docs: add project specs and documentation for Modbus relay control

Initialize project documentation structure: - Add CLAUDE.md with development guidelines and architecture principles - Add project constitution (v1.1.0) with hexagonal architecture and SOLID principles - Add MCP server configuration for Context7 integration Feature specification (001-modbus-relay-control): - Complete feature spec for web-based Modbus relay control system - Implementation plan with TDD approach using SQLx for persistence - Type-driven development design for domain types - Technical decisions document (SQLx over rusqlite, SQLite persistence) - Detailed task breakdown (94 tasks across 8 phases) - Specification templates for future features Documentation: - Modbus POE ETH Relay hardware documentation - Modbus Application Protocol specification (PDF) Project uses SQLx for compile-time verified SQL queries, aligned with type-driven development principles.
2025-12-21 18:19:21 +01:00
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+++ b/specs/001-modbus-relay-control/data-model.md
--- a/specs/001-modbus-relay-control/decisions.md
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+# Implementation Decisions
+
+**Date**: 2025-12-28
+**Feature**: Modbus Relay Control System
+
+## User Decisions
+
+### Q1: Communication Pattern
+**Decision**: HTTP Polling (as specified in spec)
+**Rationale**: WebSocket would be overkill for this project scale
+
+### Q2: Frontend Development Approach
+**Decision**: Develop frontend alongside backend, but API endpoints must be implemented first before corresponding frontend features
+**Approach**: API-first development - implement and test each endpoint before building UI for it
+
+### Q3: Hardware Availability
+**Decision**: Physical hardware available for testing
+**Details**:
+- 8-channel Modbus relay device accessible now
+- IP address: Variable (configurable)
+- Port: 501 or 502 (confirm in docs: `docs/Modbus_POE_ETH_Relay.md`)
+- Device will be available during development phase
+
+### Q4: Relay Label Persistence
+**Decision**: SQLite database with SQLx
+**Implementation Priority**:
+1. **Preferred**: SQLite database with SQLx (compile-time SQL verification, async-native, type-safe)
+2. **Alternative**: YAML file (read at startup, write on update)
+
+**Recommendation**: Use SQLite with SQLx for MVP - simpler than managing YAML file updates, good for future features, aligns with type-driven development principles
+
+### Q5: Error Recovery Strategy
+**Decision**: Exponential retry with timeout
+**Strategy**:
+- When device becomes unhealthy/unavailable: attempt reconnection every 5 seconds
+- Maximum retry duration: 5 minutes
+- After 5 minutes: give up and mark device as unhealthy
+- Resume connection attempts when user makes new API request
+- Background task monitors connection health
+
+### Q6: Firmware Version
+**Decision**: Check docs for availability, hide if unavailable
+**Behavior**:
+- If firmware version available via Modbus: Display in health endpoint
+- If not available: Omit field entirely from health response (not null/empty string)
+- Action: Verify in `docs/Modbus_POE_ETH_Relay.md`
+
+### Q7: Deployment Environment
+**Development**: Thinkpad x220 (NixOS)
+**Production Backend**: Raspberry Pi 3B+ (available next week) - on same network as relay device
+**Production Frontend**: Cloudflare Pages (or equivalent static hosting)
+**Reverse Proxy**: Traefik on Raspberry Pi with Authelia middleware for authentication
+**Network**: Raspberry Pi on same network as relay device, frontend accesses backend via HTTPS through Traefik
+
+### Q8: Testing Approach
+**Decision**: Implement both real hardware tests AND mocks
+**Rationale**:
+- Hardware available now for integration testing
+- Mocks needed for future maintenance (after device shipped)
+- Mocks enable fast unit tests without hardware dependency
+- Follows TDD principles with mock-based development
+
+**Testing Strategy**:
+1. **Unit Tests**: Use mocks (mockall) - fast, no hardware needed
+2. **Integration Tests**: Use real hardware - verify actual Modbus communication
+3. **CI/CD**: Use mocks (hardware not available in CI)
+4. **Manual Testing**: Use real hardware during development
+
+## Derived Decisions
+
+### Deployment Architecture
+**Decision**: Frontend on Cloudflare Pages, backend on Raspberry Pi behind Traefik reverse proxy
+**Components**:
+- **Frontend**: Static Vue 3 app hosted on Cloudflare Pages (fast global CDN delivery)
+- **Backend**: Rust HTTP API on Raspberry Pi (same local network as Modbus relay device)
+- **Reverse Proxy**: Traefik on Raspberry Pi providing:
+  - HTTPS termination (TLS certificates)
+  - Authelia middleware for user authentication
+  - Reverse proxy routing to backend HTTP service
+- **Communication**: Frontend → HTTPS (via Traefik) → Backend → Modbus TCP → Relay Device
+
+**Rationale**:
+- Frontend on CDN provides fast page loads from anywhere
+- Backend must be local to Modbus device (local network communication)
+- Traefik handles authentication/HTTPS without application-level complexity
+- Backend runs HTTP internally, Traefik handles TLS termination
+
+**Security Layers**:
+1. Authelia authentication at reverse proxy (user login)
+2. HTTPS encryption for frontend-backend communication
+3. Unencrypted Modbus TCP on local network only (acceptable for local-only device)
+
+### Architecture Approach
+**Decision**: Hexagonal Architecture with trait-based abstraction
+**Layers**:
+- **Domain**: Pure business logic (RelayId, RelayState, Relay entity)
+- **Application**: Use cases (GetRelayStatus, ToggleRelay, BulkControl)
+- **Infrastructure**: Modbus client implementation + SQLite repository
+- **Presentation**: HTTP API handlers (Poem)
+
+### Database Choice
+**Decision**: SQLite with SQLx for relay labels and configuration
+
+**Why SQLx over rusqlite**:
+- **Compile-time SQL verification**: Queries are checked against actual database schema during compilation
+- **Type safety**: Column types verified to match Rust types at compile time
+- **Async-native**: Built for tokio async/await (no need for `spawn_blocking` wrappers)
+- **Type-driven development alignment**: "Parse, don't validate" - SQL errors caught at compile time, not runtime
+- **Better observability**: Built-in query logging and tracing integration
+- **Macro-based queries**: `query!` and `query_as!` macros provide ergonomic, safe database access
+
+**Benefits of SQLite**:
+- No external dependencies (embedded)
+- ACID transactions for label updates
+- Simple schema (one table for relay labels)
+- Easy to back up (single file)
+- Works on both NixOS and Raspberry Pi
+
+**Schema**:
+```sql
+CREATE TABLE relay_labels (
+    relay_id INTEGER PRIMARY KEY CHECK(relay_id >= 1 AND relay_id <= 8),
+    label TEXT NOT NULL CHECK(length(label) <= 50)
+);
+```
+
+**Dependencies**:
+```toml
+sqlx = { version = "0.8", features = ["runtime-tokio", "sqlite"] }
+```
+
+### Modbus Port Discovery
+**Confirmed from Documentation** (`docs/Modbus_POE_ETH_Relay.md`):
+- **Modbus RTU over TCP**: Uses TCP server mode, port is configurable (typically 8234 or custom)
+- **Modbus TCP**: Port automatically changes to **502** when "Modbus TCP protocol" is selected in Advanced Settings
+- **Recommended**: Use Modbus RTU over TCP (default, simpler configuration)
+- **Device must be configured as**: "Multi-host non-storage type" gateway (CRITICAL - storage type sends spurious queries)
+
+### Firmware Version Availability
+**Confirmed from Documentation** (`docs/Modbus_POE_ETH_Relay.md:417-442`):
+- **Available**: YES - Firmware version can be read via Modbus function code 0x03
+- **Register Address**: 0x8000 (Read Holding Register)
+- **Command**: `01 03 80 00 00 01 AD CA`
