLE AGE deployment.apps/quote 1/1 1 1 3m5s NAME DESIRED CURRENT READY AGE replicaset.apps/quote-6c8f564ff 1 1 1 3m5s

Next, create the HPA for the quote deployment using kubectl:

kubectl apply -f 07-scaling-application-workloads/assets/manifests/hpa/metrics-server/quote-deployment-hpa-test.yaml -n hpa-external-load

Now, check if the HPA resource is in place and alive:

kubectl get hpa external-load-test -n hpa-external-load

The output looks similar to:

Output
NAME REFERENCE TARGETS MINPODS MAXPODS REPLICAS AGE external-load-test Deployment/quote 1%/20% 1 3 1 108s

Please note that in this case, there’s a different threshold value set for the CPU utilization resource metric, as well as a different scale-down behavior. The external-load-test HPA CRD looks like below:

apiVersion: autoscaling/v2beta2
kind: HorizontalPodAutoscaler
metadata:
  name: external-load-test
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: quote
  behavior:
    scaleDown:
      stabilizationWindowSeconds: 60
  minReplicas: 1
  maxReplicas: 3
  metrics:
    - type: Resource
      resource:
        name: cpu
        target:
          type: Utilization
          averageUtilization: 20

The above configuration alters the scaleDown.stabilizationWindowSeconds behavior and sets it to a lower value of 60 seconds. This is not really needed in practice, but in this case, you may want to speed up things to see more quickly how the auto scaler performs the scale-down action. By default, the HorizontalPodAutoscaler has a cool-down period of 5 minutes (or 300 seconds). This is sufficient in most of the cases and should avoid fluctuations when replicas are being scaled.

In the final step, you will run the helper script provided in this repository to create an external load for the target application (meaning the quote server). The script performs successive HTTP calls in a really short period of time, thus trying to simulate external load coming from the users (should suffice for demonstration purposes).

Please make sure to split the terminal in two separate windows in order to observe the results better (you can use tmux, for example). Then, in the first window, please invoke the quote service load test shell script (you can cancel execution anytime by pressing Ctrl+C):

./quote_service_load_test.sh

Next, in the second window, run a kubectl watch (via the -w flag) on the HPA resource:

kubectl get hpa -n hpa-external-load -w

The animation below will show you the experiment results:

Metrics Server HPA in Action

You can observe how the autoscaler kicks in when load increases (as long as the load generator script runs), and alters the quote server deployment replica set to a higher value. As soon as the load generator script is stopped, there’s a cool-down period, and after 1 minute or so, the replica set is lowered to the initial value of 1.

Next, you will learn how to scale applications on custom metrics coming from Prometheus. As an example, you can scale deployments based on the number of HTTP requests an application receives rather than CPU and/or memory utilization.

Step 4 - Scaling Applications Automatically via Prometheus Adapter

So far you learned how to horizontally scale applications based on CPU metrics (memory metrics can be used as well, but not covered in this tutorial). This is what metrics server was meant for and that’s it. But, you can also automatically scale applications on custom metrics. Thus, you can leverage Prometheus for the job, and let the HPA take decisions based on Prometheus metrics. Such an example is the number of HTTP requests hitting a web application, thus have the HPA scale your deployment based on the incoming traffic.

To accomplish this task, you need to install a special piece of software called the prometheus-adapter. The main purpose of the prometheus-adapter is to act as a bridge (or translator) between Prometheus and the Kubernetes API server.

When to choose metrics-server over prometheus-adapter or vice versa?

Well, metrics-server is fast, low on resources, and provides only the basic set of metrics (CPU and memory) for HPAs to work, thus making it a good candidate when you don’t want to use a full-blown monitoring system. On the other hand, when you need more granular control over HPAs and scale on other metrics not covered by the metrics server, then Prometheus in combination with the prometheus-adapter is a better choice.

The following steps assume that you have Prometheus Operator deployed in your cluster. Basic knowledge of ServiceMonitors is required as well. If not, please follow the Prometheus Monitoring Stack chapter from the Starter Kit repository. You also need a ServiceMonitor to scrape metrics from the application under test and send them to the Kubernetes API server via the prometheus-adapter. Then, the horizontal pod auto scaler can fetch custom metrics from the Kubernetes API Server and scale your applications appropriately.

