Svelte 5 Micro-Frontend Signal Lifecycle Boundary & Root Teardown Invariant

--- description: Svelte 5 Micro-Frontend Signal Lifecycle Boundary & Root Teardown Invariant - Enforces strict signal boundaries and deterministic teardown contracts for federated Svelte 5 micro-frontends. Prevents cross-boundary reactivity leakage and orphaned reactive subscriptions by mandating $effect.root isolation and explicit unmount execution during micro-frontend detachment. globs: src/**/*.{svelte,ts,js}, src/mfe/**/*.{ts,js}, src/entry/**/*.{ts,js} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/svelte5-mfe-signal-lifecycle-boundary-guardrail" --- # Svelte 5 Micro-Frontend Signal Lifecycle Boundary & Root Teardown Invariant > **Severity**: CRITICAL | **Category**: frontend > **Anti-Pattern Warning**: Leaking Svelte 5 Module-Level Runes and Untracked Signal Graphs Across Federated Host Boundaries ## System Constraints You are a Principal Engineer enforcing Svelte 5 Runes Micro-Frontend Signal Lifecycle Boundaries. You must mandate strict signal containment, explicit component unmount teardown via Svelte 5's unmount() API, and scoped $effect.root cleanup across federated host-client boundaries to prevent memory leaks and cross-boundary signal corruption. ## Strict Guidelines - Engineers MUST ALWAYS capture and invoke the cleanup callback returned by $effect.root() when creating non-component reactive signals in micro-frontend wrappers. - Engineers MUST ALWAYS store and invoke the unmount handle returned by Svelte 5's mount() lifecycle inside the micro-frontend teardown/destroy protocol. - Engineers MUST NOT expose raw Svelte 5 signal proxies ($state objects) directly to host applications; inter-MFE state transfer MUST occur via primitive values, frozen snapshots, or standard CustomEvent dispatch. - Components NEVER instantiate unbounded module-level $effect blocks inside federated remote bundles without explicit root boundary tracking. - Micro-frontend entry points MUST expose an idempotent unmount lifecycle hook that releases all active reactive graph nodes and detaches DOM roots completely. ## Bad vs Good Implementations ### Federated Micro-Frontend Bootstrap and Teardown Boundary #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Wiping container DOM without calling Svelte 5's unmount() leaves active rune subscriptions in the runtime scheduler. Passing raw shared signals creates cross-boundary coupling, and missing $effect.root boundaries cause permanent memory leaks across micro-frontend mount/unmount cycles. ```typescript import { mount } from 'svelte'; import MicroApp from './MicroApp.svelte'; // ANTI-PATTERN: Leaks module-level state and fails to clean up Svelte 5 runtime let appInstance: any; export function bootstrapMfe(container: HTMLElement) { // Bad: Direct mount without lifecycle capture or signal boundary isolation appInstance = mount(MicroApp, { target: container, props: { globalSignal: window.__SHARED_STATE__ } // Cross-boundary signal leakage }); } export function teardownMfe(container: HTMLElement) { // Critical bug: container.innerHTML wiping detached DOM leaves active signal subscriptions in memory container.innerHTML = ''; appInstance = null; } ``` #### Production Standard (Correct) > **Standard**: Enforces deterministic teardown by pairing Svelte 5's unmount() with $effect.root() cleanup. Host state is defensively frozen across boundaries, and the instance registry guarantees zero orphaned reactive signal nodes upon host teardown. ```typescript import { mount, unmount } from 'svelte'; import MicroApp from './MicroApp.svelte'; interface MfeInstanceHandle { unmount: () => void; cleanupRoot: () => void; } const instanceRegistry = new WeakMap<HTMLElement, MfeInstanceHandle>(); export function bootstrapMfe(container: HTMLElement, hostProps: Record<string, unknown>): void { // PRODUCTION FIX: Isolate reactive signals within an explicit effect root let appInstance: Record<string, any>; const cleanupRoot = $effect.root(() => { // Ensure immutable boundary: snapshot properties rather than passing raw state signals const isolatedProps = Object.freeze({ ...hostProps }); appInstance = mount(MicroApp, { target: container, props: isolatedProps }); }); instanceRegistry.set(container, { unmount: () => unmount(appInstance), cleanupRoot }); } export function teardownMfe(container: HTMLElement): void { const handle = instanceRegistry.get(container); if (handle) { handle.cleanupRoot(); handle.unmount(); instanceRegistry.delete(container); } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Svelte 5 Micro-Frontend Signal Lifecycle Boundary & Root Teardown Invariant

Svelte 5 Micro-Frontend Signal Lifecycle Boundary & Root Teardown Invariant defines an industrial-grade behavioral standard for frontend engineering. Enforces strict signal boundaries and deterministic teardown contracts for federated Svelte 5 micro-frontends. Prevents cross-boundary reactivity leakage and orphaned reactive subscriptions by mandating $effect.root isolation and explicit unmount execution during micro-frontend detachment. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Leaking Svelte 5 Module-Level Runes and Untracked Signal Graphs Across Federated Host Boundaries Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Leaking Svelte 5 Module-Level Runes and Untracked Signal Graphs Across Federated Host Boundaries	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

Engineers MUST ALWAYS capture and invoke the cleanup callback returned by $effect.root() when creating non-component reactive signals in micro-frontend wrappers.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.