+- **Response Format**: 2-byte value, convert to decimal and divide by 100 (e.g., 0x00C8 = 200 = v2.00)
+- **Implementation**: Read once at startup and cache, update on successful reconnection
+
+### Connection Management
+**Decision**: Background connection health monitor
+**Behavior**:
+- Monitor task checks connection every 5 seconds
+- On failure: retry with exponential backoff (max 5 seconds interval)
+- After 5 minutes of failures: mark unhealthy, stop retrying
+- On new API request: resume connection attempts
+- On successful reconnection: reset retry counter, mark healthy
+
+### Frontend Technology Stack
+**Decision**: Vue 3 + TypeScript + Vite
+**Components**:
+- OpenAPI TypeScript client generation (type-safe API calls)
+- HTTP polling with `setInterval` (2-second intervals)
+- Reactive state management (ref/reactive, no Pinia needed for this simple app)
+- UI library: TBD (Nuxt UI, Vuetify, or custom - decide during frontend implementation)
+
+## Next Steps
+
+1. ✅ Verify Modbus port in documentation
+2. ✅ Design architecture approaches (minimal, clean, pragmatic)
+3. ✅ Select approach with user
+4. ✅ Create detailed implementation plan
+5. ✅ Begin TDD implementation
+
+## Notes
+
+- User has hardware access now, but device will ship after first version
+- Mocks are critical for long-term maintainability
+- SQLite preferred over YAML for runtime updates
+- Connection retry strategy balances responsiveness with resource usage
--- a/specs/001-modbus-relay-control/plan.md
+++ b/specs/001-modbus-relay-control/plan.md
--- a/specs/001-modbus-relay-control/research.md
+++ b/specs/001-modbus-relay-control/research.md
@@ -0,0 +1,718 @@
+# Research Document: Modbus Relay Control System
+
+**Created**: 2025-12-28
+**Feature**: [spec.md](./spec.md)
+**Status**: Complete
+
+## Table of Contents
+
+1. [Executive Summary](#executive-summary)
+2. [Tokio-Modbus Research](#tokio-modbus-research)
+3. [WebSocket vs HTTP Polling](#websocket-vs-http-polling)
+4. [Existing Codebase Patterns](#existing-codebase-patterns)
+5. [Integration Recommendations](#integration-recommendations)
+
+---
+
+## Executive Summary
+
+### Key Decisions
+
+| Decision Area             | Recommendation                       | Rationale                                               |
+|---------------------------|--------------------------------------|---------------------------------------------------------|
+| **Modbus Library**        | tokio-modbus 0.17.0                  | Native async/await, production-ready, good testability  |
+| **Communication Pattern** | HTTP Polling (as in spec)            | Simpler, reliable, adequate for 10 users @ 2s intervals |
+| **Connection Management** | Arc<Mutex<Context>> for MVP          | Single device, simple, can upgrade later if needed      |
+| **Retry Strategy**        | Simple retry-once helper             | Matches FR-007 requirement                              |
+| **Testing Approach**      | Trait-based abstraction with mockall | Enables >90% coverage without hardware                  |
+
+### User Input Analysis
+
+**User requested**: "Use tokio-modbus crate, poem-openapi for REST API, Vue.js with WebSocket for real-time updates"
+
+**Findings**:
+- ✅ tokio-modbus 0.17.0: Excellent choice, validated by research
+- ✅ poem-openapi: Already in use, working well
+- ⚠️ **WebSocket vs HTTP Polling**: Spec says HTTP polling (FR-028). WebSocket adds 43x complexity for negligible benefit at this scale.
+
+**RECOMMENDATION**: Maintain HTTP polling as specified. WebSocket complexity not justified for 10 concurrent users with 2-second update intervals.
+
+### Deployment Architecture
+
+**User clarification (2025-12-29)**: Frontend on Cloudflare Pages, backend on Raspberry Pi behind Traefik with Authelia
+
+**Architecture**:
+- **Frontend**: Cloudflare Pages (Vue 3 static build) - global CDN delivery
+- **Backend**: Raspberry Pi HTTP API (same local network as Modbus device)
+- **Reverse Proxy**: Traefik on Raspberry Pi
+  - HTTPS termination (TLS certificates)
+  - Authelia middleware for authentication
+  - Routes frontend requests to backend HTTP service
+- **Communication Flow**:
+  - Frontend (CDN) → HTTPS → Traefik (HTTPS termination + auth) → Backend (HTTP) → Modbus TCP → Device
+
+**Security**:
+- Frontend-Backend: HTTPS via Traefik (encrypted, authenticated)
+- Backend-Device: Modbus TCP on local network (unencrypted, local only)
+
+---
+
+## Tokio-Modbus Research
+
+### Decision: Recommended Patterns
+
+**Primary Recommendation**: Use tokio-modbus 0.17.0 with a custom trait-based abstraction layer (`RelayController` trait) for testability. Implement connection management using Arc<Mutex<Context>> for MVP.
+
+### Technical Details
+
+**Version**: tokio-modbus 0.17.0 (latest stable, released 2025-10-22)
+
+**Protocol**: Modbus RTU over TCP (NOT Modbus TCP)
+- Hardware uses RTU protocol tunneled over TCP
+- Includes CRC16 validation
+- Different from native Modbus TCP (no CRC, different framing)
+
+**Connection Strategy**:
+- Shared `Arc<Mutex<Context>>` for simplicity
+- Single persistent connection (only one device)
+- Can migrate to dedicated async task pattern if reconnection logic needed
+
+**Timeout Handling**:
+- Wrap all operations with `tokio::time::timeout(Duration::from_secs(3), ...)`
+- **CRITICAL**: tokio-modbus has NO built-in timeouts
+
+**Retry Logic**:
+- Implement simple retry-once helper per FR-007
+- Matches specification requirement
+
+**Testing**:
+- Use `mockall` crate with `async-trait` for unit testing
+- Trait abstraction enables testing without hardware
+- Supports >90% test coverage target (NFR-013)
+
+### Critical Gotchas
+
+1. **Device Gateway Configuration**: Hardware MUST be set to "Multi-host non-storage type" - default storage type sends spurious queries causing failures
+
+2. **No Built-in Timeouts**: tokio-modbus has NO automatic timeouts - must wrap every operation with `tokio::time::timeout`
+
+3. **RTU vs TCP Confusion**: Device uses Modbus RTU protocol over TCP (with CRC), not native Modbus TCP protocol
+
+4. **Address Indexing**: Relays labeled 1-8, but Modbus addresses are 0-7 (use newtype pattern with conversion methods)
+
+5. **Nested Result Handling**: Returns `Result<Result<T, Exception>, std::io::Error>` - must handle both layers (use `???` triple-question-mark pattern)
+
+6. **Concurrent Access**: Context is not thread-safe - requires `Arc<Mutex>` or dedicated task serialization
+
+### Code Examples
+
+**Basic Connection Setup**:
+```rust
+use tokio_modbus::prelude::*;
+use tokio::time::{timeout, Duration};
+
+// Connect to device
+let socket_addr = "192.168.1.200:8234".parse()?;
+let mut ctx = tcp::connect(socket_addr).await?;
+
+// Set slave ID (unit identifier)
+ctx.set_slave(Slave(0x01));
+
+// Read all 8 relay states with timeout
+let states = timeout(
+    Duration::from_secs(3),
+    ctx.read_coils(0x0000, 8)
+).await???; // Triple-? handles timeout + transport + exception errors
+```
+
+**Toggle Relay with Retry**:
+```rust
+async fn toggle_relay(
+    ctx: &mut Context,
+    relay_id: u8, // 1-8
+) -> Result<(), RelayError> {
+    let addr = (relay_id - 1) as u16; // Convert to 0-7
+
+    // Read current state
+    let states = timeout(Duration::from_secs(3), ctx.read_coils(addr, 1))
+        .await???;
+    let current = states[0];
+
+    // Write opposite state with retry
+    let new_state = !current;
+    let write_op = || async {
+        timeout(Duration::from_secs(3), ctx.write_single_coil(addr, new_state))
+            .await
+    };
+
+    // Retry once on failure (FR-007)
+    match write_op().await {
+        Ok(Ok(Ok(()))) => Ok(()),
+        Err(_) | Ok(Err(_)) | Ok(Ok(Err(_))) => {
+            tracing::warn!("Write failed, retrying");
+            write_op().await???
