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Create an init-image for Fn

This how-to shows how to create an init-image for Fn

What is an init-image?

An init-image can be used with fn init --init-image=<init_image_name> to create a template for a function. Init-images can be used to create a HelloWorld function showing how to use a particular language, or for more complex templating.

The requirements for an init-image are:

  • it will be run with docker run -e FN_FUNCTION_NAME=<function_name> init_image_name
  • the output will be treated as a tarball
  • the files in the tarball should form a valid function which can be run using fn run, with the exception of func.yaml
  • the output should not contain a func.yaml file
  • the output must contain a file called func.init.yaml which has a subset of the keys from a func.yaml
  • the func.init.yaml must contain the following keys: runtime
  • the func.init.yaml may also contain the following keys: build, buildImage, cmd, content_type, entrypoint, expects, headers, runImage, and tests

The CLI will:

  • put the files in the archive on disk, owned by the current user and with correct permissions
  • merge the content of func.init.yaml with other properties such as name and version and create a func.yaml
  • remove the left-over func.init.yaml

Creating an example init-image

The easiest way to create an init-image is to base it on an existing function

A simple function

We'll use a really simple function as our example, one which we could create by hand that uses the string-reversing tool called rev that is bundled in Alpine Linux:

$ ls
Dockerfile  func.yaml
$ cat Dockerfile
FROM alpine:latest
CMD ["rev"]
$ cat func.yaml
name: reverser
version: 0.0.1
runtime: docker
$ echo HELLO | fn run
Building image blah:0.0.1

Creating the init-image

Firstly we need to create func.init.yaml. It'll be the same as the existing func.yaml but without name and version (these are provided by the CLI when the init-image is used).

$ cat func.init.yaml
runtime: docker

We need to create a tarball of the files we want the init-image to create:

$ tar -cf init.tar Dockerfile func.init.yaml

There is no need to use any compression here as Docker does that for us.

We'll be creating the init-image here, so we need another Dockerfile to do that:

$ cat Dockerfile-init-image
FROM alpine:latest
COPY init.tar /
CMD ["cat", "/init.tar"]

Now we can create our init-image:

$ docker build -t reverser-init -f Dockerfile-init-image .

Sending build context to Docker daemon  15.36kB
Step 1/3 : FROM alpine:latest
 ---> 3fd9065eaf02
Step 2/3 : COPY init.tar /
 ---> 03ad6bc830c3
Step 3/3 : CMD cat /init.tar
 ---> Running in b8dad41e966f
 ---> decb5e898b1d
Removing intermediate container b8dad41e966f
Successfully built decb5e898b1d
Successfully tagged reverser-init:latest

And we are done.

Testing out the init-image

Move into an empty directory, and we can create a function from the init-image:

$ mkdir new && cd new
$ fn init --init-image=reverser-init
Building from init-image: reverser-init
func.yaml created.
$ echo "YES MATE" | fn run
Building image new:0.0.1 

You can docker push this image to a registry to share it with others, and it should be clear how to create more complex init-images too. Remember that you are not limited to COPY for adding a tarball, you could create it on the fly within the init-image if you wanted to do some deeper customization.