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AWS Elastic Container Registry (ECR) cleaner

Removes old, unused images from AWS Elastic Container Registry (ECR).

Why have we built it?

The number and size of images in your docker registry grow over time and can cost a significant amount of money. It is a good practice to have a cleaning policy of unused, old images. There are ready-to-use solutions like ECR lifecycle policies, but they lack some features - mainly they don't check if an images is still in use. We consider using them dangerous - for example if your ECS Service can't find an image when scaling out, it would fail to start your app. That's why we decided to write a Lambda function that will check periodically for old, unused images.

How does it work?

The first step is scanning for images that are currently in use and putting the set in memory:

  • listing all ECS Services in all ECS Clusters and taking image ids from task definitions,
  • listing all Lambda functions with package type "Image" and taking image ids from them,
  • listing all App Runner services and taking their image ids.

The second step is iterating over all images in ECR repositories tagged with BoxCleanerEnabled set to true and for every image checking if:

  • it is older than a threshold (default 30 days),
  • it is unused (not present in the set of images that are currently in use).

Any unused, old image is being removed if the DRY_RUN environment variable is set to false. If you don't set the DRY_RUN environment variable or the value is different than false, ECR cleaner will only put line in the logs.

We strongly advise you to start with DRY_RUN=true!

Limitations

Only ECS, Lambda, and App Runner are supported. ECR cleaner will not check for any images used by any other service - for example EKS is currently not supported. Also, we do not check for containers used in different AWS accounts or different regions.

There can be some problems with RAM if you use a lot of images as we store them in memory. But we use map[string]struct{} to mitigate the risk.

Usage

ECR cleaner is designed to be used as an AWS Lambda container image. The Lambda can be triggered by AWS EventBridge Schedule (we use cron(0 0 * * ? *))

You can use our docker image ghcr.io/devopsbox-io/aws-ecr-cleaner:v0.2.0 (replace the tag with another version when appropriate) or build your own image downloading the binary in your Dockerfile (Set the ECR_CLEANER_SHA256 and ECR_CLEANER_VERSION variables appropriately):

FROM ubuntu:22.04

RUN apt-get update && apt-get install --no-install-recommends -y \
    ca-certificates \
    curl \
    && rm -rf /var/lib/apt/lists/*

ENV ECR_CLEANER_SHA256=b2be75392eb7cb382cb9a4a9308b3026d248dd90b33bec6c54a53f1f3e4a5303 \
    ECR_CLEANER_VERSION=0.2.0
RUN curl -L https://github.com/devopsbox-io/aws-ecr-cleaner/releases/download/v${ECR_CLEANER_VERSION}/aws-ecr-cleaner-${ECR_CLEANER_VERSION}-linux-amd64 \
        -o /usr/local/bin/aws-ecr-cleaner && \
    echo "${ECR_CLEANER_SHA256} /usr/local/bin/aws-ecr-cleaner" | sha256sum --check && \
    chmod +x /usr/local/bin/aws-ecr-cleaner

CMD [ "/usr/local/bin/aws-ecr-cleaner" ]

Either way, you have to push ECR cleaner image to an AWS ECR repository in your AWS account before using it in Lambda.

The Lambda IAM execution role will need a usual AWSLambdaBasicExecutionRole policy and additionally a policy with the following permissions:

{
  "Statement": [
    {
      "Action": [
        "apprunner:DescribeService",
        "apprunner:ListServices",
        "ecr:BatchDeleteImage",
        "ecr:DescribeImages",
        "ecr:DescribeRepositories",
        "ecr:ListTagsForResource",
        "ecs:DescribeServices",
        "ecs:DescribeTaskDefinition",
        "ecs:ListClusters",
        "ecs:ListServices",
        "lambda:GetFunction",
        "lambda:ListFunctions"
      ],
      "Effect": "Allow",
      "Resource": [
        "*"
      ]
    },
    {
      "Action": [
        "ssm:GetParametersByPath"
      ],
      "Effect": "Allow",
      "Resource": [
        "arn:aws:ssm:*::parameter/aws/service/global-infrastructure/services/apprunner/regions"
      ]
    }
  ],
  "Version": "2012-10-17"
}

Settings

Environment variables

  • DEFAULT_KEEP_DAYS - integer in days, default 30; ECR cleaner will not remove images younger than value of this environment variable
  • DRY_RUN - boolean, default true; if set to false, ECR cleaner will start removing images, any other value means that ECR cleaner will only put a Found unused image, should be removed line to the logs

Repository tags

  • BoxCleanerEnabled - boolean; only repositories with this tag set to true will be cleaned
  • BoxCleanerKeepDays - integer in days; you can override the DEFAULT_KEEP_DAYS for each repository using this tag

Known issues

If you have a lot of old images and there are throttling errors (error ThrottlingException: Rate exceeded), just rerun the process - it is perfectly normal.

Useful commands related to development

Generating mocks

mockgen -source=internal/pkg/aws/apprunner.go -destination=internal/pkg/aws/apprunner_mock.go -package=aws
mockgen -source=internal/pkg/aws/ecr.go -destination=internal/pkg/aws/ecr_mock.go -package=aws
mockgen -source=internal/pkg/aws/ecs.go -destination=internal/pkg/aws/ecs_mock.go -package=aws
mockgen -source=internal/pkg/aws/lambda.go -destination=internal/pkg/aws/lambda_mock.go -package=aws
mockgen -source=internal/pkg/aws/ssm.go -destination=internal/pkg/aws/ssm_mock.go -package=aws

Running unit tests with coverage

mkdir -p build/test-results \
  && go test -coverpkg=./... -coverprofile=build/test-results/coverage.out ./... \
  && go tool cover -html=build/test-results/coverage.out -o build/test-results/coverage.html