Upload build results to S3#

This guide shows how to use the Nix post-build-hook configuration option to automatically upload build results to an S3-compatible binary cache.

Implementation caveats#

This is a simple and working example, but it is not suitable for all use cases.

The post-build hook program runs after each executed build, and blocks the build loop. The build loop exits if the hook program fails.

Concretely, this implementation will make Nix slow or unusable when the network connection is slow or unreliable. A more advanced implementation might pass the store paths to a user-supplied daemon or queue for processing the store paths outside of the build loop.


This tutorial assumes you have configured an S3-compatible binary cache, and that the root user’s default AWS profile can upload to the bucket.

Set up a signing key#

Use nix-store --generate-binary-cache-key to create a pair of cryptographic keys. You will sign paths with the private key, and distribute the public key for verifying the authenticity of the paths.

$ nix-store --generate-binary-cache-key example-nix-cache-1 /etc/nix/key.private /etc/nix/key.public
$ cat /etc/nix/key.public

Then update nix.conf on any machine that will access the cache. Add the cache URL to substituters and the public key to trusted-public-keys:

substituters = https://cache.nixos.org/ s3://example-nix-cache
trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= example-nix-cache-1:1/cKDz3QCCOmwcztD2eV6Coggp6rqc9DGjWv7C0G+rM=

Machines that build for the cache must sign derivations using the private key. The path to the file containing the private key you just generated must be added to the secret-key-files field in the nix.conf of those machines:

secret-key-files = /etc/nix/key.private

You will restart the Nix daemon in a later step.

Implementing the build hook#

Write the following script to /etc/nix/upload-to-cache.sh:

set -eu
set -f # disable globbing
export IFS=' '
echo "Uploading paths" $OUT_PATHS
exec nix copy --to "s3://example-nix-cache" $OUT_PATHS


The $OUT_PATHS variable is a space-separated list of Nix store paths. In this case, we expect and want the shell to perform word splitting to make each output path its own argument to nix store sign. Nix guarantees the paths will not contain any spaces, however a store path might contain glob characters. The set -f disables globbing in the shell. Then make sure the hook program is executable by the root user:

# chmod +x /etc/nix/upload-to-cache.sh

Updating nix configuration#

Edit /etc/nix/nix.conf to run our hook, by adding the following configuration snippet at the end:

post-build-hook = /etc/nix/upload-to-cache.sh

Then, restart the nix-daemon.


Build any derivation, for example:

$ nix-build -E '(import <nixpkgs> {}).writeText "example" (builtins.toString builtins.currentTime)'
this derivation will be built:
building '/nix/store/s4pnfbkalzy5qz57qs6yybna8wylkig6-example.drv'...
running post-build-hook '/home/grahamc/projects/github.com/NixOS/nix/post-hook.sh'...
post-build-hook: Signing paths /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
post-build-hook: Uploading paths /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example

Then delete the path from the store, and try substituting it from the binary cache:

$ rm ./result
$ nix-store --delete /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example

Now, copy the path back from the cache:

$ nix-store --realise /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
copying path '/nix/store/m8bmqwrch6l3h8s0k3d673xpmipcdpsa-example from 's3://example-nix-cache'...
warning: you did not specify '--add-root'; the result might be removed by the garbage collector


We now have a Nix installation configured to automatically sign and upload every local build to a remote binary cache.

Before deploying this to production, be sure to consider the implementation caveats.