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ko: Easy Go Containers

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ko is a simple, fast container image builder for Go applications.

It's ideal for use cases where your image contains a single Go application without any/many dependencies on the OS base image (e.g., no cgo, no OS package dependencies).

ko builds images by effectively executing go build on your local machine, and as such doesn't require docker to be installed. This can make it a good fit for lightweight CI/CD use cases.

ko also includes support for simple YAML templating which makes it a powerful tool for Kubernetes applications (See below).

Setup

Install

Install from Releases

VERSION=TODO # choose the latest version
OS=Linux     # or Darwin
ARCH=x86_64  # or arm64, i386, s390x
curl -L https://github.com/google/ko/releases/download/v${VERSION}/ko_${VERSION}_${OS}_${ARCH}.tar.gz | tar xzf - ko
chmod +x ./ko

Install using Homebrew

brew install ko

Build and Install from Source

With Go 1.16+, build and install the latest released version:

go install github.com/google/ko@latest

Authenticate

ko depends on the authentication configured in your Docker config (typically ~/.docker/config.json). If you can push an image with docker push, you are already authenticated for ko.

Since ko doesn't require docker, ko login also provides a surface for logging in to a container image registry with a username and password, similar to docker login.

Choose Destination

ko depends on an environment variable, KO_DOCKER_REPO, to identify where it should push images that it builds. Typically this will be a remote registry, e.g.:

  • KO_DOCKER_REPO=gcr.io/my-project, or
  • KO_DOCKER_REPO=my-dockerhub-user

Build an Image

ko publish ./cmd/app builds and pushes a container image, and prints the resulting image digest to stdout.

ko publish ./cmd/app
...
gcr.io/my-project/app-099ba5bcefdead87f92606265fb99ac0@sha256:6e398316742b7aa4a93161dce4a23bc5c545700b862b43347b941000b112ec3e

Because the output of ko publish is an image reference, you can easily pass it to other tools that expect to take an image reference:

To run the container:

docker run -p 8080:8080 $(ko publish ./cmd/app)

Or, for example, to deploy it to other services like Cloud Run:

gcloud run deploy --image=$(ko publish ./cmd/app)

Configuration

Aside from KO_DOCKER_REPO, you can configure ko's behavior using a .ko.yaml file. The location of this file can be overridden with KO_CONFIG_PATH.

Overriding Base Images

By default, ko bases images on gcr.io/distroless/static:nonroot. This is a small image that provides the bare necessities to run your Go binary.

You can override this base image in two ways:

  1. To override the base image for all images ko builds, add this line to your .ko.yaml file:
defaultBaseImage: registry.example.com/base/image
  1. To override the base image for certain importpaths:
baseImageOverrides:
  github.com/my-user/my-repo/cmd/app: registry.example.com/base/for/app
  github.com/my-user/my-repo/cmd/foo: registry.example.com/base/for/foo

Overriding Go build settings

By default, ko builds the binary with no additional build flags other than --trimpath (depending on the Go version). You can replace the default build arguments by providing build flags and ldflags using a GoReleaser influenced builds configuration section in your .ko.yaml.

builds:
- id: foo
  main: ./foobar/foo
  env:
  - GOPRIVATE=git.internal.example.com,source.developers.google.com
  flags:
  - -tags
  - netgo
  ldflags:
  - -s -w
  - -extldflags "-static"
  - -X main.version={{.Env.VERSION}}
- id: bar
  main: ./foobar/bar/main.go
  env:
  - GOCACHE=/workspace/.gocache
  ldflags:
  - -s
  - -w

For the build, ko will pick the entry based on the respective import path being used. It will be matched against the local path that is configured using dir and main. In the context of ko, it is fine just to specify main with the intended import path.

Please note: Even though the configuration section is similar to the GoReleaser builds section, only the env, flags and ldflags fields are currently supported. Also, the templating support is currently limited to environment variables only.

