.github/workflows/cache_retain.yml

# Copyright (C) 2023 and later: Unicode, Inc. and others.
# License & terms of use: http://www.unicode.org/copyright.html
#
# This workflow is designed to keep specific caches on the main
# branch from getting evicted according to the Github Actions policy
# (currently in 2023: 7 days) in cases where the cost to construct
# the cache is high, especially in cases where there is flakiness
# (ex: network loss / throttling when downloading artifacts) involved in
# constructing the cache.
#
# Preventing a cache from eviction using this workflow requires that:
#  - the cache is not too big that it starves other caches 
#    from using the shared cache quota for the repository
#  - the cache key is specific enough to avoid cache collisions, according
#    to good cache key design
#  - the cache key is not overly specific to cause unnecessary cache misses
#    (resulting in duplicate caches values, thereby wasting space), according
#    to good cache key design
#  - For more details, see: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows

name: Retain Specific Caches

on:
  schedule:
    # Because the Github Actions cache eviction policy is every 7 days,
    # this cron schedule is set to run every 6 days to ensure retention
    - cron: '0 12 */6 * *'

jobs:
  retain-maven-cache:
    name: Run all tests with Maven
    runs-on: ubuntu-latest
    steps:
      - name: Checkout and setup
        uses: actions/checkout@v2
      - name: Set up JDK 11
        uses: actions/setup-java@v1
        with:
          java-version: 11
      - name: Cache local Maven repository
        uses: actions/cache@v2
        with:
          path: ~/.m2/repository
          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
          restore-keys: |
            ${{ runner.os }}-maven-
      - name: Set up out-of-source output dir
        run: |
          mkdir -p unicodetools/mine/Generated/BIN

      # Omitting running any build instructions.
      #
      # The point is to touch the cache before the TTL causes it to get evicted.
      # For Github Actions, the cache is preserved only when the cache action is
      # invoked and the job completes successfully.
      #
      # It seems sufficient for the CI job here to finish successfully by
      # trivially running nothing.