# Distribution Architecture Example
Proof-of-concept: thin container-bound add-on delegates to a centralized standalone script via iframe + postMessage bridge.
## Architecture
```
Container-Bound Add-on (thin shell) Standalone Script (all logic + UI)
┌──────────────────────────┐ ┌────────────────────────────────┐
│ AddOnCode.gs │ │ Code.gs │
│ onOpen() │ │ doGet(e) │
│ showSidebar() │ │ processSheet(sheetId) │
│ getActiveContext() │ │ getSheetSummary(sheetId) │
│ │ │ ping() │
│ Bridge.html │ │ │
│ <iframe src="/exec">──────────────────→ App.html │
│ postMessage bridge │ │ google.script.run → Code.gs │
│ │ │ postMessage → parent bridge │
└──────────────────────────┘ └────────────────────────────────┘
```
## Files
| File | Deploys To | Purpose |
|------|-----------|---------|
| `Code.gs` | Standalone script | doGet + all business logic |
| `App.html` | Standalone script | Full UI served via web app |
| `AddOnCode.gs` | Container-bound (manual) | Thin shell: menu, sidebar, bridge |
| `Bridge.html` | Container-bound (manual) | iframe host + postMessage bridge |
## Deployment
### 1. Deploy Standalone (via MCP GAS)
```bash
# Create standalone project
gas_project_create --title="Central App"
# Write server code
gas_write --path="[scriptId]/Code.gs" --content="<Code.gs content>"
# Write HTML (raw_write for HTML files)
gas_raw_write --path="[scriptId]/App.html" --content="<App.html content>"
# Deploy as web app
gas_deploy --scriptId="[scriptId]" --operation="promote" --environment="dev"
```
### 2. Set Up Add-on (Manual — Container-Bound)
1. Open a Google Sheet
2. Extensions > Apps Script
3. Copy `AddOnCode.gs` content into `Code.gs`
4. Create `Bridge.html` with the Bridge.html content
5. Update `STANDALONE_URL` in `AddOnCode.gs` with the standalone's `/exec` URL
6. Both scripts: Project Settings > Change GCP Project → same project number
7. Reload the Sheet — "Central App" menu appears
## Key Patterns
### postMessage Bridge
The Bridge.html in the add-on listens for messages from the iframe and routes them to the container-bound server for active context operations.
### Shared GCP Project
Both scripts must be linked to the same GCP project so that authorizing the add-on also grants scopes for the standalone. Without this, the iframe's auth prompt would be invisible and fail silently.
### setXFrameOptionsMode(ALLOWALL)
The standalone's doGet() must call `.setXFrameOptionsMode(HtmlService.XFrameOptionsMode.ALLOWALL)` to allow the sidebar iframe to embed it.
## Customization
- Replace `processSheet()` / `getSheetSummary()` with your business logic
- Add more bridge operations in Bridge.html's message handler
- Extend App.html with your full UI framework