Naming Images

ko provides a few different strategies for naming the image it pushes, to workaround certain registry limitations and user preferences:

Given KO_DOCKER_REPO=registry.example.com/repo, by default, ko publish ./cmd/app will produce an image named like registry.example.com/repo/app-<md5>, which includes the MD5 hash of the full import path, to avoid collisions.

  • --preserve-import-path (-P) will include the entire importpath: registry.example.com/repo/github.com/my-user/my-repo/cmd/app
  • --base-import-paths (-B) will omit the MD5 portion: registry.example.com/repo/app
  • --bare will only include the KO_DOCKER_REPO: registry.example.com/repo

Local Publishing Options

ko is normally used to publish images to container image registries, identified by KO_DOCKER_REPO.

ko can also publish images to a local Docker daemon, if available, by setting KO_DOCKER_REPO=ko.local, or by passing the --local (-L) flag.

Locally-published images can be used as a base image for other ko images:

defaultBaseImage: ko.local/example/base/image

ko can also publish images to a local KinD cluster, if available, by setting KO_DOCKER_REPO=kind.local. By default this publishes to the default KinD cluster name (kind). To publish to another KinD cluster, set KIND_CLUSTER_NAME=my-other-cluster.

Multi-Platform Images

Because Go supports cross-compilation to other CPU architectures and operating systems, ko excels at producing multi-platform images.

To build and push an image for all platforms supported by the configured base image, simply add --platform=all. This will instruct ko to look up all the supported platforms in the base image, execute GOOS=<os> GOARCH=<arch> GOARM=<variant> go build for each platform, and produce a manifest list containing an image for each platform.

You can also select specific platforms, for example, --platform=linux/amd64,linux/arm64

Static Assets

ko can also bundle static assets into the images it produces.

By convention, any contents of a directory named <importpath>/kodata/ will be bundled into the image, and the path where it's available in the image will be identified by the environment variable KO_DATA_PATH.

As an example, you can bundle and serve static contents in your image:

cmd/
  app/
    main.go
    kodata/
      favicon.ico
      index.html

Then, in your main.go:

func main() {
    http.Handle("/", http.FileServer(http.Dir(os.Getenv("KO_DATA_PATH"))))
    log.Fatal(http.ListenAndServe(":8080", nil))
}

You can simulate ko's behavior outside of the container image by setting the KO_DATA_PATH environment variable yourself:

KO_DATA_PATH=cmd/app/kodata/ go run ./cmd/app

Tip: Symlinks in kodata are followed and included as well. For example, you can include Git commit information in your image with:

ln -s -r .git/HEAD ./cmd/app/kodata/

Also note that http.FileServer will not serve the Last-Modified header (or validate If-Modified-Since request headers) because ko does not embed timestamps by default.

This can be supported by manually setting the KO_DATA_DATE_EPOCH environment variable during build (See below).

Kubernetes Integration

You could stop at just building and pushing images.

But, because building images is so easy with ko, and because building with ko only requires a string importpath to identify the image, we can integrate this with YAML generation to make Kubernetes use cases much simpler.

YAML Changes

Traditionally, you might have a Kubernetes deployment, defined in a YAML file, that runs an image:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-deployment
spec:
  replicas: 3
  ...
  template:
    spec:
      containers:
      - name: my-app
        image: registry.example.com/my-app:v1.2.3

...which you apply to your cluster with kubectl apply:

kubectl apply -f deployment.yaml

With ko, you can instead reference your Go binary by its importpath, prefixed with ko://:

    ...
    spec:
      containers:
      - name: my-app
        image: ko://github.com/my-user/my-repo/cmd/app

ko resolve

With this small change, running ko resolve -f deployment.yaml will instruct ko to:

  1. scan the YAML file(s) for values with the ko:// prefix,
  2. for each unique ko://-prefixed string, execute ko publish <importpath> to build and push an image,
  3. replace ko://-prefixed string(s) in the input YAML with the fully-specified image reference of the built image(s), for example:
spec:
  containers:
    - name: my-app
      image: registry.example.com/github.com/my-user/my-repo/cmd/app@sha256:deadb33f...
  1. Print the resulting resolved YAML to stdout.

