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Abhishek Gupta
Abhishek Gupta

Posted on • Originally published at abhishek1987.Medium

Write your Kubernetes Infrastructure as Go code - Combine cdk8s with AWS CDK

#kubernetes #go #tutorial #devops

In an earlier blog post you saw how to use cdk8s with AWS Controllers for Kubernetes (also known as ACK), thanks to the fact that you can import existing Kubernetes Custom Resource Definitions using cdk8s! This made is possible to deploy DynamoDB along with a client application, from using cdk8s and Kubernetes.

But, what if you continue using AWS CDK for AWS infrastructure and harness the power cdk8s (and cdk8s-plus!) to define Kubernetes resources using regular code? Thanks to the native integration between
the AWS EKS module and cdk8s, you can have the best of both worlds!

The goal of this blog post is to demonstrate that with a few examples. We will start off with a simple (nginx based) example before moving on to a full-fledged application stack (including DynamoDB etc.). Both will be using the Go programming language which is well supported in AWS CDK as well as cdk8s.

All the code discussed in this blog is available in this GitHub repo

Prerequisites

To follow along step-by-step, in addition to an AWS account, you will need following CLIs - AWS CLI, cdk8s CLI and kubectl. Also, dont' forget to install AWS CDK, the Go programming language (v1.16 or above) as well as Docker, if you don't have them already.

Keeping it simple with Nginx on EKS

As with most things in life, there are two ways - the easy way or the hard way ;) You will see both of them! Let's try things out first, see them working and then look at the code.

To start off, clone the repo and change to the right directory:

git clone https://github.com/abhirockzz/cdk8s-for-go-developers
cd cdk8s-for-go-developers/part6-cdk-eks-cdk8s/cdk-cdk8s-nginx-eks
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To setup everything, all you need is a single command:

cdk deploy
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you can also use cdk synth to generate and inspect the Cloud Formation template first

You will be prompted to confirm. Once you do that, the process will kick off - it will take some time since lots of AWS resources will be created, including VPC, EKS cluster etc.

Feel free to check the AWS Cloud Formation console to track the progress.

Once the process is complete, you need to connect to the EKS cluster using kubectl. The command required for this will be available as a result of the cdk deploy process (in the terminal) or you can refer to the Outputs section of the AWS Cloud Formation stack.

Image description

Once you've configured kubectl to point to your EKS cluster, you can check the Nginx Deployment and Service.

kubectl get deployment

# output

NAME                          READY   UP-TO-DATE   AVAILABLE   AGE
nginx-deployment-cdk8s        1/1     1            1           1m
nginx-deployment-cdk          1/1     1            1           1m
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You will see that two Deployments have been created - more on this soon. Similiarly, if you check the Service (kubectl get svc), you should see two of them - nginx-service-cdk and nginx-service-cdk8s.

To access Nginx, pick the EXTERNAL-IP of any of the two Services. For example:

APP_URL=$(kubectl get service/nginx-service-cdk -o jsonpath="{.status.loadBalancer.ingress[0].hostname}")

echo $APP_URL

# to access nginx (notice we are using port 9090)
curl -i http://$APP_URL:9090
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If you get a Could not resolve host error while accessing the LB URL, wait for a minute or so and re-try

Behind the scenes

Let's look at the code now - this will clarify why we have two Nginx Deployments.

Thanks to AWS CDK, VPC creation is a one liner with awsec2.NewVpc function and creating an EKS cluster isn't too hard either!

func NewNginxOnEKSStack(scope constructs.Construct, id string, props *CdkStackProps) awscdk.Stack {
  //...
  vpc := awsec2.NewVpc(stack, jsii.String("demo-vpc"), nil)

    eksSecurityGroup := awsec2.NewSecurityGroup(stack, jsii.String("eks-demo-sg"),
        &awsec2.SecurityGroupProps{
            Vpc:               vpc,
            SecurityGroupName: jsii.String("eks-demo-sg"),
            AllowAllOutbound:  jsii.Bool(true)})

    eksCluster := awseks.NewCluster(stack, jsii.String("demo-eks"),
        &awseks.ClusterProps{
            ClusterName:   jsii.String("demo-eks-cluster"),
            Version:       awseks.KubernetesVersion_V1_21(),
            Vpc:           vpc,
            SecurityGroup: eksSecurityGroup,
            VpcSubnets: &[]*awsec2.SubnetSelection{
                {Subnets: vpc.PrivateSubnets()}},
            DefaultCapacity:         jsii.Number(2),
            DefaultCapacityInstance: awsec2.InstanceType_Of(awsec2.InstanceClass_BURSTABLE3, awsec2.InstanceSize_SMALL), DefaultCapacityType: awseks.DefaultCapacityType_NODEGROUP,
            OutputConfigCommand: jsii.Bool(true),
            EndpointAccess:      awseks.EndpointAccess_PUBLIC()})
//...
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Nginx on Kubernetes, the hard way!

