Standard Software Architecture RFC & Decision Record (ADR) Template

# RFC-042: Migration to Client-Side WASM Document Processing Engine

**Author(s):** Alex Mercer (Staff Architect)  
**Status:** In Review (Draft / In Review / Approved / Rejected)  
**Target Review Date:** November 14, 2026  
**Target Delivery Sprint:** Q1 2027 Sprint 4  

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### 1. CONTEXT & PROBLEM STATEMENT
Our current server-side PDF manipulation architecture incurs heavy cloud compute expenses ($4,200/month) and introduces GDPR liability by handling sensitive user documents on backend worker nodes. Furthermore, during traffic spikes, server upload queues cause request latency to exceed 8 seconds.

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### 2. PROPOSED SOLUTION & ARCHITECTURE
We propose migrating 100% of PDF compression, format conversions, and metadata stripping to execute **client-side inside the user's web browser** using WebAssembly (WASM) compiled from Rust and HTML5 Canvas API.

#### Architectural Diagram:
```
[User Browser]
   ├── UI Layer (Next.js React Server & Client Components)
   ├── Web Worker Thread (Offloads heavy CPU processing)
   │     └── Rust WASM Module (pdf-engine.wasm)
   └── Volatile ArrayBuffer Memory (0 Server Uploads)
```

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### 3. GOALS & NON-GOALS

#### Explicit Goals:
- Reduce cloud server PDF processing costs to **$0.00**.
- Guarantee zero-knowledge document privacy (no file payloads ever hit our APIs).
- Maintain sub-second processing latency for documents under 25MB.

#### Non-Goals:
- Supporting legacy browsers lacking WebAssembly support (e.g. Internet Explorer 11).
- Cloud archival storage of processed user documents.

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### 4. TRADE-OFFS & CONSIDERATIONS

| Alternative Considered | Pros | Cons / Why Rejected |
| :--- | :--- | :--- |
| **Status Quo (AWS Lambda + S3)** | Known tech stack | $4,200/mo ongoing cost; privacy liability |
| **Serverless Go Microservice** | Fast cold starts | Still stores user files temporarily on disk |
| **Client-Side Rust WASM (Chosen)** | $0 compute; 100% private | Initial 1.2MB WASM bundle download |

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### 5. SECURITY & DATA PRIVACY IMPACT
- **Zero Server Egress:** No document data packets leave the client device.
- **Content Security Policy (CSP):** The WASM module is sandboxed and granted zero network socket permissions.

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### 6. MIGRATION & ROLLBACK PLAN
- **Phase 1:** Roll out WASM engine behind a 10% feature flag for beta users.
- **Phase 2:** Monitor browser crash telemetry via Sentry. If crash rate exceeds 0.2%, instant feature-flag rollback to previous engine.