The basic steps required for applications autoscaling using custom metrics served by Prometheus are as follows:

  1. First, you need the prometheus-adapter installed in your cluster.
  2. Next, you define ServiceMonitors for Prometheus to scrape custom metrics from your applications.
  3. Then, you define discovery rules for prometheus-adapter to expose your application custom metrics in the Kubernetes API.
  4. Finally, you create the HPA targeting your application deployment, and configure it to scale on custom metric(s).

Installing Prometheus Adapter

Prometheus adapter can be installed the usual way, via Helm. Please follow the below steps:

  1. First, clone the Starter Kit repository, and change the directory to your local copy:
git clone https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers.git
Kubernetes-Starter-Kit-Developers
  1. Next, add the prometheus-community Helm repo and list the available charts:
helm repo add prometheus-community https://prometheus-community.github.io/helm-chart
helm repo update prometheus-community
helm search repo prometheus-community

The output looks similar to the following:

Output
NAME CHART VERSION APP VERSION DESCRIPTION prometheus-community/alertmanager 0.18.0 v0.23.0 The Alertmanager handles alerts sent by client ... prometheus-community/kube-prometheus-stack 35.5.1 0.56.3 kube-prometheus-stack collects Kubernetes manif... prometheus-community/kube-state-metrics 4.9.2 2.5.1 Install kube-state-metrics to generate and expo... prometheus-community/prometheus 15.10.1 2.34.0 Prometheus is a monitoring system and time seri... prometheus-community/prometheus-adapter 3.3.1 v0.9.1 A Helm chart for k8s prometheus adapter ...

The chart of interest is prometheus-community/prometheus-adapter, which will install prometheus-adapter on the cluster. Please visit the prometheus-adapter chart page, for more details. 3. Then, open and inspect the prometheus-adapter Helm values file provided in the Starter Kit repository, using an editor of your choice (preferably with YAML lint support). For example, you can use VS Code:

code https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers/blob/main/07-scaling-application-workloads/assets/manifests/prometheus-adapter-values-v3.3.1.yaml
  1. Make sure to adjust the Prometheus endpoint setting based on your setup (explanations are provided in the Helm values file).
  2. Finally, save the values file and install prometheus-adapter using Helm (a dedicated prometheus-adapter namespace is being provisioned as well):
HELM_CHART_VERSION="3.3.1"
helm install prometheus-adapter prometheus-community/prometheus-adapter \
--version "$HELM_CHART_VERSION" \
--namespace prometheus-adapter \
--create-namespace \
-f "07-scaling-application-workloads/assets/manifests/prometheus-adapter-values-v${HELM_CHART_VERSION}.yaml"

A specific version for the prometheus-adapter Helm chart is used. In this case, 3.3.1 was picked, which maps to the 0.9.1 release of the Prometheus-adapter application (see the output from Step 2.). It’s good practice, in general, to lock on a specific version. This helps to have predictable results, and allows versioning control via Git.

Observations and results:

You can verify prometheus-adapter deployment status via:

helm ls -n prometheus-adapter

The output looks similar to (notice the STATUS column value set to deployed):

Output
NAME NAMESPACE REVISION UPDATED STATUS CHART APP VERSION prometheus-adapter prometheus-adapter 1 2022-03-01 12:06:22 deployed prometheus-adapter-3.3.1 v0.9.1

Next, check the Kubernetes resources status from the prometheus-adapter namespace:

kubectl get all -n prometheus-adapter

The output looks similar to (notice the deployment and replicaset resources, which should be healthy and counted as 2):

Output
NAME READY STATUS RESTARTS AGE pod/prometheus-adapter-7f475b958b-fd9sm 1/1 Running 0 2m54s pod/prometheus-adapter-7f475b958b-plzbw 1/1 Running 0 2m54s NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE service/prometheus-adapter ClusterIP 10.245.150.99 <none> 443/TCP 2m55s NAME READY UP-TO-DATE AVAILABLE AGE deployment.apps/prometheus-adapter 2/2 2 2 2m55s NAME DESIRED CURRENT READY AGE replicaset.apps/prometheus-adapter-7f475b958b 2 2 2 2m55s