Now we look at two different ways of creating Nginx, starting with the "hard" way. In this case, we use AWS CDK (not cdk8s) to define the Deployment and Service resources.

func deployNginxUsingCDK(eksCluster awseks.Cluster) {

    appLabel := map[string]*string{
        "app": jsii.String("nginx-eks-cdk"),
    }

    deployment := map[string]interface{}{
        "apiVersion": jsii.String("apps/v1"),
        "kind":       jsii.String("Deployment"),
        "metadata": map[string]*string{
            "name": jsii.String("nginx-deployment-cdk"),
        },
        "spec": map[string]interface{}{
            "replicas": jsii.Number(1),
            "selector": map[string]map[string]*string{
                "matchLabels": appLabel,
            },
            "template": map[string]interface{}{
                "metadata": map[string]map[string]*string{
                    "labels": appLabel,
                },
                "spec": map[string][]map[string]interface{}{
                    "containers": {
                        {
                            "name":  jsii.String("nginx"),
                            "image": jsii.String("nginx"),
                            "ports": []map[string]*float64{
                                {
                                    "containerPort": jsii.Number(80),
                                },
                            },
                        },
                    },
                },
            },
        },
    }

    service := map[string]interface{}{
        "apiVersion": jsii.String("v1"),
        "kind":       jsii.String("Service"),
        "metadata": map[string]*string{
            "name": jsii.String("nginx-service-cdk"),
        },
        "spec": map[string]interface{}{
            "type": jsii.String("LoadBalancer"),
            "ports": []map[string]*float64{
                {
                    "port":       jsii.Number(9090),
                    "targetPort": jsii.Number(80),
                },
            },
            "selector": appLabel,
        },
    }

    eksCluster.AddManifest(jsii.String("app-deployment"), &service, &deployment)
}
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Finally, to create this in EKS we invoke AddManifest (think of its like the programmatic equivalent of kubectl apply). This works, but there are a few gaps in this approach:

  • We are not able to reap the benefits of Go which is a strongly typed language. That's because the API is loosely typed, thanks to map[string]interface{} everywhere. This makes it very error prone (I made a few mistakes too!)
  • The verbosity is apparent as well. It seems as if we are writing YAML in Go - not too much of an improvement!

Is there a better way..?

Let's look at the second function deployNginxUsingCDK8s - by the name its obvious that we used cdk8s, not just CDK)

func deployNginxUsingCDK8s(eksCluster awseks.Cluster) {

    app := cdk8s.NewApp(nil)
    eksCluster.AddCdk8sChart(jsii.String("nginx-eks-chart"), NewNginxChart(app, "nginx-cdk8s", nil), nil)
}
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This looks "too easy" to to be true! But it's made possible due to the inter-operability between CDK and cdk8s. What this implies is that, you can use define Kubernetes resources using cdk8s Charts and apply them to an EKS cluster created with CDK (this makes it sort of a hybrind system).

The hero of our story is AddCdk8sChart function, which accepts a constructs.Construct (remember, everything is a construct!). In this case, the Construct happens to be a cdk8s.Chart thats returned by NewNginxChart function - so lets take a look at that.

func NewNginxChart(scope constructs.Construct, id string, props *MyChartProps) cdk8s.Chart {
  //....

    dep := cdk8splus22.NewDeployment(chart, jsii.String("nginx-deployment"),
        &cdk8splus22.DeploymentProps{
            Metadata: &cdk8s.ApiObjectMetadata{
                Name: jsii.String("nginx-deployment-cdk8s")}})

    dep.AddContainer(&cdk8splus22.ContainerProps{
        Name:  jsii.String("nginx-container"),
        Image: jsii.String("nginx"),
        Port:  jsii.Number(80)})

    dep.ExposeViaService(&cdk8splus22.DeploymentExposeViaServiceOptions{
        Name:        jsii.String("nginx-service-cdk8s"),
        ServiceType: cdk8splus22.ServiceType_LOAD_BALANCER,
        Ports: &[]*cdk8splus22.ServicePort{{
            Port:       jsii.Number(9090),
            TargetPort: jsii.Number(80)}}})

    return chart
}
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If you have worked with cdk8s (and Go) or read some of my previous blogs on this topic, this should look familiar - a strongly-typed, compact and expressive API! I don't even need to walk you through this since its so readable - we use cdk8s-plus to create a Nginx Deployment, add the container info and finally expose it via a Service so that we can access the Nginx from outside of EKS.

