Frequently Asked Questions

What Nix versions are supported?

Cachix supports Nix 2.0.2 or higher. Nix 2.3.5 or higher is recommended.

Does pushing a store path override an existing entry?

No. The existing entry first needs to be deleted.

Are entries immediately available after they are pushed?

Yes. See the next questions what might happen to seem otherwise.

Why is Nix not picking up on any of the pre-built artifacts?

For example, given that /nix/store/spznih45c56kfwygx8qyq1skd1rs4zv1-myproject-1.0.0 is missing, check if store path exists in the binary cache:

$ curl https://mycache.cachix.org/spznih45c56kfwygx8qyq1skd1rs4zv1.narinfo

If the entry exists:

  1. It is possible that you need to restart nix-daemon to pick up nix.conf changes. Run sudo pkill nix-daemon.

  2. If you ran nix-build before binary cache has been populated, Nix will maintain a negative cache for the entry.

    See a workaround to remove negative caching.

  3. Substitution could be disabled for that derivation via allowSubstitutes = false; attribute.

  4. On NixOS, it’s a known limitation that you first need to add cachix configuration and run nixos-rebuild switch and only after the next run of nixos-rebuild switch binary caches would be used.

If the entry does not exists:

  1. Check if the entry that you expected has a different hash.

    You can compare the difference between two derivations via nix-shell -p nix-diff --run "nix-diff drv1 drv".

    In order to have both derivations available locally you can run nix-instantiate default.nix | cachix push mycache to push the dervation into Cachix and then run nix-store -r /nix/store/hash.drv to retrieve it on another machine.

    Common reason for derivation hash differences are described in language anti-patterns.

  2. Maybe cachix push got interrupted and the whole dependency tree is not available from Cachix. In that case push again retry to make sure everything is uploaded.

  3. If the same store hash is available in https://cache.nixos.org it will count as existing upstream and won’t be present in cachix

Is there a way to cache nix-shell?

Many of CI integrations will push everything that was built during the CI run. Otherwise you can do the following.

Make sure shell dependencies are built:

$ nix-shell --run "echo OK"

Push to cachix:

$ nix-store --query --references $(nix-instantiate shell.nix) | xargs nix-store --realise | xargs nix-store --query --requisites | cachix push mycache

How to disable binary caches when working offline?

Pass --option substitute false to Nix commands.

What happens when I run cachix use (both immediately and any stateful effects for the future)?

It will modify nix.conf and append substituters and trusted-public-keys. If you’re using NixOS, it will write NixOS configuration. If you’re a trusted-user it will append to ~/.config/nix/nix.conf.

Otherwise it will either fail in case configuration cannot be written or it will append to /etc/nix/nix.conf.