# NixOS Prometheus Exporter

## Overview

Build a generic Prometheus exporter for NixOS-specific metrics. This exporter should be useful for any NixOS deployment, not just our homelab.

## Goal

Provide visibility into NixOS system state that standard exporters don't cover:
- Generation management (count, age, current vs booted)
- Flake input freshness
- Upgrade status

## Metrics

### Core Metrics

| Metric | Description | Source |
|--------|-------------|--------|
| `nixos_generation_count` | Number of system generations | Count entries in `/nix/var/nix/profiles/system-*` |
| `nixos_current_generation` | Active generation number | Parse `readlink /run/current-system` |
| `nixos_booted_generation` | Generation that was booted | Parse `/run/booted-system` |
| `nixos_generation_age_seconds` | Age of current generation | File mtime of current system profile |
| `nixos_config_mismatch` | 1 if booted != current, 0 otherwise | Compare symlink targets |

### Flake Metrics (optional collector)

| Metric | Description | Source |
|--------|-------------|--------|
| `nixos_flake_input_age_seconds` | Age of each flake.lock input | Parse `lastModified` from flake.lock |
| `nixos_flake_input_info` | Info gauge with rev label | Parse `rev` from flake.lock |

Labels: `input` (e.g., "nixpkgs", "home-manager")

### Future Metrics

| Metric | Description | Source |
|--------|-------------|--------|
| `nixos_upgrade_pending` | 1 if remote differs from local | Compare flake refs (expensive) |
| `nixos_store_size_bytes` | Size of /nix/store | `du` or filesystem stats |
| `nixos_store_path_count` | Number of store paths | Count entries |

## Architecture

Single binary with optional collectors enabled via config or flags.

```
nixos-exporter
├── main.go
├── collector/
│   ├── generation.go    # Core generation metrics
│   └── flake.go         # Flake input metrics
└── config/
    └── config.go
```

## Configuration

```yaml
listen_addr: ":9971"
collectors:
  generation:
    enabled: true
  flake:
    enabled: false
    lock_path: "/etc/nixos/flake.lock"  # or auto-detect from /run/current-system
```

Command-line alternative:
```bash
nixos-exporter --listen=:9971 --collector.flake --flake.lock-path=/etc/nixos/flake.lock
```

## NixOS Module

```nix
services.prometheus.exporters.nixos = {
  enable = true;
  port = 9971;
  collectors = [ "generation" "flake" ];
  flake.lockPath = "/etc/nixos/flake.lock";
};
```

The module should integrate with nixpkgs' existing `services.prometheus.exporters.*` pattern.

## Implementation

### Language

Go - mature prometheus client library, single static binary, easy cross-compilation.

### Phase 1: Core
1. Create git repository
2. Implement generation collector (count, current, booted, age, mismatch)
3. Basic HTTP server with `/metrics` endpoint
4. NixOS module

### Phase 2: Flake Collector
1. Parse flake.lock JSON format
2. Extract lastModified timestamps per input
3. Add input labels

### Phase 3: Packaging
1. Add to nixpkgs or publish as flake
2. Documentation
3. Example Grafana dashboard

## Example Output

```
# HELP nixos_generation_count Total number of system generations
# TYPE nixos_generation_count gauge
nixos_generation_count 47

# HELP nixos_current_generation Currently active generation number
# TYPE nixos_current_generation gauge
nixos_current_generation 47

# HELP nixos_booted_generation Generation that was booted
# TYPE nixos_booted_generation gauge
nixos_booted_generation 46

# HELP nixos_generation_age_seconds Age of current generation in seconds
# TYPE nixos_generation_age_seconds gauge
nixos_generation_age_seconds 3600

# HELP nixos_config_mismatch 1 if booted generation differs from current
# TYPE nixos_config_mismatch gauge
nixos_config_mismatch 1

# HELP nixos_flake_input_age_seconds Age of flake input in seconds
# TYPE nixos_flake_input_age_seconds gauge
nixos_flake_input_age_seconds{input="nixpkgs"} 259200
nixos_flake_input_age_seconds{input="home-manager"} 86400
```

## Alert Examples

```yaml
- alert: NixOSConfigStale
  expr: nixos_generation_age_seconds > 7 * 24 * 3600
  for: 1h
  labels:
    severity: warning
  annotations:
    summary: "NixOS config on {{ $labels.instance }} is over 7 days old"

- alert: NixOSRebootRequired
  expr: nixos_config_mismatch == 1
  for: 24h
  labels:
    severity: info
  annotations:
    summary: "{{ $labels.instance }} needs reboot to apply config"

- alert: NixpkgsInputStale
  expr: nixos_flake_input_age_seconds{input="nixpkgs"} > 30 * 24 * 3600
  for: 1d
  labels:
    severity: info
  annotations:
    summary: "nixpkgs input on {{ $labels.instance }} is over 30 days old"
```

## Open Questions

- [ ] How to detect flake.lock path automatically? (check /run/current-system for flake info)
- [ ] Should generation collector need root? (probably not, just reading symlinks)
- [ ] Include in nixpkgs or distribute as standalone flake?

## Notes

- Port 9971 suggested (9970 reserved for homelab-exporter)
- Keep scope focused on NixOS-specific metrics - don't duplicate node-exporter
- Consider submitting to prometheus exporter registry once stable