Where should caching occur in a GitHub Action? - caching

Question

What is the correct placement of caching in a GitHub Action? Specifically is in correct to place it before or after running setup of tools using another Action?
For example if I'm using something like haskell/actions/setup should my use of actions/cache precede or follow that? Put another way: if setup subsequently installs updated components on a future run of my Action, will the corresponding parts of the cache be invalidated?

Answer 1

The cache action should be placed before any step that consumes or creates that cache. This step is responsible for:
defining cache parameters.
restoring the cache, if it was cached in the past.
GitHub Actions will then run a "Post *" step after all the steps, which will store the cache for future calls.
See the example workflow from the documentation.
For example, consider this sample workflow:
name: Caching Test
on: push
jobs:
build:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout#v2
- name: Enable Cache
id: cache-action
uses: actions/cache#v2
with:
path: cache-folder
key: ${{ runner.os }}-cache-key
- name: Use or generate the cache
if: steps.cache-action.outputs.cache-hit != 'true'
run: mkdir cache-folder && touch cache-folder/hello
- name: Verify we have our cached file
run: ls cache-folder
This is how it looks on the first run:
And this is how it looks on the second run:
GitHub will not invalidate the cache. Instead, it is the responsibility of the developer to ensure that the cache key is unique to the content it represents.
On common way to do this, is to set the cache key so that it contains a hash of a file that lives in the repository, so that changes to this file, will yield a different cache key. A good example for such a behavior, is when you have lock files that list all your repository's dependencies (requirements.txt for pyrhon, Gemfile.lock for ruby, etc).
This is achieved by a syntax similar to this:
key: ${{ runner.os }}-${{ hashFiles('**/lockfiles') }}
as described in the Creating a cache key section of the documentation.