Imagine you have a single code base, which has a few different applications within it... or at least the ability to run a few different applications. Or, maybe you're Google with your mono repo?
In any case, how do you manage this on Heroku? You don't. Heroku applications assume one repo to one application.
Enter the Monorepo buildpack, which is a copy of heroku-buildpack-multi-procfile except it moves the target path in to the root, rather than just the Procfile. This helps for ruby apps etc.
This buildpack was extended by Bus.com to add support for shared gems. The expected repository layout is:
. +- app1 | | | + Gemfile | + Gemfile.lock | + Procfile | +- app2 | | | + Gemfile | + Gemfile.lock | + Procfile | +- shared | + gems | +- common | +- common.gemspec +- Gemfile
During the compile phase, APP_BASE is moved to /app, and shared/gems is moved within /app. Gemfile and Gemfile.lock are modified to move the shared/gems directory around, in case this app references the shared gems.
APP_BASE=relative/path/to/app/root, and of course:
heroku buildpacks:add -a <app> https://github.com/lstoll/heroku-buildpack-monorepo
git push firstname.lastname@example.org:<app> master
Copy the snippet above into CLI.