wild-pc/docs/design.md

Castle Design

Castle is a personal software platform. It manages independent services, tools, and frontends on a Linux machine using standard Unix primitives — systemd for process supervision, Caddy for HTTP routing, the filesystem for storage, and env vars for configuration. The castle CLI and API provide a registry and coordination layer on top.

The long-term goal: multiple Castle nodes (machines) that discover each other and coordinate, forming a personal infrastructure mesh. Each node is self-sufficient. The mesh is optional.

Principles

1. Unix-native. Use the OS. systemd, journald, filesystem, signals, env vars, DNS. Don't reimplement what Linux already provides.

2. Independence. Components never depend on Castle. They accept standard configuration (ports, data dirs, URLs) via env vars. A Castle service is just a well-behaved Unix daemon that happens to be registered in a manifest.

3. Declare capabilities, derive roles. Components say what they do (expose HTTP, run on a schedule, install to PATH). Castle infers what they are (service, job, tool, frontend). No role labels.

4. Language-agnostic above the build line. Below the build line, every language is different (uv, pnpm, cargo, go). Above it, everything is just processes, ports, files, and signals. Castle operates above the line.

5. Separate source from runtime. The repo is for development. The runtime lives in standard Unix locations (~/.castle/, /data/castle/, systemd units). Nothing running should point into the source tree.

6. AI-manageable. The CLI and API exist so that AI assistants can discover, create, and manage components programmatically. Humans use the dashboard. Agents use the CLI and API.

7. Simple until proven otherwise. Filesystem over databases. HTTP over custom protocols. Shell commands over plugin systems. Add complexity only when the simple thing actually fails.

Architecture Layers

┌─────────────────────────────────────────────┐
│  Coordination                               │
│  Node discovery, global registry, messaging │
├─────────────────────────────────────────────┤
│  Registry                                   │
│  Component spec, node config, CLI, API      │
├─────────────────────────────────────────────┤
│  Runtime                                    │
│  systemd, Caddy, filesystem, journald       │
├─────────────────────────────────────────────┤
│  Build                                      │
│  uv, pnpm, cargo, go build, etc.           │
└─────────────────────────────────────────────┘

The critical boundary is between Build and Runtime. Below it, each language has its own toolchain. Above it, everything is uniform — a process that reads env vars, listens on a port, logs to stdout, and responds to SIGTERM.

Build Layer

Transforms source code into runnable artifacts. Castle does not abstract over language toolchains — it just records the build commands and their outputs.

Language	Toolchain	Artifact
Python	uv	Entry point in venv
Node/TS	pnpm	Static bundle (frontends) or node script
Rust	cargo	Binary
Go	go build	Binary

Castle's build spec is intentionally minimal: a list of shell commands and a list of output paths. This works for any language without Castle needing to understand the toolchain.

For interpreted languages (Python, Node), Castle also needs to know the runtime wrapper — how to invoke the artifact. This is what the run spec's runner variants handle:

• python — Python (sync via uv, deploy resolves installed binary)
• node — Node.js (sync via pnpm/npm)
• command — Direct execution (compiled binaries, shell scripts)
• container — Docker/Podman
• remote — External service (no local process)

Compiled languages (Rust, Go) use command — once built, they're just binaries. No Castle-specific runner needed.

Runtime Layer

Manages running processes using standard Linux infrastructure.

systemd handles process supervision:

• Start/stop/restart services
• Restart-on-failure policies (OTP's "let it crash")
• Dependency ordering via After= / Wants=
• Scheduled execution via .timer units
• Logging via journald (stdout/stderr capture)

Caddy handles HTTP routing:

• Reverse proxy on port 9000
• Path-based routing to services (/api → port 9020)
• Static file serving for frontends
• TLS termination

Filesystem handles storage:

• Service data: /data/castle/<name>/
• Secrets: ~/.castle/secrets/
• Generated config: ~/.castle/generated/

Castle generates systemd unit files and Caddyfile entries from the registry. It doesn't run a daemon itself — it configures OS-level infrastructure and gets out of the way.

Critically, the runtime layer references only standard paths — never the source tree. Systemd units point to installed binaries (on PATH or in ~/.castle/bin/), not to repo subdirectories. Caddy serves from ~/.castle/static/, not from build output directories in the repo.

Registry Layer

The registry is the central concept in Castle. It tracks what components exist, what they can do, and how they're configured. But it's not a single thing — it's three distinct concepts:

1. Component spec — what a component is. Description, capabilities, build instructions, default configuration. This is source-level information, version-controlled in the repo. It answers: "what components could exist?"

2. Node config — what's deployed on this machine, with what concrete ports, data paths, and env vars. This is per-machine. Two Castle nodes might run different subsets of components with different parameters. It answers: "what's running here, and how?"

3. Runtime state — what's actually happening. PIDs, health, uptime, logs. This is ephemeral, owned by systemd and queried on demand. It answers: "is it working?"

