sta/specs/001-modbus-relay-control/tasks.md

# Implementation Tasks: Modbus Relay Control System

**Feature**: 001-modbus-relay-control
**Total Tasks**: 102 tasks across 9 phases
**MVP Delivery**: Phase 4 complete (Task 57)
**Parallelizable Tasks**: 39 tasks marked with `[P]`
**Approach**: Type-Driven Development (TyDD) + Test-Driven Development (TDD), Backend API first

---

## Phase 1: Setup & Foundation (0.5 days) DONE

**Purpose**: Initialize project dependencies and directory structure

- [x] **T001** [Setup] [TDD] Add Rust dependencies to Cargo.toml
  - Add: tokio-modbus = { version = "0.17.0", default-features = false, features = ["tcp"] }, sqlx = { version = "0.8", features = ["runtime-tokio", "sqlite"] }, mockall = "0.13", async-trait = "0.1"
  - **Test**: cargo check passes
  - **Complexity**: Low | **Uncertainty**: Low

- [x] **T002** [P] [Setup] [TDD] Create module structure in src/
  - Create: src/domain/, src/application/, src/infrastructure/, src/presentation/
  - **Test**: Module declarations compile without errors
  - **Complexity**: Low | **Uncertainty**: Low

- [x] **T003** [P] [Setup] [TDD] Update settings.rs with Modbus configuration
  - Add ModbusSettings struct with `host`, `port`, `slave_id`, `timeout_secs` fields
  - Add RelaySettings struct with `label_max_length` field
  - Update Settings struct to include modbus and relay fields
  - **Test**: Settings loads from settings/base.yaml with test Modbus config
  - **Complexity**: Low | **Uncertainty**: Low

- [x] **T004** [P] [Setup] [TDD] Create settings/base.yaml with Modbus defaults
  - Add modbus section: `host: "192.168.0.200"`, `port: 502`, `slave_id: 0`, `timeout_secs: 5`
  - Add relay section: `label_max_length: 8`
  - **Test**: Settings::new() loads config without errors
  - **Complexity**: Low | **Uncertainty**: Low

- [x] **T005** [P] [Setup] [TDD] Add SQLite schema file
  - Create infrastructure/persistence/schema.sql with relay_labels table
  - Table: `relay_labels (relay_id INTEGER PRIMARY KEY CHECK(relay_id BETWEEN 1 AND 8), label TEXT NOT NULL CHECK(length(label) <= 50))`
  - **Test**: Schema file syntax is valid SQL
  - **Complexity**: Low | **Uncertainty**: Low

- [x] **T006** [P] [Setup] [TDD] Initialize SQLite database module
  - Create infrastructure/persistence/mod.rs
  - Create infrastructure/persistence/sqlite_repository.rs with SqliteRelayLabelRepository struct
  - Implement SqliteRelayLabelRepository::new(path) using SqlitePool
  - **Test**: SqliteRelayLabelRepository::in_memory() creates in-memory DB with schema
  - **Complexity**: Medium | **Uncertainty**: Low

- [x] **T007** [P] [Setup] [TDD] Add frontend project scaffolding
  - Create frontend/ directory with Vite + Vue 3 + TypeScript
  - Run: `npm create vite@latest frontend -- --template vue-ts`
  - Install: axios, @types/node
  - **Test**: `npm run dev` starts frontend dev server
  - **Complexity**: Low | **Uncertainty**: Low

- [x] **T008** [P] [Setup] [TDD] Generate TypeScript API client from OpenAPI
  - Add poem-openapi spec generation in startup.rs
  - Generate frontend/src/api/client.ts from OpenAPI spec
  - **Test**: TypeScript client compiles without errors
  - **Complexity**: Medium | **Uncertainty**: Medium
  - **Note**: May need manual adjustments to generated code

---

## Phase 0.5: CORS Configuration & Production Security (0.5 days) DONE

**Purpose**: Replace permissive `Cors::new()` with configurable production-ready CORS

**⚠️ TDD CRITICAL**: Write failing tests FIRST for every configuration and function

- [x] **T009** [P] [Setup] [TDD] Write tests for CorsSettings struct
  - Test: CorsSettings deserializes from YAML correctly ✓
  - Test: Default CorsSettings has empty allowed_origins (restrictive fail-safe) ✓
  - Test: CorsSettings with wildcard origin deserializes correctly ✓
  - Test: Settings::new() loads cors section from development.yaml ✓
  - Test: CorsSettings with partial fields uses defaults ✓
  - **File**: backend/src/settings.rs (in tests module)
  - **Complexity**: Low | **Uncertainty**: Low
  - **Tests Written**: 5 tests (cors_settings_deserialize_from_yaml, cors_settings_default_has_empty_origins, cors_settings_with_wildcard_deserializes, settings_loads_cors_section_from_yaml, cors_settings_deserialize_with_defaults)

