Add documentation section with commands to query the cache

web/content/hut/_index.md

@@ -82,3 +82,44 @@ By adding the configuration below to a `flake.nix`, when someone uses the flake,
   };
 }
 ```
+
+### Querying the cache
+
+Check if the cache is available:
+```sh
+$ curl https://jungle.bsc.es/cache/nix-cache-info
+StoreDir: /nix/store
+WantMassQuery: 1
+Priority: 30
+```
+
+Prevent nix from building locally:
+```bash
+nix build --max-jobs 0 <...>
+```
+
+Check if a package is in cache:
+```bash
+# Do a raw eval on the <package>.outPath (this should not build the package)
+$ nix eval --raw jungle#openmp.outPath
+/nix/store/dwnn4dgm1m4184l4xbi0qfrprji9wjmi-openmp-2024.11
+# Take the hash (everything from / to - in the basename) and curl <hash>.narinfo
+# if it exists in the cache, it will return HTTP 200 and some information
+# if not, it will return 404
+$ curl https://jungle.bsc.es/cache/dwnn4dgm1m4184l4xbi0qfrprji9wjmi.narinfo
+StorePath: /nix/store/dwnn4dgm1m4184l4xbi0qfrprji9wjmi-openmp-2024.11
+URL: nar/dwnn4dgm1m4184l4xbi0qfrprji9wjmi-17imkdfqzmnb013d14dx234bx17bnvws8baf3ii1xra5qi2y1wiz.nar
+Compression: none
+NarHash: sha256:17imkdfqzmnb013d14dx234bx17bnvws8baf3ii1xra5qi2y1wiz
+NarSize: 1519328
+References: 4gk773fqcsv4fh2rfkhs9bgfih86fdq8-gcc-13.3.0-lib nqb2ns2d1lahnd5ncwmn6k84qfd7vx2k-glibc-2.40-36
+Deriver: vcn0x8hikc4mvxdkvrdxp61bwa5r7lr6-openmp-2024.11.drv
+Sig: jungle.bsc.es:GDTOUEs1jl91wpLbb+gcKsAZjpKdARO9j5IQqb3micBeqzX2M/NDtKvgCS1YyiudOUdcjwa3j+hyzV2njokcCA==
+# In oneline:
+$ curl "https://jungle.bsc.es/cache/$(nix eval --raw jungle#<package>.outPath | cut -d '/' -f4 | cut -d '-' -f1).narinfo"
+```
+
+#### References
+
+- https://nix.dev/guides/recipes/add-binary-cache.html
+- https://nixos.wiki/wiki/Binary_Cache