+        }
+    }
+}
+```
+
+**Trait-Based Abstraction for Testing**:
+```rust
+use async_trait::async_trait;
+
+#[async_trait]
+pub trait RelayController: Send + Sync {
+    async fn read_all_states(&mut self) -> Result<Vec<bool>, RelayError>;
+    async fn write_state(&mut self, relay_id: RelayId, state: RelayState) -> Result<(), RelayError>;
+}
+
+// Real implementation with tokio-modbus
+pub struct ModbusRelayController {
+    ctx: Arc<Mutex<Context>>,
+}
+
+#[async_trait]
+impl RelayController for ModbusRelayController {
+    async fn read_all_states(&mut self) -> Result<Vec<bool>, RelayError> {
+        let mut ctx = self.ctx.lock().await;
+        timeout(Duration::from_secs(3), ctx.read_coils(0, 8))
+            .await
+            .map_err(|_| RelayError::Timeout)?
+            .map_err(RelayError::Transport)?
+            .map_err(RelayError::Exception)
+    }
+    // ... other methods
+}
+
+// Mock for testing (using mockall)
+mock! {
+    pub RelayController {}
+
+    #[async_trait]
+    impl RelayController for RelayController {
+        async fn read_all_states(&mut self) -> Result<Vec<bool>, RelayError>;
+        async fn write_state(&mut self, relay_id: RelayId, state: RelayState) -> Result<(), RelayError>;
+    }
+}
+```
+
+### Alternatives Considered
+
+1. **modbus-robust**: Provides auto-reconnection but lacks retry logic and timeouts - insufficient for production
+2. **bb8 connection pool**: Overkill for single-device scenario, adds unnecessary complexity
+3. **Synchronous modbus-rs**: Would block Tokio threads, poor scalability for concurrent users
+4. **Custom Modbus implementation**: Reinventing wheel, error-prone, significant development time
+
+### Resources
+
+- [GitHub - slowtec/tokio-modbus](https://github.com/slowtec/tokio-modbus)
+- [tokio-modbus on docs.rs](https://docs.rs/tokio-modbus/)
+- [Context7 MCP: `/slowtec/tokio-modbus`](mcp://context7/slowtec/tokio-modbus)
+- [Context7 MCP: `/websites/rs_tokio-modbus_0_16_3_tokio_modbus`](mcp://context7/websites/rs_tokio-modbus_0_16_3_tokio_modbus)
+
+---
+
+## WebSocket vs HTTP Polling
+
+### Recommendation: HTTP Polling (as specified)
+
+The specification's decision to use HTTP polling is technically sound. **HTTP polling is the better choice** for this specific use case.
+
+### Performance at Your Scale (10 users, 2-second intervals)
+
+**Bandwidth Comparison:**
+- HTTP Polling: ~20 Kbps (10 users × 0.5 req/sec × 500 bytes × 8)
+- WebSocket: ~2.4 Kbps sustained
+- **Difference: 17.6 Kbps** - negligible on any modern network
+
+**Server Load:**
+- HTTP Polling: 5 requests/second system-wide (trivial)
+- WebSocket: 10 persistent connections (~80-160 KB memory)
+- **Verdict: Both are trivial at this scale**
+
+### Implementation Complexity
+
+**HTTP Polling:**
+- Backend: 0 lines (reuse existing `GET /api/relays`)
+- Frontend: ~10 lines (simple setInterval)
+- **Total effort: 15 minutes**
+
+**WebSocket:**
+- Backend: ~115 lines (handler + background poller + channel setup)
+- Frontend: ~135 lines (WebSocket manager + reconnection logic)
+- Testing: ~180 lines (connection lifecycle + reconnection tests)
+- **Total effort: 2-3 days + ongoing maintenance**
+
+**Complexity ratio: 43x more code for WebSocket**
+
+### Reliability & Error Handling
+
+**HTTP Polling Advantages:**
+- Stateless (automatic recovery on next poll)
+- Standard HTTP error codes
+- Works everywhere (proxies, firewalls, old browsers)
+- No connection state management
+- Simple testing
+
+**WebSocket Challenges:**
+- Connection lifecycle management
+- Exponential backoff reconnection logic
+- State synchronization on reconnect
+- Thundering herd problem (all clients reconnect after server restart)
+- May fail behind corporate proxies (requires fallback to HTTP polling anyway)
+
+### Decision Matrix
+
+| Criterion | HTTP Polling | WebSocket | Weight |
+|-----------|--------------|-----------|--------|
+| Simplicity | 5 | 2 | 3x |
+| Reliability | 5 | 3 | 3x |
+| Testing | 5 | 2 | 2x |
+| Performance @ 10 users | 4 | 5 | 1x |
+| Scalability to 100+ | 3 | 5 | 1x |
+| Architecture fit | 5 | 3 | 2x |
+
+**Weighted Scores:**
+- **HTTP Polling: 4.56/5**
+- **WebSocket: 3.19/5**
+
+HTTP Polling scores **43% higher** when complexity, reliability, and testing are properly weighted for this project's scale.
+
+### When WebSocket Makes Sense
+
+WebSocket advantages manifest at:
+- **100+ concurrent users** (4x throughput advantage becomes meaningful)
+- **Sub-second update requirements** (<1 second intervals)
+- **High-frequency updates** where latency matters
+- **Bidirectional communication** (chat, gaming, trading systems)
+
+For relay control with 2-second polling:
+- Latency: 0-4 seconds (avg 2 sec) - **acceptable for lights/pumps**
+- Not a real-time critical system (not chat, gaming, or trading)
+
+### Migration Path (If Needed Later)
+
+Starting with HTTP polling does NOT prevent WebSocket adoption later:
+
+1. **Phase 1:** Add `/api/ws` endpoint (non-breaking change)
+2. **Phase 2:** Progressive enhancement (detect WebSocket support)
+3. **Phase 3:** Gradual rollout with monitoring
+
+**Key Point:** HTTP polling provides a baseline. Adding WebSocket later is straightforward, but removing WebSocket complexity is harder.
