nixos-servers/docs/plans/nats-deploy-service.md

# NATS-Based Deployment Service

## Overview

Create a message-based deployment system that allows triggering NixOS configuration updates on-demand, rather than waiting for the daily auto-upgrade timer. This enables faster iteration when testing changes and immediate fleet-wide deployments.

## Goals

1. **On-demand deployment** - Trigger config updates immediately via NATS message
2. **Targeted deployment** - Deploy to specific hosts or all hosts
3. **Branch/revision support** - Test feature branches before merging to master
4. **MCP integration** - Allow Claude Code to trigger deployments during development

## Current State

- **Auto-upgrade**: All hosts run `nixos-upgrade.service` daily, pulling from master
- **Manual testing**: `nixos-rebuild-test <action> <branch>` helper exists on all hosts
- **NATS**: Running on nats1 with JetStream enabled, using NKey authentication
- **Accounts**: ADMIN (system) and HOMELAB (user workloads with JetStream)

## Architecture

```
┌─────────────┐                        ┌─────────────┐
│  MCP Tool   │  deploy.test.>         │  Admin CLI  │  deploy.test.> + deploy.prod.>
│  (claude)   │────────────┐     ┌─────│  (torjus)   │
└─────────────┘            │     │     └─────────────┘
                           ▼     ▼
                      ┌──────────────┐
                      │    nats1     │
                      │  (authz)     │
                      └──────┬───────┘
                             │
           ┌─────────────────┼─────────────────┐
           │                 │                 │
           ▼                 ▼                 ▼
     ┌──────────┐      ┌──────────┐      ┌──────────┐
     │ template1│      │   ns1    │      │   ha1    │
     │ tier=test│      │ tier=prod│      │ tier=prod│
     └──────────┘      └──────────┘      └──────────┘
```

## Repository Structure

The project lives in a **separate repository** (e.g., `homelab-deploy`) containing:

```
homelab-deploy/
├── flake.nix           # Nix flake with Go package + NixOS module
├── go.mod
├── go.sum
├── cmd/
│   └── homelab-deploy/
│       └── main.go     # CLI entrypoint with subcommands
├── internal/
│   ├── listener/       # Listener mode logic
│   ├── mcp/            # MCP server mode logic
│   └── deploy/         # Shared deployment logic
└── nixos/
    └── module.nix      # NixOS module for listener service
```

This repo imports the flake as an input and uses the NixOS module.

## Single Binary with Subcommands

The `homelab-deploy` binary supports multiple modes:

```bash
# Run as listener on a host (systemd service)
homelab-deploy listener --hostname ns1 --nats-url nats://nats1:4222

# Run as MCP server (for Claude Code)
homelab-deploy mcp --nats-url nats://nats1:4222

# CLI commands for manual use
homelab-deploy deploy ns1 --branch feature-x --action switch
homelab-deploy deploy --all --action boot
homelab-deploy status
```

## Components

### Listener Mode

A systemd service on each host that:
- Subscribes to `deploy.<tier>.<hostname>` and `deploy.<tier>.all` subjects
- Validates incoming messages (revision, action)
- Executes `nixos-rebuild` with specified parameters
- Reports status back via NATS

**NixOS module configuration:**
```nix
services.homelab-deploy.listener = {
  enable = true;
  timeout = 600;  # seconds, default 10 minutes
};
```

The listener reads its tier from `config.homelab.host.tier` (see Host Metadata below) and subscribes to tier-specific subjects (e.g., `deploy.prod.ns1` and `deploy.prod.all`).

**Request message format:**
```json
{
  "action": "switch" | "boot" | "test" | "dry-activate",
  "revision": "master" | "feature-branch" | "abc123...",
  "reply_to": "deploy.responses.<request-id>"
}
```

**Response message format:**
```json
{
  "status": "accepted" | "rejected" | "started" | "completed" | "failed",
  "error": "invalid_revision" | "already_running" | "build_failed" | null,
  "message": "human-readable details"
}
```

**Request/Reply flow:**
1. MCP/CLI sends deploy request with unique `reply_to` subject
2. Listener validates request (e.g., `git ls-remote` to check revision exists)
3. Listener sends immediate response:
   - `{"status": "rejected", "error": "invalid_revision", "message": "branch 'foo' not found"}`, or
   - `{"status": "started", "message": "starting nixos-rebuild switch"}`
4. If started, listener runs nixos-rebuild
5. Listener sends final response:
   - `{"status": "completed", "message": "successfully switched to generation 42"}`, or
   - `{"status": "failed", "error": "build_failed", "message": "nixos-rebuild exited with code 1"}`

This provides immediate feedback on validation errors (bad revision, already running) without waiting for the build to fail.

### MCP Mode

Runs as an MCP server providing tools for Claude Code:
- `deploy` - Deploy to specific host(s) with optional revision
- `deploy_status` - Check deployment status/history
- `list_hosts` - List available deployment targets

The MCP server runs with limited credentials (test-tier only), so Claude can deploy to test hosts but not production.