After a few moments, you can query the custom.metrics.k8s.io API and redirect the output to a file:

kubectl get --raw /apis/custom.metrics.k8s.io/v1beta1 > custom_metrics_k8s_io.json

Looking at the custom_metrics_k8s_io.json file contents, you can observe some of the custom metrics exposed by the Kubernetes API server:

{
 "kind": "APIResourceList",
 "apiVersion": "v1",
 "groupVersion": "custom.metrics.k8s.io/v1beta1",
 "resources": [
     {
         "name": "jobs.batch/prometheus_sd_kubernetes_events",
         "singularName": "",
         "namespaced": true,
         "kind": "MetricValueList",
         "verbs": [
             "get"
         ]
     },
     {
         "name": "services/kube_statefulset_labels",
         "singularName": "",
         "namespaced": true,
         "kind": "MetricValueList",
         "verbs": [
             "get"
         ]
     },
...

If the output is similar to the above, then you installed and configured the prometheus-adapter correctly. Next, you will learn how to set up a sample application deployment providing a custom metrics endpoint. Then, you will configure a Prometheus ServiceMonitor to scrape the metrics endpoint.

Creating a Sample Application to Test Prometheus Adapter

In this step, you will deploy the prometheus-example application which exposes the following custom metrics:

  • http_requests_total - total number of incoming HTTP requests.
  • http_request_duration_seconds - duration of all HTTP requests.
  • http_request_duration_seconds_count- total count of all incoming HTTP requests.
  • http_request_duration_seconds_sum - total duration in seconds of all incoming HTTP requests.
  • http_request_duration_seconds_bucket - a histogram representation of the duration of the incoming HTTP requests.

After the prometheus-example application is deployed, you can test the custom metrics HPA setup. Then, you can fire multiple HTTP requests against the prometheus-example service and see how autoscaling works based on the http_requests_total metric.

First, change the directory where Starter Kit was cloned on your local machine:

cd Kubernetes-Starter-Kit-Developers

Then, please deploy the prometheus-example application using the YAML manifest from the Starter Kit repository. The provided manifest will create the prometheus-example deployment resource, as well as the associated Kubernetes service. A separate namespace named prometheus-custom-metrics-test is created as well, to test the whole setup:

kubectl create ns prometheus-custom-metrics-test
kubectl apply -f 07-scaling-application-workloads/assets/manifests/hpa/prometheus-adapter/prometheus-example-deployment.yaml -n prometheus-custom-metrics-test

Next, verify the resources created in the prometheus-custom-metrics-test namespace:

kubectl get all -n prometheus-custom-metrics-test

The output looks similar to (notice the prometheus-example-app Pod which must be up and running, as well as the associated service):

Output
NAME READY STATUS RESTARTS AGE pod/prometheus-example-app-7455d5c48f-wbshc 1/1 Running 0 9s NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE service/prometheus-example-app ClusterIP 10.245.103.235 <none> 80/TCP 10s NAME READY UP-TO-DATE AVAILABLE AGE deployment.apps/prometheus-example-app 1/1 1 1 10s NAME DESIRED CURRENT READY AGE replicaset.apps/prometheus-example-app-7455d5c48f 1 1 1 10s

Creating a ServiceMonitor for the Application under Test

Before creating the ServiceMonitor resource, you have to check the serviceMonitorSelector configuration of your Prometheus instance. Please follow the steps below:

  1. First, check what Prometheus instance(s) is available in your cluster (the monitoring namespace is used by the Starter Kit, so please adjust according to your setup):
kubectl get prometheus -n monitoring

The output looks similar to:

Output
NAME VERSION REPLICAS AGE kube-prom-stack-kube-prome-prometheus v2.35.0 1 7h4m
  1. Now, pick the prometheus instance discovered in the previous step (if you have multiple instances, just pick the correct one based on your setup), and fetch the serviceMonitorSelector.matchLabels field value:
kubectl get prometheus kube-prom-stack-kube-prome-prometheus -n monitoring -o jsonpath='{.spec.serviceMonitorSelector.matchLabels}'

The output looks similar to (notice that there’s one label named release):

{"release":"kube-prom-stack"}

By default, each Prometheus instance is configured to discover only service monitors that match a specific label. So, in order for the prometheus-example-app ServiceMonitor to be discovered by Prometheus, you need to label it using release: kube-prom-stack.

