Amazon EKS Node Drainer

This sample code provides a means to gracefully terminate nodes of an Amazon Elastic Container Service for Kubernetes (Amazon EKS) cluster when managed as part of an Amazon EC2 Auto Scaling Group.

The code provides an AWS Lambda function that integrates as an Amazon EC2 Auto Scaling Lifecycle Hook. When called, the Lambda function calls the Kubernetes API to cordon and evict all evictable pods from the node being terminated. It will then wait until all pods have been evicted before the Auto Scaling group continues to terminate the EC2 instance. The lambda may be killed by the function timeout before all evictions complete successfully, in which case the lifecycle hook may re-execute the lambda to try again. If the lifecycle heartbeat expires then termination of the EC2 instance will continue regardless of whether or not draining was successful. You may need to increase the function and heartbeat timeouts in template.yaml if you have very long grace periods.

Using this approach can minimise disruption to the services running in your cluster by allowing Kubernetes to reschedule the pod prior to the instance being terminated enters the TERMINATING state. It works by using Amazon EC2 Auto Scaling Lifecycle Hooks to trigger an AWS Lambda function that uses the Kubernetes API to cordon the node and evict the pods.

NB: The lambda function created assumes that the Amazon EKS cluster's Kubernetes API server endpoint has public access enabled, if your endpoint only has private access enabled then you must modify the template.yml file to ensure the lambda function is running in the correct VPC and subnet.

This lambda can also be used against a non-EKS Kubernetes cluster by reading a kubeconfig file from an S3 bucket specified by the KUBE_CONFIG_BUCKET and KUBE_CONFIG_OBJECT environment variables. If these two variables are passed in then Drainer function will assume this is a non-EKS cluster and the IAM authenticator signatures will not be added to Kubernetes API requests. It is recommended to apply the principle of least privilege to the IAM role that governs access between the Lambda function and S3 bucket.

Below is a brief explanation of the folder structure of the project:

.
├── README.md                   <-- This instructions file
├── build_deploy.sh             <-- Deployment script
├── drainer                     <-- Source code for the lambda function
│   ├── __init__.py
│   ├── handler.py              <-- Lambda function code
│   ├── requirements.txt        <-- Lambda Python dependencies
│   ├── k8s_utils.py
├── k8s_rbac/                   <-- Kubernetes RBAC configuration
├── template.yaml               <-- SAM Template
└── tests                       <-- Unit tests
    └── drainer
        ├── __init__.py
        └── test_handler.py

Requirements

SAM CLI
AWS CLI already configured with Administrator permission
Python 3
Docker
Pipenv (Only if you wish to run the tests)

Setup process

Local development

Invoking function locally using a local sample payload

sam local invoke DrainerFunction --event event.json

Packaging and deployment

AWS Lambda Python runtime requires a flat folder with all dependencies including the application. SAM will use CodeUri property to know where to look up for both application and dependencies:

...
    DrainerFunction:
        Type: AWS::Serverless::Function
        Properties:
            CodeUri: drainer/
            ...

Firstly, we need a S3 bucket where we can upload our Lambda functions packaged as ZIP before we deploy anything - If you don't have a S3 bucket to store code artifacts then this is a good time to create one:

Note: The S3 bucket needs to be in the AWS region used to deploy the Lambda.

aws s3 mb s3://${BUCKET_NAME}

Run the following commands to build and package our Lambda function to S3:

sam build --use-container --skip-pull-image
sam package \
    --output-template-file packaged.yaml \
    --s3-bucket ${BUCKET_NAME}

Next, the following command will create a Cloudformation Stack and deploy your SAM resources.

sam deploy \
    --template-file packaged.yaml \
    --stack-name k8s-drainer \
    --capabilities CAPABILITY_IAM \
    --parameter-overrides AutoScalingGroup=${YOUR_AUTOSCALING_GROUP_NAME} EksCluster=${YOUR_CLUSTER_NAME}

See Serverless Application Model (SAM) HOWTO Guide for more details in how to get started.

There is a convenience script in the root directory called build_deploy.sh that wraps these three commands and takes your AWS profile as an argument (it will use the default profile if a profile is not provided) and the S3 bucket created above.

./build_deploy.sh 
    ${BUCKET_NAME} \
    ${YOUR_AUTOSCALING_GROUP_NAME} \
    ${YOUR_CLUSTER_NAME} \
    ${YOUR_AWS_PROFILE}

After deployment is complete you can run the following command to retrieve the API Gateway Endpoint URL:

aws cloudformation describe-stacks \
    --stack-name k8s-drainer \
    --output table

Kubernetes Permissions

After deployment there will be an IAM role associated with the lambda that needs to be mapped to a user or group in the EKS cluster. To create the Kubernetes ClusterRole and ClusterRoleBinding run the following shell command from the root directory of the project:

kubectl apply -R -f k8s_rbac/

You may now create the mapping to the IAM role created when deploying the Drainer function. You can find this role by checking the DrainerRole output of the CloudFormation stack created by the sam deploy command above. Run kubectl edit -n kube-system configmap/aws-auth and add the following yaml:

mapRoles: | 
# ...
    - rolearn: <DrainerFunction IAM role>
      username: lambda

Testing the Drainer function

Run the following command to simulate an EC2 instance being terminated as part of a scale-in event:

aws autoscaling terminate-instance-in-auto-scaling-group --no-should-decrement-desired-capacity --instance-id <instance-id>

You must use this command for Auto Scaling Lifecycle hooks to be used. Terminating the instance via the EC2 Console or APIs will immediately terminate the instance, bypassing the lifecycle hooks.

Fetch, tail, and filter Lambda function logs

To simplify troubleshooting, SAM CLI provides a command called sam logs. sam logs lets you fetch logs generated by your Lambda function from the command line. In addition to printing the logs on the terminal, this command has several features to help you quickly find the bug.

NOTE: This command works for all AWS Lambda functions; not just the ones you deploy using SAM.

sam logs -n DrainerFunction --stack-name k8s-drainer --tail

You can find more information and examples about filtering Lambda function logs in the SAM CLI Documentation.

Unit Tests

To run the unit tests, install the test dependencies and run pytest against the tests directory:

pipenv install --dev --ignore-pipfile
pipenv run py.test --cov=drainer

Cleanup

In order to remove the EKS Node Drainer Lambda function and Lifecycle Hook you can use the following AWS CLI Command:

aws cloudformation delete-stack --stack-name k8s-drainer

To remove the Kubernetes ClusterRole and ClusterRoleBinding, run the following commands:

kubectl delete clusterrolebinding lambda-user-cluster-role-binding

kubectl delete clusterrole lambda-cluster-access

License Summary

This sample code is made available under a modified MIT license. See the LICENSE file.

Appendix

Building the project

AWS Lambda requires a flat folder with the application as well as its dependencies in the deployment package. When you make changes to the source code or dependency manifest, run the following command to build your project local testing and deployment:

sam build

If your dependencies contain native modules that need to be compiled specifically for the operating system running on AWS Lambda, use this command to build inside a Lambda-like Docker container instead:

sam build --use-container

By default, built artifacts are written to the .aws-sam/build directory.

Limitations

This solution works on a per cluster per autoscaling group basis, multiple autoscaling groups will require a separate deployment for each group.

Certain types of pod cannot be evicted from a node, so this lambda will not attempt to evict DaemonSets or mirror pods.

Scout24/amazon-k8s-node-drainer