### Tiered Permissions

Authorization is enforced at the NATS layer using subject-based permissions. Different deployer credentials have different publish rights:

**NATS user configuration (on nats1):**
```nix
accounts = {
  HOMELAB = {
    users = [
      # MCP/Claude - test tier only
      {
        nkey = "UABC...";  # mcp-deployer
        permissions = {
          publish = [ "deploy.test.>" ];
          subscribe = [ "deploy.responses.>" ];
        };
      }
      # Admin - full access to all tiers
      {
        nkey = "UXYZ...";  # admin-deployer
        permissions = {
          publish = [ "deploy.test.>" "deploy.prod.>" ];
          subscribe = [ "deploy.responses.>" ];
        };
      }
      # Host listeners - subscribe to their tier, publish responses
      {
        nkey = "UDEF...";  # host-listener (one per host)
        permissions = {
          subscribe = [ "deploy.*.>" ];
          publish = [ "deploy.responses.>" ];
        };
      }
    ];
  };
};
```

**Host tier assignments** (via `homelab.host.tier`):
| Tier | Hosts |
|------|-------|
| test | template1, nix-cache01, future test hosts |
| prod | ns1, ns2, ha1, monitoring01, http-proxy, etc. |

**How it works:**
1. MCP tries to deploy to ns1 → publishes to `deploy.prod.ns1`
2. NATS server rejects publish (mcp-deployer lacks `deploy.prod.>` permission)
3. MCP tries to deploy to template1 → publishes to `deploy.test.template1`
4. NATS allows it, listener receives and executes

All NKeys stored in Vault - MCP gets limited credentials, admin CLI gets full-access credentials.

### Host Metadata

Rather than defining `tier` in the listener config, use a central `homelab.host` module that provides host metadata for multiple consumers. This aligns with the approach proposed in `docs/plans/prometheus-scrape-target-labels.md`.

**Module definition (in `modules/homelab/host.nix`):**
```nix
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";
  };
};
```

**Consumers:**
- `homelab-deploy` listener reads `config.homelab.host.tier` for subject subscription
- Prometheus scrape config reads `priority`, `role`, `labels` for target labels
- Future services can consume the same metadata

**Example host config:**
```nix
# hosts/nix-cache01/configuration.nix
homelab.host = {
  tier = "test";      # can be deployed by MCP
  priority = "low";   # relaxed alerting thresholds
  role = "build-host";
};

# hosts/ns1/configuration.nix
homelab.host = {
  tier = "prod";      # requires admin credentials
  priority = "high";
  role = "dns";
  labels.dns_role = "primary";
};
```

## Implementation Steps

### Phase 1: Core Binary + Listener

1. **Create homelab-deploy repository**
   - Initialize Go module
   - Set up flake.nix with Go package build

2. **Implement listener mode**
   - NATS subscription logic
   - nixos-rebuild execution
   - Status reporting via NATS reply

3. **Create NixOS module**
   - Systemd service definition
   - Configuration options (hostname, NATS URL, NKey path)
   - Vault secret integration for NKeys

4. **Create `homelab.host` module** (in nixos-servers)
   - Define `tier`, `priority`, `role`, `labels` options
   - This module is shared with Prometheus label work (see `docs/plans/prometheus-scrape-target-labels.md`)

5. **Integrate with nixos-servers**
   - Add flake input for homelab-deploy
   - Import listener module in `system/`
   - Set `homelab.host.tier` per host (test vs prod)

6. **Configure NATS tiered permissions**
   - Add deployer users to nats1 config (mcp-deployer, admin-deployer)
   - Set up subject ACLs per user (test-only vs full access)
   - Add deployer NKeys to Vault
   - Create Terraform resources for NKey secrets

### Phase 2: MCP + CLI

7. **Implement MCP mode**
   - MCP server with deploy/status tools
   - Request/reply pattern for deployment feedback

8. **Implement CLI commands**
   - `deploy` command for manual deployments
   - `status` command to check deployment state

9. **Configure Claude Code**
   - Add MCP server to configuration
   - Document usage

### Phase 3: Enhancements

10. Add deployment locking (prevent concurrent deploys)
11. Prometheus metrics for deployment status

## Security Considerations

- **Privilege escalation**: Listener runs as root to execute nixos-rebuild
- **Input validation**: Strictly validate revision format (branch name or commit hash)
- **Rate limiting**: Prevent rapid-fire deployments
- **Audit logging**: Log all deployment requests with source identity
- **Network isolation**: NATS only accessible from internal network

## Decisions

All open questions have been resolved. See Notes section for decision rationale.

## Notes

- The existing `nixos-rebuild-test` helper provides a good reference for the rebuild logic
- Uses NATS request/reply pattern for immediate validation feedback and completion status
- Consider using NATS headers for metadata (request ID, timestamp)
- **Timeout decision**: Metrics show no-change upgrades complete in 5-55 seconds. A 10-minute default provides ample headroom for actual updates with package downloads. Per-host override available for hosts with known longer build times.
- **Rollback**: Not needed as a separate feature - deploy an older commit hash to effectively rollback.
- **Offline hosts**: No message persistence - if host is offline, deploy fails. Daily auto-upgrade is the safety net. Avoids complexity of JetStream deduplication (host coming online and applying 10 queued updates instead of just the latest).
- **Deploy history**: Use existing Loki - listener logs deployments to journald, queryable via Loki. No need for separate JetStream persistence.
- **Naming**: `homelab-deploy` - ties it to the infrastructure rather than implementation details.