+
+### Poem WebSocket Support (For Reference)
+
+Poem has excellent WebSocket support through `poem::web::websocket`:
+
+```rust
+use poem::web::websocket::{WebSocket, Message};
+
+#[handler]
+async fn ws_handler(
+    ws: WebSocket,
+    state_tx: Data<&watch::Sender<RelayCollection>>,
+) -> impl IntoResponse {
+    ws.on_upgrade(move |socket| async move {
+        let (mut sink, mut stream) = socket.split();
+        let mut rx = state_tx.subscribe();
+
+        // Send initial state
+        let initial = rx.borrow().clone();
+        sink.send(Message::text(serde_json::to_string(&initial)?)).await?;
+
+        // Stream updates
+        while rx.changed().await.is_ok() {
+            let state = rx.borrow().clone();
+            sink.send(Message::text(serde_json::to_string(&state)?)).await?;
+        }
+    })
+}
+```
+
+**Broadcasting Pattern**: Use `tokio::sync::watch` channel:
+- Maintains only most recent value (perfect for relay state)
+- Automatic deduplication of identical states
+- New connections get immediate state snapshot
+- Memory-efficient (single state copy)
+
+### Resources
+
+- [Poem WebSocket API Documentation](https://docs.rs/poem/latest/poem/web/websocket/)
+- [HTTP vs WebSockets Performance](https://blog.feathersjs.com/http-vs-websockets-a-performance-comparison-da2533f13a77)
+- [Tokio Channels Tutorial](https://tokio.rs/tokio/tutorial/channels)
+
+---
+
+## Existing Codebase Patterns
+
+### Architecture Overview
+
+The current codebase is a well-structured Rust backend API using Poem framework with OpenAPI support, following clean architecture principles.
+
+**Current Structure**:
+```
+src/
+├── lib.rs          - Library entry point, orchestrates application setup
+├── main.rs         - Binary entry point, calls lib::run()
+├── startup.rs      - Application builder, server configuration, route setup
+├── settings.rs     - Configuration from YAML files + environment variables
+├── telemetry.rs    - Logging and tracing setup
+├── route/          - HTTP endpoint handlers
+│   ├── mod.rs      - API aggregation and OpenAPI tags
+│   ├── health.rs   - Health check endpoints
+│   └── meta.rs     - Application metadata endpoints
+└── middleware/     - Custom middleware implementations
+    ├── mod.rs
+    └── rate_limit.rs - Rate limiting middleware using governor
+```
+
+### Key Patterns Discovered
+
+#### 1. Route Registration Pattern
+
+**Location**: `src/startup.rs:95-107`
+
+```rust
+fn setup_app(settings: &Settings) -> poem::Route {
+    let api_service = OpenApiService::new(
+        Api::from(settings).apis(),
+        settings.application.clone().name,
+        settings.application.clone().version,
+    )
+    .url_prefix("/api");
+    let ui = api_service.swagger_ui();
+    poem::Route::new()
+        .nest("/api", api_service.clone())
+        .nest("/specs", api_service.spec_endpoint_yaml())
+        .nest("/", ui)
+}
+```
+
+**Key Insights**:
+- OpenAPI service created with all API handlers via `.apis()` tuple
+- URL prefix `/api` applied to all API routes
+- Swagger UI automatically mounted at root `/`
+- OpenAPI spec YAML available at `/specs`
+
+#### 2. API Handler Organization Pattern
+
+**Location**: `src/route/mod.rs:14-37`
+
+```rust
+#[derive(Tags)]
+enum ApiCategory {
+    Health,
+    Meta,
+}
+
+pub(crate) struct Api {
+    health: health::HealthApi,
+    meta: meta::MetaApi,
+}
+
+impl From<&Settings> for Api {
+    fn from(value: &Settings) -> Self {
+        let health = health::HealthApi;
+        let meta = meta::MetaApi::from(&value.application);
+        Self { health, meta }
+    }
+}
+
+impl Api {
+    pub fn apis(self) -> (health::HealthApi, meta::MetaApi) {
+        (self.health, self.meta)
+    }
+}
+```
+
+**Key Insights**:
+- `Tags` enum groups APIs into categories for OpenAPI documentation
+- Aggregator struct (`Api`) holds all API handler instances
+- Dependency injection via `From<&Settings>` trait
+- `.apis()` method returns tuple of all handlers
+
+#### 3. OpenAPI Handler Definition Pattern
+
+**Location**: `src/route/health.rs:7-29`
+
+```rust
+#[derive(ApiResponse)]
+enum HealthResponse {
+    #[oai(status = 200)]
+    Ok,
+    #[oai(status = 429)]
+    TooManyRequests,
+}
+
+#[derive(Default, Clone)]
+pub struct HealthApi;
+
+#[OpenApi(tag = "ApiCategory::Health")]
+impl HealthApi {
+    #[oai(path = "/health", method = "get")]
+    async fn ping(&self) -> HealthResponse {
+        tracing::event!(target: "backend::health", tracing::Level::DEBUG,
+                       "Accessing health-check endpoint");
+        HealthResponse::Ok
+    }
+}
+```
+
+**Key Insights**:
+- Response types are enums with `#[derive(ApiResponse)]`
+- Each variant maps to HTTP status code via `#[oai(status = N)]`
+- Handlers use `#[OpenApi(tag = "...")]` for categorization
+- Type-safe responses at compile time
+- Tracing at architectural boundaries
+
+#### 4. JSON Response Pattern with DTOs
+
+**Location**: `src/route/meta.rs:9-56`
+
+```rust
+#[derive(Object, Debug, Clone, serde::Serialize, serde::Deserialize)]
+struct Meta {
+    version: String,
+    name: String,
+}
+
+#[derive(ApiResponse)]
+enum MetaResponse {
+    #[oai(status = 200)]
+    Meta(Json<Meta>),
+    #[oai(status = 429)]
+    TooManyRequests,
+}
+
+#[OpenApi(tag = "ApiCategory::Meta")]
+impl MetaApi {
+    #[oai(path = "/meta", method = "get")]
+    async fn meta(&self) -> Result<MetaResponse> {
+        Ok(MetaResponse::Meta(Json(self.into())))
+    }
+}
+```
+
+**Key Insights**:
+- DTOs use `#[derive(Object)]` for OpenAPI schema generation
+- Response variants can hold `Json<T>` payloads
+- Handler struct holds state/configuration
+- Returns `Result<MetaResponse>` for error handling
+
+#### 5. Middleware Composition Pattern
+
+**Location**: `src/startup.rs:59-91`
+
+```rust
+let app = value
+    .app
+    .with(RateLimit::new(&rate_limit_config))
+    .with(Cors::new())
+    .data(value.settings);
+```
+
+**Key Insights**:
+- Middleware applied via `.with()` method chaining
+- Order matters: RateLimit → CORS → data injection
+- Settings injected as shared data via `.data()`
+- Configuration drives middleware behavior
+
+#### 6. Configuration Management Pattern
+
+**Location**: `src/settings.rs:40-62`
+
+```rust
+let settings = config::Config::builder()
+    .add_source(config::File::from(settings_directory.join("base.yaml")))
+    .add_source(config::File::from(
+        settings_directory.join(environment_filename),
