nixos-servers/docs/plans/prometheus-scrape-target-labels.md

# Prometheus Scrape Target Labels

## Goal

Add support for custom per-host labels on Prometheus scrape targets, enabling alert rules to reference host metadata (priority, role) instead of hardcoding instance names.

**Related:** This plan shares the `homelab.host` module with `docs/plans/nats-deploy-service.md`, which uses the same metadata for deployment tier assignment.

## Motivation

Some hosts have workloads that make generic alert thresholds inappropriate. For example, `nix-cache01` regularly hits high CPU during builds, requiring a longer `for` duration on `high_cpu_load`. Currently this is handled by excluding specific instance names in PromQL expressions, which is brittle and doesn't scale.

With per-host labels, alert rules can use semantic filters like `{priority!="low"}` instead of `{instance!="nix-cache01.home.2rjus.net:9100"}`.

## Proposed Labels

### `priority`

Indicates alerting importance. Hosts with `priority = "low"` can have relaxed thresholds or longer durations in alert rules.

Values: `"high"` (default), `"low"`

### `role`

Describes the function of the host. Useful for grouping in dashboards and targeting role-specific alert rules.

Values: free-form string, e.g. `"dns"`, `"build-host"`, `"database"`, `"monitoring"`

**Note on multiple roles:** Prometheus labels are strictly string values, not lists. For hosts that serve multiple roles there are a few options:

- **Separate boolean labels:** `role_build_host = "true"`, `role_cache_server = "true"` -- flexible but verbose, and requires updating the module when new roles are added.
- **Delimited string:** `role = "build-host,cache-server"` -- works with regex matchers (`{role=~".*build-host.*"}`), but regex matching is less clean and more error-prone.
- **Pick a primary role:** `role = "build-host"` -- simplest, and probably sufficient since most hosts have one primary role.

Recommendation: start with a single primary role string. If multi-role matching becomes a real need, switch to separate boolean labels.

### `dns_role`

For DNS servers specifically, distinguish between primary and secondary resolvers. The secondary resolver (ns2) receives very little traffic and has a cold cache, making generic cache hit ratio alerts inappropriate.

Values: `"primary"`, `"secondary"`

Example use case: The `unbound_low_cache_hit_ratio` alert fires on ns2 because its cache hit ratio (~62%) is lower than ns1 (~90%). This is expected behavior since ns2 gets ~100x less traffic. With a `dns_role` label, the alert can either exclude secondaries or use different thresholds:

```promql
# Only alert on primary DNS
unbound_cache_hit_ratio < 0.7 and on(instance) unbound_up{dns_role="primary"}

# Or use different thresholds
(unbound_cache_hit_ratio < 0.7 and on(instance) unbound_up{dns_role="primary"})
or
(unbound_cache_hit_ratio < 0.5 and on(instance) unbound_up{dns_role="secondary"})
```

## Implementation

This implementation uses a shared `homelab.host` module that provides host metadata for multiple consumers (Prometheus labels, deployment tiers, etc.). See also `docs/plans/nats-deploy-service.md` which uses the same module for deployment tier assignment.

### 1. Create `homelab.host` module

**Status:** Step 1 (Create `homelab.host` module) is complete. The module is in
`modules/homelab/host.nix` with tier, priority, role, and labels options.

Create `modules/homelab/host.nix` with shared host metadata options:

```nix
{ lib, ... }:
{
  options.homelab.host = {
    tier = lib.mkOption {
      type = lib.types.enum [ "test" "prod" ];
      default = "prod";
      description = "Deployment tier - controls which credentials can deploy to this host";
    };

    priority = lib.mkOption {
      type = lib.types.enum [ "high" "low" ];
      default = "high";
      description = "Alerting priority - low priority hosts have relaxed thresholds";
    };

    role = lib.mkOption {
      type = lib.types.nullOr lib.types.str;
      default = null;
      description = "Primary role of this host (dns, database, monitoring, etc.)";
    };

    labels = lib.mkOption {
      type = lib.types.attrsOf lib.types.str;
      default = { };
      description = "Additional free-form labels (e.g., dns_role = 'primary')";
    };
  };
}
```

Import this module in `modules/homelab/default.nix`.

### 2. Update `lib/monitoring.nix`

- `extractHostMonitoring` should also extract `homelab.host` values (priority, role, labels).
- Build the combined label set from `homelab.host`:

```nix
# Combine structured options + free-form labels
effectiveLabels =
  (lib.optionalAttrs (host.priority != "high") { priority = host.priority; })
  // (lib.optionalAttrs (host.role != null) { role = host.role; })
  // host.labels;
```

- `generateNodeExporterTargets` returns structured `static_configs` entries, grouping targets by their label sets:

```nix
# Before (flat list):
["ns1.home.2rjus.net:9100", "ns2.home.2rjus.net:9100", ...]

# After (grouped by labels):
[
  { targets = ["ns1.home.2rjus.net:9100", "ns2.home.2rjus.net:9100", ...]; }
  { targets = ["nix-cache01.home.2rjus.net:9100"]; labels = { priority = "low"; role = "build-host"; }; }
]
```

This requires grouping hosts by their label attrset and producing one `static_configs` entry per unique label combination. Hosts with default values (priority=high, no role, no labels) get grouped together with no extra labels (preserving current behavior).

### 3. Update `services/monitoring/prometheus.nix`

Change the node-exporter scrape config to use the new structured output:

```nix
# Before:
static_configs = [{ targets = nodeExporterTargets; }];

# After:
static_configs = nodeExporterTargets;
```

### 4. Set metadata on hosts

Example in `hosts/nix-cache01/configuration.nix`:

```nix
homelab.host = {
  tier = "test";       # can be deployed by MCP (used by homelab-deploy)
  priority = "low";    # relaxed alerting thresholds
  role = "build-host";
};
```

Example in `hosts/ns1/configuration.nix`:

```nix
homelab.host = {
  tier = "prod";
  priority = "high";
  role = "dns";
  labels.dns_role = "primary";
};
```

### 5. Update alert rules

After implementing labels, review and update `services/monitoring/rules.yml`:

- Replace instance-name exclusions with label-based filters (e.g. `{priority!="low"}` instead of `{instance!="nix-cache01.home.2rjus.net:9100"}`).
- Consider whether any other rules should differentiate by priority or role.

Specifically, the `high_cpu_load` rule currently has a nix-cache01 exclusion that should be replaced with a `priority`-based filter.

### 6. Consider labels for `generateScrapeConfigs` (service targets)

The same label propagation could be applied to service-level scrape targets. This is optional and can be deferred -- service targets are more specialized and less likely to need generic label-based filtering.