Update chaching strategy

.github/workflows/main.yml

@@ -108,13 +108,30 @@ jobs:
       - name: use mobility.api.opendatahub.testingmachine.eu for testing
         run: sed -i 's/mobility.api.opendatahub.com/mobility.api.opendatahub.testingmachine.eu/' */config.yml
 
+      # elevation data does not change so we cache it forever
       - name: Cache OSM and elevation data
         uses: actions/cache@v4
         with:
           path: |
             data/srtm*
           key: elevation
 
+      # OSM data changes every day, so we cache a monthly download and apply the daily updates to it
+      # ideally we also put the updated file back into the cache but Github CI doesn't allow you to
+      # do that without jumping through many hoops.
+      # Therefore, we take the version from the beginning of the month and apply more and more daily
+      # updates until the next month triggers a complete download.
+      - name: Set variable OSM_CACHE_VERSION
+        run: |
+          echo "OSM_CACHE_VERSION=$(date +%Y-%m)" >> $GITHUB_ENV
+
+      - name: Cache OSM and elevation data
+        uses: actions/cache@v4
+        with:
+          path: |
+            data/italy-nord-est.osm.pbf
+          key: osm-${{ env.OSM_CACHE_VERSION }}
+
       - name: Build graph
         run: bash build-graph.sh