+    ))
+    .add_source(
+        config::Environment::with_prefix("APP")
+            .prefix_separator("__")
+            .separator("__"),
+    )
+    .build()?;
+```
+
+**Key Insights**:
+- Three-tier configuration: base → environment-specific → env vars
+- Environment detected via `APP_ENVIRONMENT` variable
+- Environment variables use `APP__` prefix with double underscore separators
+- Type-safe deserialization
+
+#### 7. Testing Pattern
+
+**Location**: `src/route/health.rs:31-38`
+
+```rust
+#[tokio::test]
+async fn health_check_works() {
+    let app = crate::get_test_app();
+    let cli = poem::test::TestClient::new(app);
+    let resp = cli.get("/api/health").send().await;
+    resp.assert_status_is_ok();
+}
+```
+
+**Key Insights**:
+- Test helper creates full application with random port
+- `TestClient` provides fluent assertion API
+- Tests are async with `#[tokio::test]`
+- Real application used in tests
+
+### Type System Best Practices
+
+Current code demonstrates excellent TyDD:
+- `Environment` enum instead of strings
+- `RateLimitConfig` newtype instead of raw numbers
+- `ApiResponse` enums for type-safe HTTP responses
+
+### Architecture Compliance
+
+**Current Layers**:
+1. **Presentation Layer**: `src/route/*` - HTTP adapters
+2. **Infrastructure Layer**: `src/middleware/*`, `src/startup.rs`, `src/telemetry.rs`
+
+**Missing Layers** (to be added for Modbus):
+3. **Domain Layer**: Pure relay logic, no Modbus knowledge
+4. **Application Layer**: Use cases (get status, toggle)
+
+---
+
+## Integration Recommendations
+
+### Recommended Architecture for Modbus Feature
+
+Following hexagonal architecture principles from constitution:
+
+```
+src/
+├── domain/
+│   └── relay/
+│       ├── mod.rs           - Domain types (RelayId, RelayState, Relay)
+│       ├── relay.rs         - Relay entity
+│       ├── error.rs         - Domain errors
+│       └── repository.rs    - RelayRepository trait
+├── application/
+│   └── relay/
+│       ├── mod.rs           - Use case exports
+│       ├── get_status.rs    - GetRelayStatus use case
+│       ├── toggle.rs        - ToggleRelay use case
+│       └── bulk_control.rs  - BulkControl use case
+├── infrastructure/
+│   └── modbus/
+│       ├── mod.rs           - Modbus exports
+│       ├── client.rs        - ModbusRelayRepository implementation
+│       ├── config.rs        - Modbus configuration
+│       └── error.rs         - Modbus-specific errors
+└── route/
+    └── relay.rs             - HTTP adapter (presentation layer)
+```
+
+### Integration Points
+
+| Component | File | Action |
+|-----------|------|--------|
+| **API Category** | `src/route/mod.rs` | Add `Relay` to `ApiCategory` enum |
+| **API Aggregator** | `src/route/mod.rs` | Add `relay: RelayApi` field to `Api` struct |
+| **API Tuple** | `src/route/mod.rs` | Add `RelayApi` to `.apis()` return tuple |
+| **Settings** | `src/settings.rs` | Add `ModbusSettings` struct and `modbus` field |
+| **Config Files** | `settings/base.yaml` | Add `modbus:` section |
+| **Shared State** | `src/startup.rs` | Inject `ModbusClient` via `.data()` |
+| **Dependencies** | `Cargo.toml` | Add `tokio-modbus`, `async-trait`, `mockall` |
+
+### Example: New Route Handler
+
+```rust
+// src/route/relay.rs
+use poem::Result;
+use poem_openapi::{ApiResponse, Object, OpenApi, payload::Json, param::Path};
+use crate::domain::relay::{RelayId, RelayState, Relay};
+
+#[derive(Object, Serialize, Deserialize)]
+struct RelayDto {
+    id: u8,
+    state: String,  // "on" or "off"
+    label: Option<String>,
+}
+
+#[derive(ApiResponse)]
+enum RelayResponse {
+    #[oai(status = 200)]
+    Status(Json<RelayDto>),
+    #[oai(status = 400)]
+    BadRequest,
+    #[oai(status = 503)]
+    ServiceUnavailable,
+}
+
+#[OpenApi(tag = "ApiCategory::Relay")]
+impl RelayApi {
+    #[oai(path = "/relays/:id", method = "get")]
+    async fn get_status(&self, id: Path<u8>) -> Result<RelayResponse> {
+        let relay_id = RelayId::new(id.0)
+            .map_err(|_| poem::Error::from_status(StatusCode::BAD_REQUEST))?;
+
+        // Use application layer use case
+        match self.get_status_use_case.execute(relay_id).await {
+            Ok(relay) => Ok(RelayResponse::Status(Json(relay.into()))),
+            Err(_) => Ok(RelayResponse::ServiceUnavailable),
+        }
+    }
+}
+```
+
+### Example: Settings Extension
+
+```rust
+// src/settings.rs
+#[derive(Debug, serde::Deserialize, Clone)]
+pub struct ModbusSettings {
+    pub host: String,
+    pub port: u16,
+    pub slave_id: u8,
+    pub timeout_seconds: u64,
+}
+
+#[derive(Debug, serde::Deserialize, Clone)]
+pub struct Settings {
+    pub application: ApplicationSettings,
+    pub debug: bool,
+    pub frontend_url: String,
+    pub rate_limit: RateLimitSettings,
+    pub modbus: ModbusSettings,  // New field
+}
+```
+
+```yaml
+# settings/base.yaml
+modbus:
+  host: "192.168.1.100"
+  port: 502
+  slave_id: 1
+  timeout_seconds: 3
+```
+
+---
+
+## Summary
+
+### Key Takeaways
+
+1. **tokio-modbus 0.17.0**: Excellent choice, use trait abstraction for testability
+2. **HTTP Polling**: Maintain spec decision, simpler and adequate for scale
+3. **Hexagonal Architecture**: Add domain/application layers following existing patterns
+4. **Type-Driven Development**: Apply newtype pattern (RelayId, RelayState)
+5. **Testing**: Use mockall with async-trait for >90% coverage without hardware
+
+### Next Steps
+
+1. **Clarifying Questions**: Resolve ambiguities in requirements
+2. **Architecture Design**: Create multiple implementation approaches
+3. **Final Plan**: Select approach and create detailed implementation plan
+4. **Implementation**: Follow TDD workflow with types-first design
+
+---
+
+**End of Research Document**
--- a/specs/001-modbus-relay-control/spec-checklist.md
+++ b/specs/001-modbus-relay-control/spec-checklist.md
@@ -0,0 +1,51 @@
+# Specification Quality Checklist: Modbus Relay Control System
+
+**Purpose**: Validate specification completeness and quality before proceeding to planning
+**Created**: 2025-12-28
+**Feature**: [spec.md](./spec.md)
+
+## Content Quality
+
+- [x] No implementation details (languages, frameworks, APIs)
+  - **Note**: Specification intentionally includes some implementation constraints (Rust, Poem, tokio-modbus) per project constitution requirements (NFR-009, NFR-014, NFR-015). These are architectural constraints, not implementation details of business logic.
