Using CNB buildpacks in builder-classic

[ipmb]

I was surprised to see heroku/builder-classic:22 defaults to the shimmed heroku/nodejs buildpack instead of the CNB one. I use the heroku/python buildpack, so the classic builder is the only option for me currently. AFAICT, there's no way for me to take advantage of the latest heroku/nodejs without looking up the current version on each build. On heroku/buildpacks:20, mixing CNB and shimmed buildpacks worked as expected.

Expected

The latest heroku/nodejs buildpack is selected on both heroku/builder:22 and heroku/builder-classic:22. I expected the classic one to look like this:

$ pack builder inspect heroku/builder:22 --output json | jq '.remote_info.buildpacks[] | select(.id == "heroku/nodejs")'
{
  "id": "heroku/nodejs",
  "name": "Node.js",
  "version": "0.5.11",
  "homepage": "https://github.com/heroku/buildpacks-nodejs"
}

Actual

$ pack builder inspect heroku/builder-classic:22 --output json | jq '.remote_info.buildpacks[] | select(.id == "heroku/nodejs")'
{
  "id": "heroku/nodejs",
  "name": "Node.js",
  "version": "0.0.0"
}

I assumed the classic builder was so you could use shimmed builpacks, not so you could exclude all CNB buildpacks. Is there any reason a user would want to use a shimmed buildpack when CNB exists?

[edmorley]

@ipmb Hi again :-)

So this is by design, and is mentioned here:
https://github.com/heroku/builder/blame/8f01e3e6ee5f5e172547b3d1322ec1e52a6abbc8/README.md#L15-L19

The reasoning for this current design is that:

• we generally try and avoid breaking changes for a specific stack
• we know there will be breaking changes between the classic shimmed CNBs and their upcoming native CNB replacements (as legacy features/behaviour are dropped + UX improvements introduced)
• so to avoid future breaking changes for Heroku-22 we've temporarily omitted any languages that don't yet have a native CNB from heroku/builder:22, rather than add a shimmed version and have to swap it out later
• we also wanted to provide a way for people to get a "close to current Heroku" build experience (which will also give an alternate route for people who need a legacy feature intentionally cut from the new CNBs), which is why the heroku/builder-classic:22 image exists.

If you want to use a shimmed + not shimmed CNB at the same time, you can use --buildpack to specify the buildpacks:

pack build --builder heroku/builder-classic:22 --buildpack urn:cnb:registry:heroku/nodejs --buildpack heroku/python myapp

The urn:cnb:registry syntax refers to this registry listing:
https://registry.buildpacks.io/buildpacks/heroku/nodejs

The docs for the various prefixes supported are here:
https://buildpacks.io/docs/app-developer-guide/specify-buildpacks/#uri-examples

Otherwise, another option would be to continue to use builder heroku/buildpacks:20 and wait until more languages have a native CNB before switching to Heroku-22.

[ipmb]

Ah, I wasn't aware of the urn:cnb:registry prefix. That is a reasonable workaround for me until Python gets a proper CNB.

Thanks @edmorley !