Change the directory where Starter Kit was cloned on your local machine:

cd Kubernetes-Starter-Kit-Developers

Now open the prometheus-example-service-monitor manifest provided in the Starter Kit repository, using an editor of your choice (preferably with YAML lint support). For example, you can use VS Code:

code 07-scaling-application-workloads/assets/manifests/hpa/prometheus-adapter/prometheus-example-service-monitor.yaml

In the metadata.labels section, make sure to add the label discovered earlier (release: kube-prom-stack). The ServiceMonitor manifest looks similar to:

kind: ServiceMonitor
apiVersion: monitoring.coreos.com/v1
metadata:
  name: prometheus-example-app
  labels:
    app: prometheus-example-app
    release: kube-prom-stack
spec:
  selector:
    matchLabels:
      app.kubernetes.io/name: prometheus-example-app
  namespaceSelector:
    matchNames:
      - prometheus-custom-metrics-test
  endpoints:
    - port: web

Finally, create the required ServiceMonitor for Prometheus to scrape the /metrics endpoint of the prometheus-example-app:

kubectl apply -f 07-scaling-application-workloads/assets/manifests/hpa/prometheus-adapter/prometheus-example-service-monitor.yaml -n prometheus-custom-metrics-test

After completing the above steps, you should see a new target being present in the Targets panel from the Prometheus dashboard. First, you need to port-forward the main Prometheus service to access the dashboard (the below sample command is using the Starter Kit naming conventions, so please adjust based on your current setup):

kubectl port-forward svc/kube-prom-stack-kube-prome-prometheus 9090:9090 -n monitoring

Open a web browser on localhost:9090. Once in, you can go to Status -> Targets, and see that the target has been added.

The output looks similar to (notice the prometheus-example-app present in the discovered targets list):

prometheus-example-app target

Defining Discovery Rules for Prometheus Adapter

Although Prometheus is able to scrape and see your application custom metrics, the prometheus-adapter is not going to expose them in the custom metrics API of Kubernetes. You need to define a set of discovery rules to help Prometheus-adapter expose your application’s custom metrics.

Quoting from the official documentation:

The adapter determines which metrics to expose and how to expose them through a set of “discovery” rules. Each rule is executed independently (so make sure that your rules are mutually exclusive), and specifies each of the steps the adapter needs to take to expose a metric in the API.

Each rule can be broken down into roughly four parts:

  • Discovery, which specifies how the adapter should find all Prometheus metrics for this rule.
  • Association, which specifies how the adapter should determine which Kubernetes resources a particular metric is associated with.
  • Naming, which specifies how the adapter should expose the metric in the custom metrics API.
  • Querying, which specifies how a request for a particular metric on one or more Kubernetes objects should be turned into a query to Prometheus.

Typical discovery rule definition looks similar to:

rules:
  custom:
    - seriesQuery: 'http_requests_total{pod!=""}'
      resources:
        template: "<<.Resource>>"
      name:
        matches: "^(.*)_total"
        as: "${1}_per_second"
      metricsQuery: "sum(rate(<<.Series>>{<<.LabelMatchers>>}[2m])) by (<<.GroupBy>>)"

You can break the above configuration into pieces, to understand it better:

  • seriesQuery: Represents the metric you’re interested in, as exposed by the application /metrics endpoint (vectorial representation). This is the Discovery part of the rule definition, and tells prometheus-adapter to select the http_requests_total metric for all your application Pods that are not null (pod!="").
  • resources.template: This is a Prometheus template, and represents the resource that exposes the metrics (e.g. Pod). This is the Association part of the rule definition.
  • name: Defines a name for the rule. Basically you’re telling prometheus-adapter to rename http_requests_total to http_requests_per_second. In the end you’re interested in the number of http requests per second, and not just a simple counter. This is the Naming part of the rule definition.
  • metricsQuery: Represents a parameterized Prometheus query. It should look familiar because it’s just PromQL. Here, we compute the http requests rate on average, over a predefined period (e.g. 2 minutes). This is the Querying part of the rule definition.