The result can be redirected to a file, to distribute to others:

ko resolve -f config/ > release.yaml

Taken together, ko resolve aims to make packaging, pushing, and referencing container images an invisible implementation detail of your Kubernetes deployment, and let you focus on writing code in Go.

ko apply

To apply the resulting resolved YAML config, you can redirect the output of ko resolve to kubectl apply:

ko resolve -f config/ | kubectl apply -f -

Since this is a relatively common use case, the same functionality is available using ko apply:

ko apply -f config/

NB: This requires that kubectl is available.

ko delete

To teardown resources applied using ko apply, you can run ko delete:

ko delete -f config/

This is purely a convenient alias for kubectl delete, and doesn't perform any builds, or delete any previously built images.

Frequently Asked Questions

How can I set ldflags?

Using -ldflags is a common way to embed version info in go binaries (In fact, we do this for ko!). Unfortunately, because ko wraps go build, it's not possible to use this flag directly; however, you can use the GOFLAGS environment variable instead:

GOFLAGS="-ldflags=-X=main.version=1.2.3" ko publish .

How can I set multiple ldflags?

Currently, there is a limitation that does not allow to set multiple arguments in ldflags using GOFLAGS. Using -ldflags multiple times also does not work. In this use case, it works best to use the builds section in the .ko.yaml file.

Why are my images all created in 1970?

In order to support reproducible builds, ko doesn't embed timestamps in the images it produces by default.

However, ko does respect the SOURCE_DATE_EPOCH environment variable, which will set the container image's timestamp accordingly.

Similarly, the KO_DATA_DATE_EPOCH environment variable can be used to set the modtime timestamp of the files in KO_DATA_PATH.

For example, you can set the container image's timestamp to the current timestamp by executing:

export SOURCE_DATE_EPOCH=$(date +%s)

or set the timestamp of the files in KO_DATA_PATH to the latest git commit's timestamp with:

export KO_DATA_DATE_EPOCH=$(git log -1 --format='%ct')

Can I build Windows containers?

Yes, but support for Windows containers is new, experimental, and tenuous. Be prepared to file bugs. 🐛

The default base image does not provide a Windows image. You can try out building a Windows container image by setting the base image to a Windows base image and building with --platform=windows/amd64 or --platform=all:

For example, to build a Windows container image for ko, from within this repo:

KO_DEFAULTBASEIMAGE=mcr.microsoft.com/windows/nanoserver:1809 ko publish ./ --platform=windows/amd64

Known issues 🐛

  • Symlinks in kodata are ignored when building Windows images; only regular files and directories will be included in the Windows image.

Can I optimize images for eStargz support?

Yes! Set the environment variable GGCR_EXPERIMENT_ESTARGZ=1 to produce eStargz-optimized images.

Does ko support autocompletion?

Yes! ko completion generates a Bash completion script, which you can add to your bash_completion directory:

ko completion > /usr/local/etc/bash_completion.d/ko

Or, you can source it directly:

source <(ko completion)

Does ko work with Kustomize?

Yes! ko resolve -f - will read and process input from stdin, so you can have ko easily process the output of the kustomize command.

kustomize build config | ko resolve -f -

Does ko work with OpenShift Internal Registry?

Yes! Follow these steps:

oc registry login --to=$HOME/.docker/config.json
  • Create a namespace where you will push your images, i.e: ko-images
  • Execute this command to set KO_DOCKER_REPO to publish images to the internal registry.
   export KO_DOCKER_REPO=$(oc registry info --public)/ko-images

Acknowledgements

This work is based heavily on learnings from having built the Docker and Kubernetes support for Bazel. That work was presented here.

Discuss

Questions? Comments? Ideas? Come discuss ko with us in the #ko-project channel on the Kubernetes Slack! See you there!