> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sdk.fdo.alexvwan.me/llms.txt
> Use this file to discover all available pages before exploring further.

# Runtime Model

> Understand backend runtime, iframe runtime, and the host boundary.

# Runtime Model

The SDK has two runtimes and one host boundary.

## Backend Runtime

This is where your plugin class runs.

Use it for:

* `init()`
* registering handlers
* storage access
* logging
* diagnostics
* backend orchestration

Do not assume browser-only globals exist here.

## Iframe UI Runtime

This is where your UI executes after `render()` output is consumed by FDO.

Use it for:

* DOM work
* event listeners
* UI feedback
* host-injected helpers and libraries
* calling your backend through `window.createBackendReq(...)`

## Host Boundary

The host owns:

* iframe sandboxing
* capability grants
* privileged action execution
* network and filesystem boundaries
* session/request infrastructure for connector patterns

Your plugin should treat host boundaries as explicit contracts, not hidden implementation details.

## What This Means In Practice

* backend code should not depend on iframe-only libraries
* iframe UI should not assume direct privileged host access
* privileged work should go through documented host-mediated contracts
* provider-specific semantics should stay in plugin code unless the host contract explicitly owns them

## Related Reading

* `docs/ARCHITECTURE.md`
* `docs/RENDER_RUNTIME_CONTRACT.md`
* `docs/SAFE_PLUGIN_AUTHORING.md`