This was a simple enough example to help bring out difference between the two approaches. The next scenario is different - in addition to the EKS cluster it has DynamoDB along with a URL shortener application that will be deployed to EKS.

End to end example: DynamoDB along with an application on EKS

Instead of creating a new EKS cluster from scratch, we will re-use the existing cluster created as a result of the previous example - this is a good opportunity to take a look at how you can reference an existing EKS cluster in your CDK code. As expected, we need to create DynamoDB table as well.

Just like in the previous example, let's try out the solution first. Change into the right directory first:

cd part6-cdk-eks-cdk8s/cdk-cdk8s-dynamodb-app-eks
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Since the URL shortener application has to make API calls to DynamoDB, we need to configure IAM Roles for Service Accounts (also known as IRSA).

Refer to https://docs.aws.amazon.com/eks/latest/userguide/create-service-account-iam-policy-and-role.html

Define IAM roles for the application

Start by creating a Kubernetes Service Account:

kubectl apply -f - <<EOF
apiVersion: v1
kind: ServiceAccount
metadata:
  name: eks-dynamodb-app-sa
EOF
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To confirm - kubectl get serviceaccount/eks-dynamodb-app-sa -o yaml

Set your AWS Account ID and OIDC Identity provider as environment variables:

ACCOUNT_ID=$(aws sts get-caller-identity --query "Account" --output text)

export EKS_CLUSTER_NAME=<enter cluster name>
export AWS_REGION=<enter region e.g. us-east-1>

OIDC_PROVIDER=$(aws eks describe-cluster --name $EKS_CLUSTER_NAME --query "cluster.identity.oidc.issuer" --output text | sed -e "s/^https:\/\///")
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Create a JSON file with Trusted Entities for the role:

read -r -d '' TRUST_RELATIONSHIP <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Federated": "arn:aws:iam::${ACCOUNT_ID}:oidc-provider/${OIDC_PROVIDER}"
      },
      "Action": "sts:AssumeRoleWithWebIdentity",
      "Condition": {
        "StringEquals": {
          "${OIDC_PROVIDER}:aud": "sts.amazonaws.com",
          "${OIDC_PROVIDER}:sub": "system:serviceaccount:default:eks-dynamodb-app-sa"
        }
      }
    }
  ]
}
EOF
echo "${TRUST_RELATIONSHIP}" > trust.json
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Check - cat trust.json

Now, create the IAM role:

export ROLE_NAME=dynamodb-app-irsa

aws iam create-role --role-name $ROLE_NAME --assume-role-policy-document file://trust.json --description "IRSA for DynamoDB app on EKS"
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We will need to create and attach policy to role since we only want to allow PutItem and GetItem operations from our application. Here is the policy JSON file:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "PutandGet",
            "Effect": "Allow",
            "Action": [
                "dynamodb:PutItem",
                "dynamodb:GetItem"
            ],
            "Resource": "arn:aws:dynamodb:*:*:table/urls"
        }
    ]
}
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Create and attach the policy to the role we just created:

aws iam create-policy --policy-name dynamodb-irsa-policy --policy-document file://policy.json

aws iam attach-role-policy --role-name $ROLE_NAME --policy-arn=arn:aws:iam::<enter AWS account ID>:policy/dynamodb-irsa-policy
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Finally, we need to associate the IAM role and Service Account:

kubectl annotate serviceaccount -n default eks-dynamodb-app-sa eks.amazonaws.com/role-arn=arn:aws:iam::<enter AWS account ID>:role/dynamodb-app-irsa
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Get the EKS kubectl role ARN

To reference existing EKS cluster in AWS CDK, you need EKS cluster name and kubectl role ARN.

You can find the role ARN in the Outputs section of the AWS Cloud Formation stack.