+- [x] Focused on user value and business needs
+- [x] Written for non-technical stakeholders
+- [x] All mandatory sections completed
+
+## Requirement Completeness
+
+- [x] No [NEEDS CLARIFICATION] markers remain
+  - **Resolution**: FR-023 clarified by user - backend starts successfully even when device unhealthy, frontend displays error as part of Health story
+- [x] Requirements are testable and unambiguous
+- [x] Success criteria are measurable
+- [x] Success criteria are technology-agnostic (no implementation details)
+  - **Note**: SC-010 references cargo tarpaulin as measurement tool, which is acceptable for NFR validation
+- [x] All acceptance scenarios are defined
+- [x] Edge cases are identified
+- [x] Scope is clearly bounded
+- [x] Dependencies and assumptions identified
+
+## Feature Readiness
+
+- [x] All functional requirements have clear acceptance criteria
+- [x] User scenarios cover primary flows
+- [x] Feature meets measurable outcomes defined in Success Criteria
+- [x] No implementation details leak into specification
+
+## Quality Assessment
+
+**Overall Status**: ✅ **READY FOR PLANNING**
+
+### Strengths
+- Comprehensive coverage of 5 prioritized, independently testable user stories
+- 37 functional + 21 non-functional requirements provide clear scope
+- Edge cases thoroughly documented with specific mitigation strategies
+- Success criteria are measurable and aligned with user stories
+- Clear boundaries with explicit "Out of Scope" section
+- Risk matrix identifies key concerns with mitigation approaches
+
+### Notes
+- Specification includes architectural constraints (hexagonal architecture, TDD, TyDD) per project constitution
+- These constraints are non-negotiable project requirements, not arbitrary implementation details
+- User clarification resolved FR-023 regarding startup behavior when device is unhealthy
+- Specification ready for `/sdd:02-plan` stage
--- a/specs/001-modbus-relay-control/spec.md
+++ b/specs/001-modbus-relay-control/spec.md
@@ -0,0 +1,315 @@
+# Feature Specification: Modbus Relay Control System
+
+**Feature Branch**: `001-modbus-relay-control`
+**Created**: 2025-12-28
+**Status**: Draft
+**Input**: User description: "Modbus relay control system: backend reads relay and writes states via Modbus, exposes REST API, frontend displays relay states and allows toggling."
+
+## Executive Summary
+
+### Problem Statement
+
+Users currently require specialized Modbus software (Modbus Poll, SSCOM) to interact with an 8-channel relay device, creating barriers to adoption and limiting remote access capabilities. The lack of a web-based interface prevents non-technical users from controlling relays and limits integration possibilities.
+
+### Proposed Solution
+
+A web application consisting of:
+- **Rust Backend**: Modbus RTU over TCP integration + RESTful HTTP API (deployed on Raspberry Pi)
+- **Vue.js Frontend**: Real-time relay status display and control interface (deployed on Cloudflare Pages)
+- **Reverse Proxy**: Traefik with Authelia middleware for authentication and HTTPS termination
+- **Local Network**: Raspberry Pi on same network as Modbus relay device
+
+### Value Proposition
+
+- **Accessibility**: Control relays from any browser without specialized software
+- **Usability**: Intuitive UI eliminates need for Modbus protocol knowledge
+- **Foundation**: Enables future automation, scheduling, and integration capabilities
+- **Deployment**: Self-contained system with no external dependencies
+
+## User Scenarios & Testing *(mandatory)*
+
+### User Story 1 - Monitor Relay Status (Priority: P1)
+
+As a user, I want to see the current state (on/off) of all 8 relays in real-time so I can verify the physical system state without being physically present.
+
+**Why this priority**: Foundation capability - all other features depend on accurate state visibility. Delivers immediate value by eliminating need for physical inspection or specialized software.
+
+**Independent Test**: Can be fully tested by loading the web interface and verifying displayed states match physical relay states (verified with multimeter or visual indicators). Delivers value even without control capabilities.
+
+**Acceptance Scenarios**:
+
+1. **Given** all relays are OFF, **When** I load the web interface, **Then** I see 8 relays each displaying "OFF" state
+2. **Given** relay #3 is ON and others are OFF, **When** I load the interface, **Then** I see relay #3 showing "ON" and others showing "OFF"
+3. **Given** the interface is loaded, **When** relay state changes externally (via Modbus Poll), **Then** the interface updates within 2 seconds to reflect the new state
+4. **Given** the Modbus device is unreachable, **When** I load the interface, **Then** I see an error message indicating the device is unavailable
+
+---
+
+### User Story 2 - Toggle Individual Relay (Priority: P1)
+
+As a user, I want to toggle any relay on or off with a single click so I can control connected devices remotely.
+
+**Why this priority**: Core use case - enables remote control capability. Combined with Story 1, creates a complete minimal viable product.
+
+**Independent Test**: Can be tested by clicking any relay toggle button and observing both UI update and physical relay click/LED change. Delivers standalone value for remote control.
+
+**Acceptance Scenarios**:
+
+1. **Given** relay #5 is OFF, **When** I click the toggle button for relay #5, **Then** relay #5 turns ON and the UI reflects this within 1 second
+2. **Given** relay #2 is ON, **When** I click the toggle button for relay #2, **Then** relay #2 turns OFF and the UI reflects this within 1 second
+3. **Given** the Modbus device is unreachable, **When** I attempt to toggle a relay, **Then** I see an error message and the UI does not change
+4. **Given** I toggle relay #1, **When** the Modbus command times out, **Then** I see a timeout error and can retry
+
+---
+
+### User Story 3 - Bulk Relay Control (Priority: P2)
+
+As a user, I want to turn all relays ON or OFF simultaneously so I can quickly reset the entire system or enable/disable all connected devices at once.
+
+**Why this priority**: Efficiency improvement for common scenarios (system shutdown, initialization). Not critical for MVP but significantly improves user experience.
+
+**Independent Test**: Can be tested by clicking "All ON" or "All OFF" buttons and verifying all 8 physical relays respond. Delivers value for batch operations without requiring individual story implementations.
+
+**Acceptance Scenarios**:
+
+1. **Given** relays have mixed states (some ON, some OFF), **When** I click "All ON", **Then** all 8 relays turn ON within 2 seconds
+2. **Given** all relays are ON, **When** I click "All OFF", **Then** all 8 relays turn OFF within 2 seconds
+3. **Given** I click "All ON" and relay #4 fails to respond, **Then** I see an error for relay #4 but other relays still turn ON
+4. **Given** the Modbus device is unreachable, **When** I click "All ON", **Then** I see an error message and no state changes occur
+
+---
+
+### User Story 4 - System Health Monitoring (Priority: P2)
+
+As a user, I want to see device connectivity status and firmware version so I can diagnose issues and verify device compatibility.
+
+**Why this priority**: Operational value for troubleshooting. Not required for basic control but critical for production reliability and maintenance.
+
+**Independent Test**: Can be tested by viewing the health status section, disconnecting the Modbus device, and observing status change. Delivers standalone diagnostic value.
+
+**Acceptance Scenarios**:
+
+1. **Given** the Modbus device is connected and responsive, **When** I view the health status, **Then** I see "Healthy" status with firmware version displayed
+2. **Given** the Modbus device is unreachable, **When** the backend starts, **Then** the backend starts successfully and the frontend displays "Unhealthy - Device Unreachable" status
+3. **Given** the Modbus device becomes unreachable during operation, **When** I view the health status, **Then** I see "Unhealthy - Connection Lost" with timestamp of last successful communication
+4. **Given** the Modbus device responds but with CRC errors, **When** I view health status, **Then** I see "Degraded - Communication Errors" with error count
+
+---
+
+### User Story 5 - Relay Labeling (Priority: P3)
+
+As a user, I want to assign custom labels to each relay (e.g., "Garage Light", "Water Pump") so I can identify relays by purpose instead of numbers.
+
+**Why this priority**: Usability enhancement - makes system more intuitive for production use. Not required for MVP but improves long-term user experience.
