When you git push to Heroku, your code is received by the slug compiler, which transforms your repository into a slug.

Compilation

The slug compiler is invoked by a git pre-receive hook, which follows these steps:

1. Create a fresh checkout of HEAD from the master branch.
2. Download, build, and install local dependencies as specified in your build file (e.g. Gemfile, package.json, requirements.txt, etc.)
3. Remove unused files, including the .git directory, anything in log and tmp, local build dependencies such as .gem files in the bundled gems directory, and anything specified in .slugignore.
4. Write the environment into the slug. (The slug is rebuilt any time a config var or add-on is changed)
5. Package into the final slug archive.

Ignoring Files

If your repository contains files not necessary to run your app, you may wish to add these to a .slugignore file in the root of your repository. Examples of files you may wish to exclude from the slug:

• Unit tests or specs
• Art sources (like .psd files)
• Design documents (like .pdf files)
• Test data

The format is roughly similar to .gitignore. Here’s an example .slugignore:

*.psd
*.pdf
test
spec

Slug Size

Your slug size is displayed at the end of a successful compile. You can roughly estimate slug size locally by doing a fresh checkout of your app, deleting the .git directory, and running du -hsc.

Smaller slugs can be transferred across the dyno grid more quickly, allowing for a faster spin-up speed on your dynos. Generally speaking, any slug under 15MB is small and nimble; 30MB is average; and 40MB or above is weighty. If you find your app getting into the 40MB+ range, you may want to look into some techniques (such as removing unneeded dependencies or excluding files via .slugignore) to reduce the size.