Image description

We are ready to deploy the application using CDK. Set the required environment variables followed by cdk deploy:

you can also use cdk synth to generate and inspect the Cloud Formation template first

export EKS_CLUSTER_NAME=<enter name of EKS cluster>
export KUBECTL_ROLE_ARN=<enter kubectl role ARN>
export SERVICE_ACCOUNT_NAME=eks-dynamodb-app-sa
export APP_PORT=8080
export AWS_REGION=<enter region e.g. us-east-1>

cdk deploy
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CDK (and cdk8s) will do all the heavy lifting (we will look at the code very soon):

  • New DynamoDB table will be created
  • The docker image for our application will be built and pushed to ECR
  • Kubernetes resources for the URL shortener application will be deployed to the existing EKS cluster

Once the stack creation is complete, check the Kubernetes Deployment and Service:

kubectl get deployment/dynamodb-app
kubectl get pods
kubectl get service/dynamodb-app-service
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Testing the URL shortener service is easy. But I will not repeat it here since its already covered in [this blog](https://dev.to/abhirockzz/write-your-kubernetes-infrastructure-as-go-code-manage-aws-services-3pgi. All you need is the load balancer URL to access the service and use yout browser or curl to save and access URLs.

Back to exploring Go code again

Within the stack, we define the DynamoDB table (using awsdynamodb.NewTable) along with the docker image for our application (with awsecrassets.NewDockerImageAsset)

func NewDynamoDBAppStack(scope constructs.Construct, id string, props *CdkStackProps) awscdk.Stack {
  //...
    table := awsdynamodb.NewTable(stack, jsii.String("dynamodb-table"),
        &awsdynamodb.TableProps{
            TableName: jsii.String(tableName),
            PartitionKey: &awsdynamodb.Attribute{
                Name: jsii.String(dynamoDBPartitionKey),
                Type: awsdynamodb.AttributeType_STRING,
            },
            BillingMode:   awsdynamodb.BillingMode_PAY_PER_REQUEST,
            RemovalPolicy: awscdk.RemovalPolicy_DESTROY,
        })

    appDockerImage := awsecrassets.NewDockerImageAsset(stack, jsii.String("app-image"),
        &awsecrassets.DockerImageAssetProps{
            Directory: jsii.String(appDirectory)})
  //...
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Then comes the interesting part where we get the reference to our exsiting EKS cluster and use AddCdk8sChart (just like before) to deploy the application to EKS.

//...
  eksCluster := awseks.Cluster_FromClusterAttributes(stack, jsii.String("existing cluster"),
        &awseks.ClusterAttributes{
            ClusterName:    jsii.String(eksClusterName),
            KubectlRoleArn: jsii.String(kubectlRoleARN)})

    app := cdk8s.NewApp(nil)
    appProps := NewAppChartProps(appDockerImage.ImageUri(), table.TableName())

    eksCluster.AddCdk8sChart(jsii.String("dynamodbapp-chart"), NewDynamoDBAppChart(app, "dynamodb-cdk8s", &appProps), nil)
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The NewDynamoDBAppChart function defines the Deployment and Service. Unlike the earlier Nginx example which had static values, this application takes in dynamic values - specifically the DynamoDB table name (which is used as the container environment variable TABLE_NAME). Also notice the fact that we explicitly add the the name of the Kubernetes service account (for IRSA) that we had created in the previous step.

func NewDynamoDBAppChart(scope constructs.Construct, id string, props *AppChartProps) cdk8s.Chart {
  //...
    dep := cdk8splus22.NewDeployment(chart, jsii.String("dynamodb-app-deployment"), &cdk8splus22.DeploymentProps{
        Metadata: &cdk8s.ApiObjectMetadata{
            Name: jsii.String("dynamodb-app")},
        ServiceAccount: cdk8splus22.ServiceAccount_FromServiceAccountName(
            chart,
            jsii.String("aws-irsa"),
            jsii.String(props.serviceAccountName))})

    container := dep.AddContainer(//.. omitted for brevity)

    container.Env().AddVariable(jsii.String("TABLE_NAME"), cdk8splus22.EnvValue_FromValue(props.tableName))
    container.Env().AddVariable(jsii.String("AWS_REGION"), cdk8splus22.EnvValue_FromValue(&props.region))

    dep.ExposeViaService(//.. omitted for brevity)
    return chart
}
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Wrap up

That's all for this blog! We started off with a simple example to showcase the integration between AWS CDK and cdk8s and how easy it makes things (compared to just using CDK to deploy apps to EKS). Then, we moved on to explore a full fledged scenario where you deployed the infrastructure (DynamoDB etc.) along with the client application on EKS.

Happy Building!

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