# Magic Nix Cache

Save 30-50%+ of CI time without any effort or cost.
Use Magic Nix Cache, a totally free and zero-configuration binary cache for Nix on GitHub Actions.

Magix Nix Cache uses the GitHub Actions [built-in cache][ghacache] to share builds between Workflow runs, and has many advantages over alternatives.

1. Totally free: backed by GitHub Actions' cache, there is no additional service to pay for.
1. Zero configuration: add our action to your workflow. That's it.
1. No secrets: Forks and pull requests benefit from the cache, too.
1. Secure: Magic Nix Cache follows the [same semantics as the GitHub Actions cache][semantics], and malicious pull requests cannot pollute your project.
1. Private: The cache is stored in the GitHub Actions cache, not with an additional third party.

> **Note:** the Magic Nix Cache doesn't offer a publically available cache.
> This means the cache is only usable in CI.
> Zero to Nix has an article on binary caching if you want to [share Nix builds][z2ncache] with users outside of CI.

## Development

This project depends on the GitHub Actions Cache API.
For local development, see `gha-cache/README.md` for more details on how to obtain the required tokens.

```
cargo run -- -c creds.json --upstream https://cache.nixos.org
cargo build --release --target x86_64-unknown-linux-musl
cargo build --release --target aarch64-unknown-linux-musl
nix copy --to 'http://127.0.0.1:3000' $(which bash)
nix-store --store $PWD/test-root --extra-substituters 'http://localhost:3000' --option require-sigs false -r $(which bash)
```

## Telemetry

The goal of Magic Nix Cache is to help teams save time in CI.
The cache daemon collects a little bit of telemetry information to help us make that true.

Here is a table of the [telemetry data we collect][telemetry]:

| Field                            | Use                                                                                                              |
| -------------------------------- | ---------------------------------------------------------------------------------------------------------------- |
| `distinct_id`                    | An opaque string that represents your project, anonymized by sha256 hashing repository and organization details. |
| `version`                        | The version of Magic Nix Cache.                                                                                  |
| `is_ci`                          | Whether the Magic Nix Cache is being used in CI (i.e.: GitHub Actions).                                          |
| `elapsed_seconds`                | How long the cache daemon was running.                                                                           |
| `narinfos_served`                | Number of narinfos served from the cache daemon.                                                                 |
| `narinfos_sent_upstream`         | Number of narinfo requests forwarded to the upstream cache.                                                      |
| `narinfos_negative_cache_hits`   | Effectiveness of an internal data structure which minimizes cache requests.                                      |
| `narinfos_negative_cache_misses` | Effectiveness of an internal data structure which minimizes cache requests.                                      |
| `narinfos_uploaded`              | Number of new narinfo files cached during this run.                                                              |
| `nars_served`                    | Number of nars served from the cache daemon.                                                                     |
| `nars_sent_upstream`             | Number of nar requests forwarded to the upstream cache.                                                          |
| `nars_uploaded`                  | Number of nars uploaded during this run.                                                                         |
| `num_original_paths`             | Number of store paths that existed on startup.                                                                   |
| `num_final_paths`                | Number of store paths that existed on shutdown.                                                                  |
| `num_new_paths`                  | The diference between `num_original_paths` and `num_final_paths`.                                                |

To disable diagnostic reporting, set the diagnostics URL to an empty string by passing `--diagnostic-endpoint=""`.

You can read the full privacy policy for [Determinate Systems][detsys], the creators of this tool and the Magic Nix Cache, [here][privacy].

[ghacache]: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows
[privacy]: https://determinate.systems/privacy
[telemetry]: https://github.com/DeterminateSystems/magic-nix-cache/blob/main/src/telemetry.rs
[semantics]: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#restrictions-for-accessing-a-cache
[z2ncache]: https://zero-to-nix.com/concepts/caching#binary-caches