Source vs. runtime split

These map to two files:

castle.yaml (in the repo, version-controlled) — Component specs:

components:
  central-context:
    description: Content storage API
    source: components/central-context
    run:
      runner: python
      tool: central-context
    expose:
      http:
        internal: { port: 9001 }
        health_path: /health
    proxy:
      caddy:
        path_prefix: /central-context
    manage:
      systemd: {}

The spec says what the component is and what it needs — a port, a data directory, HTTP exposure. Convention-based env vars (<PREFIX>_PORT, <PREFIX>_DATA_DIR) are generated automatically during deploy. Only non-convention values need defaults.env.

~/.castle/registry.yaml (per-node, not in the repo) — Node config:

node:
  hostname: tower
  castle_root: /data/repos/castle
  gateway_port: 9000
deployed:
  central-context:
    runner: python
    run_cmd: [/home/user/.local/bin/central-context]
    env:
      CENTRAL_CONTEXT_DATA_DIR: /data/castle/central-context
      CENTRAL_CONTEXT_PORT: "9001"
    roles: [service]
    port: 9001
    health_path: /health
    proxy_path: /central-context
    managed: true

The node config says what's deployed here and with what concrete values. castle deploy reads the spec from the repo, generates convention-based env vars, resolves secrets, resolves binary paths, and writes the registry. Systemd units and Caddyfile are then generated from the registry — never from the spec directly.

This separation means:

• The repo is just a repo. git pull doesn't affect running services.
• Multi-node works: sync the spec + deploy on each node, no repo needed.
• The spec is portable and version-controlled. The node config is local.
• AI agents read the node registry to know what's deployed and running.

Interfaces

Three interfaces expose the registry:

• CLI (castle) — For AI agents and terminal users. Structured output via --json. Commands for listing, inspecting, creating, and managing components.
• API (castle-api) — For programmatic access over HTTP. Used by the dashboard, other nodes, and remote agents.
• Dashboard (castle-app) — For human discoverability. Visual overview of what's running, health status, logs.

Coordination Layer

Partially built. This section describes the target architecture.

Coordination handles discovery and communication — both between components on a single node and across multiple Castle nodes.

Intra-node coordination (current):

• Components find each other through the gateway (path-based routing) or direct port access via env vars.
• The registry (CLI/API) provides discoverability.
• No service mesh or message broker required for basic operation.

Inter-node coordination (future):

• Each Castle node runs the API, which exposes its component registry.
• Nodes discover each other via MQTT retained messages or mDNS/DNS-SD (Avahi) for LAN environments.
• The gateway on each node can proxy to services on other nodes, preserving path-based routing. Components don't know which node they're talking to.
• MQTT provides pub/sub messaging for events, status, and coordination across nodes.

Why MQTT over custom gossip:

• Standard protocol, every language has a client library.
• Retained messages give new nodes an immediate view of the network.
• Topic-based routing maps naturally to castle/{node}/{component}.
• Works across networks (not just LAN like mDNS).
• Mosquitto is a single binary, simple to run as a Castle component.

Why mDNS/DNS-SD as a complement:

• Zero-config LAN discovery via Avahi (already on most Linux systems).
• Each node advertises _castle._tcp — standard tooling works (avahi-browse).
• Good for bootstrapping: find the MQTT broker without hardcoding its address.

Component Contract

Every Castle component, regardless of language, must satisfy a minimal contract. This is what makes the system uniform above the build line.

Services (long-running daemons)

Requirement	Mechanism
Accept configuration	Env vars (prefixed by service name)
Declare its port	Env var, registered in expose.http.internal.port
Health endpoint	GET /health returns 200
Data storage	Read *_DATA_DIR env var, write there
Logging	stdout for output, stderr for errors
Graceful shutdown	Handle SIGTERM, exit cleanly
Secrets	Read from env vars (Castle resolves ${secret:NAME})
No Castle dependency	Must run standalone with just env vars set

Tools (CLI utilities)

Requirement	Mechanism
Input	File argument or stdin
Output	stdout (pipeable)
Errors/status	stderr
Exit codes	0 success, non-zero failure
No interactive prompts	Scriptable by default

Jobs (scheduled tasks)

Same contract as tools, plus:

Requirement	Mechanism
Idempotent	Safe to re-run or run concurrently
Short-lived	Exit when done (oneshot systemd unit)

Component Lifecycle

The path from source to managed process:

source → [build] → artifact → [install] → available → [deploy] → managed

Each step is distinct:

1. Build — Language-specific. Produces an artifact (binary, venv entry point, static bundle). Castle records the commands but doesn't execute them implicitly.

2. Install — Makes the artifact available on the system. For tools: uv tool install or compiled binary placed in ~/.castle/bin/. For services: same — the binary or entry point is on PATH or in a known location. For frontends: built assets copied to ~/.castle/static/.

3. Deploy — Materializes the runtime configuration. Reads the component spec, merges with node config, generates systemd units and Caddyfile entries that reference installed artifacts — never the source tree. Enables and starts services.

