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Generating a run.Dockerfile

Examine curl extension

detect

cat $PWD/samples/extensions/curl/bin/detect

The extension always detects (because its exit code is 0) and provides a dependency called curl.

generate

cat $PWD/samples/extensions/curl/bin/generate

The extension generates a run.Dockerfile that switches the run image to reference run-image-curl.

Build a run image for curl extension to use

cat $PWD/samples/stacks/alpine/run/curl.Dockerfile

This is a simple Dockerfile that creates a CNB run image from the curl base image by adding the required CNB user configuration and io.buildpacks.stack.id label.

The Dockerfile could come from anywhere, but we include it in the stacks directory for convenience.

Build the run image:

docker build \
  --file $PWD/samples/stacks/alpine/run/curl.Dockerfile \
  --tag run-image-curl .

Re-create our builder with hello-extensions updated to require curl

Edit $PWD/samples/buildpacks/hello-extensions/bin/detect to uncomment the second set of lines that output [[requires]] to the build plan:

sed -i "14,15s/#//" $PWD/samples/buildpacks/hello-extensions/bin/detect

(On Mac, use sed -i '' "14,15s/#//" $PWD/samples/buildpacks/hello-extensions/bin/detect)

Re-create the builder:

pack builder create localhost:5000/extensions-builder \
  --config $PWD/samples/builders/alpine/builder.toml \
  --publish

Re-build the application image

pack build hello-extensions \
  --builder localhost:5000/extensions-builder \
  --path $PWD/samples/apps/java-maven \
  --pull-policy always \
  --network host \
  --verbose

Note that --network host is necessary when publishing to a local registry.

You should see:

[detector] ======== Results ========
[detector] pass: samples/tree@0.0.1
[detector] pass: samples/curl@0.0.1
[detector] pass: samples/hello-extensions@0.0.1
[detector] Resolving plan... (try #1)
[detector] samples/tree             0.0.1
[detector] samples/curl             0.0.1
[detector] samples/hello-extensions 0.0.1
[detector] Running generate for extension samples/tree@0.0.1
...
[detector] Running generate for extension samples/curl@0.0.1
...
[detector] Checking for new run image
[detector] Found a run.Dockerfile configuring image 'run-image-curl' from extension with id 'samples/curl'
...
[extender] Found build Dockerfile for extension 'samples/tree'
[extender] Applying the Dockerfile at /layers/generated/build/samples_tree/Dockerfile...
...
[extender] Running build command
[extender] ---> Hello Extensions Buildpack
[extender] tree v1.8.0 (c) 1996 - 2018 by Steve Baker, Thomas Moore, Francesc Rocher, Florian Sesser, Kyosuke Tokoro
...
Successfully built image hello-extensions

See the image run successfully

docker run --rm hello-extensions

You should see something akin to:

curl 7.85.0-DEV (x86_64-pc-linux-musl) ... more stuff here ...

What happened: now that hello-extensions requires both tree and curl in its build plan, both extensions are included in the build and provide the needed dependencies for build and launch, respectively

  • The tree extension installs tree at build time, as before
  • The curl extension switches the run image to run-image-curl, which has curl installed

Now our curl process can succeed!

What’s next?

The tree and curl examples are very simple, but we can unlock powerful new features with this functionality.

Platforms could have several run images available, each tailored to a specific language family, thus limiting the number of installed dependencies for each image to the minimum necessary to support the targeted language. Image extensions could be used to switch the run image to that most appropriate for the current application.

Similarly, builder images could be kept lean if image extensions are used to dynamically install the needed dependencies for each application.

In the future, both run image switching and run image modification will be supported, opening the door to other use cases. Consult the RFC for further information.

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