# CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

## Project Overview

nixos-exporter is a Prometheus exporter for NixOS-specific metrics. It exposes system state information that standard exporters don't cover: generation management, flake input freshness, and upgrade status.

**Status**: Implementation complete.

## Build Commands

Run commands through the Nix development shell using `nix develop -c`:

```bash
# Build
nix develop -c go build ./...

# Run tests
nix develop -c go test ./...

# Run single test
nix develop -c go test -run TestName ./path/to/package

# Lint
nix develop -c golangci-lint run

# Vulnerability check
nix develop -c govulncheck ./...

# Test Nix build
nix build

# Run the binary (prefer this over go build + running binary)
# To pass arguments, use -- before them: nix run .#default -- --help
nix run .#default
```

## Testing Procedures

Before committing, run the following checks:

1. `nix develop -c go test ./...` - Unit tests
2. `nix develop -c golangci-lint run` - Linting
3. `nix develop -c govulncheck ./...` - Vulnerability scanning
4. `nix build` - Verify nix build works

## Architecture

Single binary with pluggable collectors:

```
nixos-exporter/
├── main.go                 # Entry point, HTTP server on :9971
├── collector/
│   ├── generation.go       # Core metrics (count, current, booted, age, mismatch)
│   └── flake.go            # Flake input freshness metrics (optional)
└── config/
    └── config.go           # YAML + CLI flag handling
```

### Collectors

**Generation collector** (always enabled): Reads symlinks in `/nix/var/nix/profiles/` and `/run/current-system` to determine generation state.

**Flake collector** (optional): Parses `flake.lock` JSON to extract `lastModified` timestamps per input.

### Configuration

Supports YAML config file, CLI flags (`--listen`, `--collector.flake`, `--flake.lock-path`), and NixOS module integration via `services.prometheus.exporters.nixos`.

## Commit Message Format

Use conventional commit format:

```
feat: add new feature
fix: fix a bug
docs: update documentation
refactor: refactor code without changing behavior
test: add or update tests
chore: maintenance tasks
```

## Version Bumping

Follow semantic versioning:

- **Patch** (0.0.x): Bugfixes
- **Minor** (0.x.0): Non-breaking changes adding features
- **Major** (x.0.0): Breaking changes

Update the `const version` in `main.go`. The Nix build extracts the version from there automatically.

**When to bump**: If any Go code has changed, bump the version before committing. Do this automatically when asked to commit. On feature branches, only bump once per branch (check if version has already been bumped compared to master).

## Key Design Decisions

- Go chosen for mature Prometheus client library and static binary output
- Port 9971 allocated for this exporter
- Follows nixpkgs `services.prometheus.exporters.*` module pattern
- No root required - only reads symlinks and files