slugcmplr

slugcmplr allows you to compile Heroku compatible slugs using official and custom buildpacks, in the same manner as the Heroku Slug Compiler

This enables more control over the build process and allows for detaching building and releasing your Heroku applications.

Install

With the go toolchain installed:

# In order to use this as a library
go get github.com/cga1123/slugcmplr

# In order to install as a CLI
go install github.com/cga1123/slugcmplr@latest

There are also precompiled binaries and their associated checksums are available attached to tagged releases.

Info

slugcmplr has 3 steps/sub-commands:

prepare [APPLICATION] --build-dir [BUILD-DIR] --source-dir [SOURCE-DIR]

In the prepare step, slugcmplr will fetch the metadata required to compile your application. It will copy your project SOURCE-DIR into BUILD-DIR/app.

It will fetch the buildpacks as defined by your Heroku application, download, and decompress them into BUILD-DIR/buildpacks. If using official buildpacks (e.g. heroku/go, heroku/ruby) slugcmplr will use the same buildpack as currently deployed to Heroku's production environment.

It will fetch the config vars as defined by your Heroku application and put them into the BUILD-DIR/environment directory.

Finally, prepare writes metadata (such as the application name, stack, buildpack order, source version) to the BUILD-DIR/compile.json file to allow the compile step to bootstrap itself.

compile --build-dir [BUILD-DIR] --cache-dir [CACHE-DIR]

In the compile step, slugcmplr executes your buildpacks in the specified order, outputs your Heroku slug, and uploads it to Heroku for future release.

compile will output the slug for your application to BUILD-DIR/app.tgz

compile will output metadata about the compilation to BUILD-DIR/release.tgz, this contains information such as the slug ID as uploaded to Heroku.

The CACHE-DIR will be used by the buildpacks as their cache argument to speed up builds in the future, as per the Buildpack API

To guarantee full compatibility, it is recommended to run this step using Heroku's build containers. e.g. heroku/heroku:24-build.

release --build-dir [BUILD-DIR]

In the release step, slugcmplr triggers a release of your previously compiled slug.

It uses the BUILD-DIR/release.json file in order to fetch metadata in order to create this release.

You can optionally pass --app [APPLICATION] to target an application that is different from the one you built from. This will work as long as the applications are in the same Heroku team. (This is becuase the slug must be accessible to the application).

You can optionally pass --commit [COMMIT] to associate this release with a separate commit from the one used to build this slug initially.

Authentication

The slugcmplr CLI looks for credentials to api.heroku.com in your .netrc file, this is the same technique used by heroku/cli and so if you are currently making use of the heroku command during CI, you should already be logging in somehow and have no issues.

Otherwise populating your .netrc should be a case of adding something equivalent to the following script (assuming the HEROKU_EMAIL and HEROKU_API_KEY are correctly populated environment variables):

cat << EOF >> ${HOME}/.netrc
machine api.heroku.com
  login ${HEROKU_EMAIL}
  password ${HEROKU_API_KEY}
EOF

By default, slugcmplr will look in ${HOME}/.netrc for the credentials, however it will respect the ${NETRC} environment variable if set and non-empty.

Testing

The majority of tests for this project are acceptance tests that will create and release live Heroku applications. These require the correct credentials to be set in the environment as well as setting a sentinel value to execute the acceptance tests:

SLUGCMPLR_ACC=true \
  SLUGCMPLR_ACC_HEROKU_PASS=<HEROKU-API-KEY> \
  SLUGCMPLR_ACC_HEROKU_EMAIL=<HEROKU-EMAIL> \
  go test -v

Fixture application are hosted in separate repositories which will be cloned and created by using the withHarness function. Fixture repositories are expected to contain a app.json file which describes the Heroku applications.

See app.json Schema

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/CGA1123/slugcmplr

See issues if you want any inspiration as to what to help with or of any pending discussion/work.

This project is Codespaces compatible! If you want to get started quickly spin one up.

The script directory should help you get going with building and testing.

Some buildpacks are not compatible with arm/arm64 architectures, you may experience issues testing slugcmplr on an M1 Chip, even if using docker containers.

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For more background on this you might find this Medium article helpful.