For compiled languages (Rust, Go), build produces a standalone binary and install is just placing it in ~/.castle/bin/. For interpreted languages (Python, Node), the runtime wrapper (uv, node) handles finding the installed artifact.

Runtime Filesystem Layout

What already exists and what the target looks like:

~/.castle/                      ← Castle runtime home
├── registry.yaml               ← Node config (what's deployed here)
├── generated/                  ← Generated Caddyfile
│   └── Caddyfile
├── secrets/                    ← Secret files (NAME → value)
│   └── PROTONMAIL_API_KEY
├── bin/                        ← Compiled binaries, shims
│   └── my-go-tool
└── static/                     ← Built frontend assets
    └── castle-app/
        └── dist/

/data/castle/                   ← Persistent service data
└── <name>/

~/.config/systemd/user/         ← Systemd units (standard location)
├── castle-central-context.service
├── castle-protonmail.service
├── castle-protonmail.timer
└── ...

Source (the repo) is referenced only during build and install. Everything the runtime touches lives in ~/.castle/, /data/castle/, or standard systemd paths.

OTP as Design Guide

Castle's architecture parallels Erlang/OTP, mapped onto Unix:

OTP Concept	Castle Equivalent
Application	Component (independent, self-contained)
Application resource file	Component spec in castle.yaml
Release config (sys.config)	Node config in ~/.castle/registry.yaml
Release assembly	castle deploy (spec + node config → runtime)
Supervisor	systemd (restart policies, ordering)
Process	Running service/worker/job
Application env	Env vars
Node	A machine running Castle
epmd	mDNS / MQTT discovery
Distribution	Inter-node coordination via MQTT + gateway proxying
"Let it crash"	restart: on-failure in systemd
Global registry	Merged node registries via MQTT retained messages

The mapping is conceptual, not literal. Castle doesn't implement OTP semantics — it uses OTP's thinking to guide which Unix primitives to compose and how.

Key OTP ideas that apply:

• Isolation. Components don't share state. Communication is through explicit interfaces (HTTP, MQTT, filesystem paths).
• Let it crash. Services don't need elaborate error recovery. systemd restarts them. Design for restartability, not immortality.
• Supervision hierarchy. systemd's dependency ordering provides this. Services declare what they need to start after.
• Location transparency. Components talk to paths (/api, /central-context), not to specific hosts or ports. The gateway can remap these across nodes.
• Spec vs. config. In OTP, an application defines its structure (the .app file) and a release provides the deployment config (sys.config). Castle mirrors this: the component spec defines structure, the node config provides deployment values.

Current State

What exists today:

• CLI — castle command, installed via uv tool install --editable cli/
• Three packages — castle-core (models, config, generators), castle-cli (commands), castle-api (HTTP API)
• Source/runtime split — castle.yaml (spec) → castle deploy → ~/.castle/registry.yaml (node config). Systemd units and Caddyfile generated from registry with fully resolved paths. No repo references in runtime artifacts.
• Convention-based env generation — castle deploy auto-generates <PREFIX>_DATA_DIR=/data/castle/<name> and <PREFIX>_PORT from the manifest. Only non-convention values need defaults.env.
• Gateway — Caddy on port 9000, Caddyfile generated from registry
• API — castle-api on port 9020, reads from registry (optional castle.yaml fallback for non-deployed components)
• Dashboard — castle-app React/Vite frontend, static assets served from ~/.castle/static/castle-app/
• Services — central-context (content storage), notification-bridge (desktop notification forwarder)
• Jobs — protonmail (email sync every 5 min), backup-collect (nightly), backup-data (nightly restic backup)
• Tools — ~15 CLI utilities (pdf2md, docx2md, search, gpt, etc.)
• Manifest — castle.yaml with typed Pydantic models

What doesn't exist yet:

• Multi-language support — Rust and Go components (the abstractions support them via command runner, but no examples exist yet)
• Inter-node coordination — MQTT broker, node discovery, cross-node routing
• Build automation — Castle records build specs but doesn't orchestrate builds (each project builds independently)

Technology Map

Concern	Technology	Status
Process supervision	systemd (user units)	Active
HTTP routing	Caddy (port 9000)	Active
Component specs	castle.yaml + Pydantic models	Active
Node config	~/.castle/registry.yaml	Active
CLI	castle (Python, uv)	Active
API	castle-api (FastAPI)	Active
Dashboard	castle-app (React, Vite, shadcn/ui)	Active
Python packaging	uv	Active
Node packaging	pnpm	Active
Linting	ruff (Python), ESLint (TS)	Active
Type checking	pyright (Python), tsc (TS)	Active
Testing	pytest (Python), Vitest (TS)	Active
Secrets	~/.castle/secrets/ file-based	Active
Data storage	Filesystem (/data/castle/)	Active
Messaging	MQTT (Mosquitto)	Planned
Node discovery	mDNS/Avahi + MQTT	Planned
Rust packaging	cargo	Planned
Go packaging	go build	Planned