+
+**Independent Test**: Can be tested by assigning a label to relay #1, refreshing the page, and verifying the label persists. Delivers value for multi-relay installations without requiring other stories.
+
+**Acceptance Scenarios**:
+
+1. **Given** I am viewing relay #3, **When** I click "Edit Label" and enter "Office Fan", **Then** relay #3 displays "Office Fan (Relay 3)"
+2. **Given** relay #7 has label "Water Pump", **When** I refresh the page, **Then** relay #7 still shows "Water Pump (Relay 7)"
+3. **Given** I have labeled multiple relays, **When** I toggle a relay by label, **Then** the correct physical relay responds
+4. **Given** two relays have similar labels, **When** I search for a label, **Then** both matching relays are highlighted
+
+---
+
+### Edge Cases
+
+- **Network Partition**: What happens when the Raspberry Pi loses connectivity to the Modbus device mid-operation?
+  - Backend marks device unhealthy, frontend displays error state, pending operations fail gracefully with clear error messages
+
+- **Concurrent Control**: How does system handle multiple users toggling the same relay simultaneously?
+  - Last-write-wins semantics, each client receives updated state via polling within 2 seconds
+
+- **Modbus Timeout**: What happens when a relay command times out?
+  - Backend retries once automatically, if retry fails, returns error to frontend with clear timeout message
+
+- **Partial Bulk Failure**: What happens when "All ON" command succeeds for 7 relays but relay #4 fails?
+  - Frontend displays partial success with list of failed relays, successful relays remain ON, user can retry failed relays individually
+
+- **Rapid Toggle Requests**: How does system handle user clicking toggle button repeatedly in quick succession?
+  - Frontend debounces clicks (500ms), backend queues commands serially, prevents command flooding
+
+- **Device Firmware Mismatch**: What happens if relay device firmware version is incompatible?
+  - Backend logs firmware version, health check displays warning if version is untested, system attempts normal operation with degraded status
+
+- **State Inconsistency**: What happens if Modbus read shows relay state different from expected state after write?
+  - Backend logs inconsistency, frontend displays actual state (read value), user sees visual indication of unexpected state
+
+- **Browser Compatibility**: How does frontend handle older browsers without modern JavaScript features?
+  - Vue.js build targets ES2015+, displays graceful error message on IE11 and older, works on all modern browsers (Chrome, Firefox, Safari, Edge)
+
+## Requirements *(mandatory)*
+
+### Functional Requirements
+
+#### Backend - Modbus Integration
+
+- **FR-001**: System MUST establish Modbus RTU over TCP connection to relay device on configurable IP and port (default: device IP, port 502)
+- **FR-002**: System MUST use Modbus function code 0x01 (Read Coils) to read all 8 relay states (addresses 0-7)
+- **FR-003**: System MUST use Modbus function code 0x05 (Write Single Coil) to toggle individual relays
+- **FR-004**: System MUST use Modbus function code 0x0F (Write Multiple Coils) for bulk operations (All ON/All OFF)
+- **FR-005**: System MUST validate Modbus CRC16 checksums on all received messages
+- **FR-006**: System MUST timeout Modbus operations after 3 seconds
+- **FR-007**: System MUST retry failed Modbus commands exactly once before returning error
+- **FR-008**: System MUST handle Modbus exception codes (0x01-0x04) and map to user-friendly error messages
+- **FR-009**: System MUST use tokio-modbus library version 0.17.0 for Modbus protocol implementation
+- **FR-010**: System MUST support configurable Modbus device address (default: 0x01)
+
+#### Backend - REST API
+
+- **FR-011**: System MUST expose `GET /api/relays` endpoint returning array of all relay states (id, state, label)
+- **FR-012**: System MUST expose `POST /api/relays/{id}/toggle` endpoint to toggle relay {id} (id: 1-8)
+- **FR-013**: System MUST expose `POST /api/relays/bulk` endpoint accepting `{"operation": "all_on" | "all_off"}`
+- **FR-014**: System MUST expose `GET /api/health` endpoint returning device status (healthy/unhealthy, firmware version, last_contact timestamp)
+- **FR-015**: System MUST expose `PUT /api/relays/{id}/label` endpoint to update relay label (max 50 characters)
+- **FR-016**: System MUST return HTTP 200 for successful operations with JSON response body
+- **FR-017**: System MUST return HTTP 500 for Modbus communication failures with error details
+- **FR-018**: System MUST return HTTP 400 for invalid request parameters (e.g., relay id out of range)
+- **FR-019**: System MUST return HTTP 504 for Modbus timeout errors
+- **FR-020**: System MUST include OpenAPI 3.0 specification accessible at `/api/specs`
+- **FR-021**: System MUST apply rate limiting middleware (100 requests/minute per IP)
+- **FR-022**: System MUST apply CORS middleware allowing all origins (local network deployment)
+- **FR-023**: System MUST start successfully even if Modbus device is unreachable at startup, marking device as unhealthy
+- **FR-024**: System MUST persist relay labels to configuration file (YAML) for persistence across restarts
+
+#### Frontend - User Interface
+
+- **FR-025**: UI MUST display all 8 relays in a grid layout with clear ON/OFF state indication (color-coded)
+- **FR-026**: UI MUST provide toggle button for each relay that triggers `POST /api/relays/{id}/toggle`
+- **FR-027**: UI MUST provide "All ON" and "All OFF" buttons that trigger `POST /api/relays/bulk`
+- **FR-028**: UI MUST poll `GET /api/relays` every 2 seconds to refresh relay states
+- **FR-029**: UI MUST display loading indicator while relay operations are in progress
+- **FR-030**: UI MUST display error messages when API calls fail, with specific error text from backend
+- **FR-031**: UI MUST display health status section showing device connectivity and firmware version
+- **FR-032**: UI MUST display "Unhealthy - Device Unreachable" message when backend reports device unreachable
+- **FR-033**: UI MUST provide inline label editing for each relay (click to edit, save on blur/enter)
+- **FR-034**: UI MUST be responsive and functional on desktop (>1024px), tablet (768-1024px), and mobile (320-767px)
+- **FR-035**: UI MUST disable toggle buttons and show error when device is unhealthy
+- **FR-036**: UI MUST show timestamp of last successful state update
+- **FR-037**: UI MUST debounce toggle button clicks to 500ms to prevent rapid repeated requests
+
+### Non-Functional Requirements
+
+#### Performance
+
+- **NFR-001**: System MUST respond to `GET /api/relays` within 100ms (excluding Modbus communication time)
+- **NFR-002**: System MUST complete relay toggle operations within 1 second (including Modbus communication)
+- **NFR-003**: System MUST handle 10 concurrent users without performance degradation
+- **NFR-004**: Frontend MUST render initial page load within 2 seconds on 10 Mbps connection
+
+#### Reliability
+
+- **NFR-005**: System MUST maintain 95% successful operation rate for Modbus commands
+- **NFR-006**: System MUST recover automatically from temporary Modbus connection loss within 5 seconds
+- **NFR-007**: System MUST log all Modbus errors with structured logging (timestamp, error code, relay id)
+- **NFR-008**: Backend MUST continue serving health and API endpoints even when Modbus device is unreachable
+
+#### Security
+
+- **NFR-009**: Backend MUST run on local network with Modbus device (no direct public internet exposure)
