checking-that-caching-works

Checking That Caching Works

All the instantiation related commands (read building commands) support the --dry-run flag even though it's not documented.

nix-build --dry-run monorepo.nix

Such invocations will output two lists - first the list of derivations that will be fetched from caches and second the list of all derivations that will be built from source.

TODO add example output

NOTE

While this is very usefull to debug caching issues. I haven't found any way to make this command ignore derivations that are already in the local store (I haven't managed to specify an alternative store). This is painfull when you finally manage to build your whole project and want to verify how much will your colleagues have to build vs. fetch from caches. My only solution was to start a Docker image with the project mounted and then running the above command such that I knew the machine's store was completely empty.

docker run -it --rm -v $(readlink -f .):/monorepo -v $(readlink -f ~/.ssh):/root/.ssh fghibellini/nix nix-build --dry-run /monorepo/nix/monorepo.nix

Nix cache debugging

You can test that the cache is reachable by querying the /nix-cache-info endpoint. This should return something along the lines of:

StoreDir: /nix/store
WantMassQuery: 1
Priority: 30

The info about a derivation is exposed at "/<hash of the derivation>.narinfo". So if you're trying to build /nix/store/d4vqy3sh0ngm9hsp7wwils7j6bms52xc-foundation-0.0.23 and your cache is at http://10.0.0.3:5000 Nix will first query http://10.0.0.3:5000/d4vqy3sh0ngm9hsp7wwils7j6bms52xc.narinfo. Assuming the cache contains the package the output will look something like:

StorePath: /nix/store/d4vqy3sh0ngm9hsp7wwils7j6bms52xc-foundation-0.0.23
URL: nar/d4vqy3sh0ngm9hsp7wwils7j6bms52xc.nar
Compression: none
NarHash: sha256:1nbv1cci1vrh1h2wxxbd0wdmh9jr47kkhz8207jjg9b9fw60vwsv
NarSize: 36341824
References: 0r9m0qi58jj14lv03ny7s4h76yg1rhfh-basement-0.0.10 681354n3k44r8z90m35hm8945vsp95h1-glibc-2.27 79gnjysbw3zkjqawx2ajs29w92q1v44a-ncurses-6.1-20190112 9b6qi566l7icp4z5l0fn86zfcbw8ws3f-gmp-6.1.2 d4vqy3sh0ngm9hsp7wwils7j6bms52xc-foundation-0.0.23 hrm96mjpxqdwlfzmilqi3029322cg1s9-foundation-0.0.23-doc icjdj6azjh3yblaghimlcznikjp0vraw-ghc-8.4.4
Deriver: fcv7anjpgh9hnznf3nir0nh1z5bjzp4m-foundation-0.0.23.drv
Sig: appuipieds-1:LFSKXu7VGR/4tIyNMVdQXLyilp0WMW6cylJJhNIdMdAGB+ZP975v9IWG9BXfG7PhHMuKtT4jmBj/tunyZ7E4Dg==

If the cache doesn't have the desired derivation it will simply return a 404.

In the next chapter we will write a script to mitigate first-time executions of Nix.