Now that you know how to set up discovery rules for prometheus-adapter, it’s time to put all knowledge into practice. Please follow below steps to instruct prometheus-adapter to use your custom rule(s):

  1. First, change the directory where Starter Kit was cloned on your local machine:
cd Kubernetes-Starter-Kit-Developers
  1. Next, please open the prometheus-adapter Helm values file provided in the Starter Kit repository, using an editor of your choice (preferably with YAML lint support). For example, you can use VS Code:
code https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers/blob/main/07-scaling-application-workloads/assets/manifests/prometheus-adapter-values-v3.3.1.yaml
  1. Look for the rules section, and uncomment everything. It should look like below:
rules:
  custom:
    - seriesQuery: 'http_requests_total{pod!=""}'
      resources:
        template: "<<.Resource>>"
      name:
        matches: "^(.*)_total"
        as: "${1}_per_second"
      metricsQuery: "sum(rate(<<.Series>>{<<.LabelMatchers>>}[2m])) by (<<.GroupBy>>)"
  1. Save the values file, and apply changes via a Helm upgrade:

    HELM_CHART_VERSION="3.3.1"
    helm upgrade prometheus-adapter prometheus-community/prometheus-adapter \
      --version "$HELM_CHART_VERSION" \
      --namespace prometheus-adapter \
      -f "07-scaling-application-workloads/assets/manifests/prometheus-adapter-values-v${HELM_CHART_VERSION}.yaml"
    

Note: Please note that with no incoming requests, only the version metric is reported. To generate some HTTP requests you need to:

kubectl port-forward svc/prometheus-example-app 8080:8080 -n prometheus-custom-metrics-test

Open a web browser on http://localhost:8080 and refresh the application’s homepage a few times.

If everything goes well, you can query the custom metrics API and observe the new metric (you can install jq, and have the results printed nicely):

kubectl get --raw="/apis/custom.metrics.k8s.io/v1beta1/namespaces/prometheus-custom-metrics-test/pods/*/http_requests_per_second" | jq .

The output looks similar to:

{
  "kind": "MetricValueList",
  "apiVersion": "custom.metrics.k8s.io/v1beta1",
  "metadata": {
    "selfLink": "/apis/custom.metrics.k8s.io/v1beta1/namespaces/prometheus-custom-metrics-test/pods/%2A/http_requests_per_second"
  },
  "items": [
    {
      "describedObject": {
        "kind": "Pod",
        "namespace": "prometheus-custom-metrics-test",
        "name": "prometheus-example-app-7455d5c48f-wbshc",
        "apiVersion": "/v1"
      },
      "metricName": "http_requests_per_second",
      "timestamp": "2022-03-02T13:33:32Z",
      "value": "0",
      "selector": null
    }
  ]
}

Looking at the above output, you can notice the custom http_requests_per_second metric with a value of 0. This is normal because you haven’t generated any load for the application yet.

Now, you can move to the final step and create an HPA for the Prometheus-example-app deployment used in this tutorial. Then, you will create a load on the application using the custom_metrics_service_load_test script provided in the Starter Kit repository.

Creating and Testing HPAs using Custom Metrics

Defining an HPA for automatically scaling applications on custom metrics is similar to the metrics server examples. The only difference is the metrics field, which in this case is using custom metrics to help HPA make decisions (e.g., http_requests_per_second).