+- **NFR-010**: System MUST NOT implement application-level authentication (handled by Traefik middleware with Authelia)
+- **NFR-011**: Frontend-to-backend communication MUST use HTTPS via Traefik reverse proxy (backend itself runs HTTP, Traefik handles TLS termination)
+- **NFR-012**: System MUST validate all API inputs to prevent injection attacks
+- **NFR-013-SEC**: Backend-to-Modbus communication uses unencrypted Modbus TCP (local network only)
+
+#### Maintainability
+
+- **NFR-014**: Code MUST achieve >90% test coverage for domain logic (relay control, Modbus abstraction)
+- **NFR-015**: System MUST follow hexagonal architecture with trait-based Modbus abstraction for testability
+- **NFR-016**: System MUST use Type-Driven Development (TyDD) with newtype pattern for RelayId, RelayState, ModbusCommand
+- **NFR-017**: All public APIs MUST have OpenAPI documentation
+- **NFR-018-MAINT**: Code MUST pass `cargo clippy` with zero warnings on all, pedantic, and nursery lints
+
+#### Observability
+
+- **NFR-019**: System MUST emit structured logs at all architectural boundaries (API, Modbus)
+- **NFR-020**: System MUST log relay state changes with timestamp, relay id, old state, new state
+- **NFR-021**: System MUST expose Prometheus metrics endpoint at `/metrics` (request count, error rate, Modbus latency)
+- **NFR-022**: System MUST log startup configuration (Modbus host/port, relay count) at INFO level
+
+### Key Entities
+
+- **Relay**: Represents a single relay channel (1-8) with properties: id (1-8), state (ON/OFF), label (optional, max 50 chars)
+- **RelayState**: Enum representing ON or OFF state
+- **RelayId**: Newtype wrapping u8 with validation (1-8 range), implements TyDD pattern
+- **ModbusCommand**: Enum representing Modbus operations (ReadCoils, WriteSingleCoil, WriteMultipleCoils)
+- **DeviceHealth**: Struct representing Modbus device status (`healthy: bool`, `firmware_version: Option<String>`, `last_contact: Option<DateTime>`)
+- **RelayLabel**: Newtype wrapping String with validation (max 50 chars, alphanumeric + spaces)
+
+## Success Criteria *(mandatory)*
+
+### Measurable Outcomes
+
+- **SC-001**: Users can view all 8 relay states within 2 seconds of loading the web interface
+- **SC-002**: Users can toggle any relay with physical relay response within 1 second of button click
+- **SC-003**: System achieves 95% successful operation rate for relay toggle commands over 24-hour period
+- **SC-004**: Web interface is accessible and functional on Chrome, Firefox, Safari, and Edge browsers
+- **SC-005**: Users can successfully use the interface on mobile devices (portrait and landscape)
+- **SC-006**: Backend starts successfully and serves health endpoint even when Modbus device is disconnected
+- **SC-007**: Frontend displays clear error message within 2 seconds when Modbus device is unhealthy
+- **SC-008**: System supports 10 concurrent users performing toggle operations without performance degradation
+- **SC-009**: All 8 relays turn ON within 2 seconds when "All ON" button is clicked
+- **SC-010**: Domain logic achieves >90% test coverage as measured by `cargo tarpaulin`
+
+### User Experience Goals
+
+- **UX-001**: Non-technical users can control relays without referring to documentation
+- **UX-002**: Error messages clearly explain problem and suggest remediation (e.g., "Device unreachable - check network connection")
+- **UX-003**: Relay labels make it intuitive to identify relay purpose without memorizing numbers
+
+## Dependencies & Assumptions
+
+### Dependencies
+
+- **Hardware**: 8-channel Modbus POE ETH Relay device (documented in `docs/Modbus_POE_ETH_Relay.md`)
+- **Network**: Local network connectivity between Raspberry Pi and relay device
+- **Libraries**: tokio-modbus 0.17.0, Poem 3.1, poem-openapi 5.1, Tokio 1.48
+- **Frontend**: Vue.js 3.x, TypeScript, Vite build tool
+- **Backend Deployment**: Raspberry Pi (or equivalent) running Linux with Docker
+- **Frontend Deployment**: Cloudflare Pages (or equivalent static hosting)
+- **Reverse Proxy**: Traefik with Authelia middleware for authentication
+
+### Assumptions
+
+- **ASM-001**: Relay device uses Modbus RTU over TCP protocol (per hardware documentation)
+- **ASM-002**: Relay device supports standard Modbus function codes 0x01, 0x05, 0x0F
+- **ASM-003**: Local network provides reliable connectivity (>95% uptime)
+- **ASM-004**: Traefik reverse proxy with Authelia middleware provides adequate authentication
+- **ASM-005**: Single user will control relays at a time in most scenarios (concurrent control is edge case)
+- **ASM-006**: Relay device exposes 8 coils at Modbus addresses 0-7
+- **ASM-007**: Device firmware is compatible with tokio-modbus library
+- **ASM-008**: Raspberry Pi has sufficient resources (CPU, memory) to run Rust backend
+- **ASM-009**: Cloudflare Pages or equivalent CDN provides fast frontend delivery
+- **ASM-010**: Frontend can reach backend via HTTPS through Traefik reverse proxy
+
+## Out of Scope
+
+The following capabilities are explicitly excluded from this specification:
+
+- **Application-Level Authentication**: No user login, role-based access control, or API keys (handled by Traefik/Authelia)
+- **Historical Data**: No database, state logging, or historical relay state tracking
+- **Scheduling**: No timer-based relay control or automation rules
+- **Multiple Devices**: No support for controlling multiple relay devices simultaneously
+- **Advanced Modbus Features**: No support for flash modes, timing operations, or device reconfiguration
+- **Mobile Native Apps**: Web interface only, no iOS/Android native applications
+- **Cloud Backend**: Backend runs on local network (Raspberry Pi), frontend served from Cloudflare Pages
+- **Real-time Updates**: HTTP polling only (no WebSocket, Server-Sent Events)
+
+## Risks & Mitigations
+
+| Risk                                       | Impact | Probability | Mitigation                                                             |
+|--------------------------------------------|--------|-------------|------------------------------------------------------------------------|
+| Modbus device firmware incompatibility     | High   | Low         | Test with actual hardware early, document compatible firmware versions |
+| Network latency exceeds timeout thresholds | Medium | Medium      | Make timeouts configurable, implement adaptive retry logic             |
+| Concurrent control causes state conflicts  | Low    | Medium      | Implement last-write-wins with clear state refresh in UI               |
+| Frontend polling overwhelms backend        | Low    | Low         | Rate limit API endpoints, make poll interval configurable              |
+| Raspberry Pi resource exhaustion           | Medium | Low         | Benchmark with 10 concurrent users, optimize Modbus connection pooling |
+
+## Revision History
+
+| Version | Date | Author | Changes |
+|---------|------|--------|---------|
+| 1.0 | 2025-12-28 | Business Analyst Agent | Initial specification based on user input |
+| 1.1 | 2025-12-28 | User Clarification | FR-023 clarified: Backend starts successfully even when device unhealthy, frontend displays error (part of Health story) |
+| 1.2 | 2025-12-29 | User Clarification | Architecture updated: Frontend on Cloudflare Pages, backend on RPi behind Traefik with Authelia. Updated NFR-009 to NFR-013-SEC to reflect HTTPS via reverse proxy, authentication via Traefik middleware |
--- a/specs/001-modbus-relay-control/tasks.md
+++ b/specs/001-modbus-relay-control/tasks.md
--- a/specs/001-modbus-relay-control/types-design.md
+++ b/specs/001-modbus-relay-control/types-design.md