Typical HPA definition based on custom metrics looks like below (explanations for the important fields can be found inline):

kind: HorizontalPodAutoscaler
apiVersion: autoscaling/v2beta2
metadata:
  name: prometheus-example-app
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: prometheus-example-app
  minReplicas: 1
  maxReplicas: 5
  metrics:
    # use a "Pods" metric, which takes the average of the
    # given metric across all pods controlled by the autoscaling target
    - type: Pods
      pods:
        metricName: http_requests_per_second
        # target 500 milli-requests per second,
        # which is 1 request every two seconds
        targetAverageValue: 500m

First, change directory where Starter Kit was cloned on your local machine:

cd Kubernetes-Starter-Kit-Developers

Next, create the prometheus-custom-metrics-hpa resource in your cluster, via kubectl:

kubectl apply -f https://github.com/digitalocean/Kubernetes-Starter-Kit-Developers/blob/06eec522f859bba957297d3068341df089468e97/07-scaling-application-workloads/assets/manifests/hpa/prometheus-adapter/prometheus-custom-metrics-hpa.yaml

The above command will create a HPA resource, targeting the sample deployment created earlier. You can check the HPA state via:

kubectl get hpa -n prometheus-custom-metrics-test

The output looks similar to (notice the REFERENCE column targeting the prometheus-example-app deployment, as well as the TARGETS column showing current http_requests_per_second versus threshold value):

Output
NAME REFERENCE TARGETS MINPODS MAXPODS REPLICAS AGE prometheus-custom-metrics Deployment/prometheus-example-app 0/500m 1 5 1 19s

In the final step, you will run the helper script provided in this repository to create a load on the target (meaning the Prometheus-example-app). The script performs successive HTTP calls in a short period, thus simulating external load coming from the users (should suffice for demonstration purposes).

Please make sure to split the terminal in two separate windows in order to observe the results better (you can use tmux, for example). Then, in the first window, please invoke the custom_metrics_service_load_test shell script (you can cancel execution anytime by pressing Ctrl+C):

./custom_metrics_service_load_test.sh

Next, in the second window, set a kubectl watch (via the -w flag) for the HPA resource:

kubectl get hpa -n prometheus-custom-metrics-test -w

The below screenshot will show you the experiment results:

Custom Metrics HPA in Action

You can observe how the autoscaler kicks in when load increases (as long as the load generator script runs), and alters the prometheus-example-app deployment replica set to a higher value. As soon as the load generator script is stopped, there’s a cool-down period, and after about 5 minutes or so, the replica set is lowered to the initial value of 1.

The following phases are highlighted:

  • Phase 1: This is the ramp-up phase. Notice how HAP alters the replica set, and it’s almost gradually increased from a value of 1 to a value of 8. The initial load is around 2140 milli-requests per second, then decreases to a more manageable value of 620 milli-requests per second as more application Pods are added.
  • Phase 2: This is where things are getting more stable. The current load has minor fluctuations between 520-540 milli-requests per second.
  • Phase 3: In this phase, you can notice a bump of more than 10% of the threshold value. The current load increased momentarily to a value of 562 milli-requests per second. We’re out of the hysteresis window, so the HPA will add more replicas (9) to compensate and stabilize the system. This has almost an immediate effect, and the load starts to decrease under the threshold value, around 480 milli-requests per second.
  • Phase 4: Here, the load generator script is stopped. You can observe that the load seen by the application is decreasing pretty fast. In about 5 minutes or so (default cooldown value), the replica set is adjusted back to the minimum value of 1.

What happens in between? The word hysteresis was mentioned at some point. Suppose the average number of HTTP requests can be stabilized to a value closer to the threshold (assuming a maximum deviation allowed of ± 10%, for example). In that case, the HPA won’t increase the replica set anymore (even though the upper limit was not reached). This is called hysteresis, and it’s part of the HPA logic to avoid fluctuations for the replica set. As a matter of fact, maintaining hysteresis is important in every system where stability is required, thus avoiding oscillations.

Conclusion

In this tutorial you learned how to scale on resource requests metrics using metrics-server, as well as on custom metrics (via prometheus-adapter and Prometheus). Although setting up prometheus-adapter and scaling applications using custom metrics seems a little bit more involved, it offers more flexibility, thus rewarding the effort.

You also simulated some real-world scenarios. Then, you observed how HPAs do their magic and try to stabilize your application (by scaling it automatically), and compensate for the increased load (or heavy traffic).

You can learn more about HPAs using the below resources:

To automate everything in the Starter Kit, please refer to Section 14 - Continuous Delivery using GitOps.

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About the authors
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Cristian Marius Tiutiu

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Sr Technical Writer


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