- [x] **T010** [P] [Setup] [TDD] Add CorsSettings struct to settings.rs
  - Struct fields: `allowed_origins: Vec<String>`, `allow_credentials: bool`, `max_age_secs: i32`
  - Implement Default with restrictive settings: `allowed_origins: vec![]`, `allow_credentials: false`, `max_age_secs: 3600`
  - Add `#[derive(Debug, serde::Deserialize, Clone)]` to struct
  - Add `#[serde(default)]` attribute to Settings.cors field
  - Update Settings struct to include `pub cors: CorsSettings`
  - **File**: backend/src/settings.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [x] **T011** [Setup] [TDD] Update development.yaml with permissive CORS settings
  - Add cors section with `allowed_origins: ["*"]`, `allow_credentials: false`, `max_age_secs: 3600`
  - Update `frontend_url` from `http://localhost:3000` to `http://localhost:5173` (Vite default port)
  - **Test**: cargo run loads development config without errors
  - **File**: backend/settings/development.yaml
  - **Complexity**: Low | **Uncertainty**: Low

- [x] **T012** [P] [Setup] [TDD] Create production.yaml with restrictive CORS settings
  - Add cors section with `allowed_origins: ["REACTED"]`, `allow_credentials: true`, `max_age_secs: 3600`
  - Add `frontend_url: "https://REDACTED"`
  - Add production-specific application settings (protocol: https, host: 0.0.0.0)
  - **Test**: Settings::new() with APP_ENVIRONMENT=production loads config
  - **File**: backend/settings/production.yaml
  - **Complexity**: Low | **Uncertainty**: Low

- [x] **T013** [Setup] [TDD] Write tests for build_cors() function
  - Test: build_cors() with wildcard origin creates permissive Cors (allows any origin) ✓
  - Test: build_cors() with specific origin creates restrictive Cors ✓
  - Test: build_cors() with `credentials=true` and wildcard origin returns error (browser constraint violation) ✓
  - Test: build_cors() sets correct methods (GET, POST, PUT, PATCH, DELETE, OPTIONS) ✓
  - Test: build_cors() sets correct headers (content-type, authorization) ✓
  - Test: build_cors() sets max_age from settings ✓
  - **File**: backend/src/startup.rs (in tests module)
  - **Complexity**: Medium | **Uncertainty**: Low

- [x] **T014** [Setup] [TDD] Implement build_cors() free function in startup.rs
  - Function signature: `fn build_cors(settings: &CorsSettings) -> Cors`
  - Validate: if `allow_credentials=true` AND `allowed_origins` contains "*", panic with clear error message
  - Iterate over `allowed_origins` and call `cors.allow_origin()` for each
  - Hardcode methods: `vec![Method::GET, Method::POST, Method::PUT, Method::PATCH, Method::DELETE, Method::OPTIONS]`
  - Hardcode headers: `vec![header::CONTENT_TYPE, header::AUTHORIZATION]`
  - Set `allow_credentials` from settings
  - Set `max_age` from settings.max_age_secs
  - Add structured logging: `tracing::info!(allowed_origins = ?settings.allowed_origins, allow_credentials = settings.allow_credentials, "CORS middleware configured")`
  - **File**: backend/src/startup.rs
  - **Complexity**: Medium | **Uncertainty**: Low

  **Pseudocode**:
  ```rust
  fn build_cors(settings: &CorsSettings) -> Cors {
      // Validation
      if settings.allow_credentials && settings.allowed_origins.contains(&"*".to_string()) {
          panic!("CORS misconfiguration: wildcard origin not allowed with credentials=true");
      }

      let mut cors = Cors::new();

      // Configure origins
      for origin in &settings.allowed_origins {
          cors = cors.allow_origin(origin.as_str());
      }

      // Hardcoded methods (API-specific)
      cors = cors.allow_methods(vec![
          Method::GET, Method::POST, Method::PUT,
          Method::PATCH, Method::DELETE, Method::OPTIONS
      ]);

      // Hardcoded headers (minimum for API)
      cors = cors.allow_headers(vec![
          header::CONTENT_TYPE,
          header::AUTHORIZATION,
      ]);

      // Configure from settings
      cors = cors
          .allow_credentials(settings.allow_credentials)
          .max_age(settings.max_age_secs);

      tracing::info!(
          target: "backend::startup",
          allowed_origins = ?settings.allowed_origins,
          allow_credentials = settings.allow_credentials,
          max_age_secs = settings.max_age_secs,
          "CORS middleware configured"
      );

      cors
  }
  ```

- [x] **T015** [Setup] [TDD] Replace Cors::new() with build_cors() in middleware chain
  - In `From<Application> for RunnableApplication`, replace `.with(Cors::new())` with `.with(Cors::from(value.settings.cors.clone()))` ✓
  - CORS is applied after rate limiting (order: RateLimit → CORS → Data) ✓
  - **Test**: Unit test verifies CORS middleware uses settings ✓
  - **File**: backend/src/startup.rs (line 84)
  - **Complexity**: Low | **Uncertainty**: Low
  - **Note**: Used `From<CorsSettings> for Cors` trait instead of `build_cors()` function (better design pattern)

- [x] **T016** [P] [Setup] [TDD] Write integration tests for CORS headers
  - Test: OPTIONS preflight request to `/api/health` returns correct CORS headers ✓
  - Test: GET `/api/health` with Origin header returns `Access-Control-Allow-Origin` header ✓
  - Test: Preflight response includes `Access-Control-Max-Age` matching configuration ✓
  - Test: Response includes `Access-Control-Allow-Credentials` when configured ✓
  - Test: Response includes correct `Access-Control-Allow-Methods` (GET, POST, PUT, PATCH, DELETE, OPTIONS) ✓
  - Test: Wildcard origin behavior verified ✓
  - Test: Multiple origins are supported ✓
  - Test: Unauthorized origins are rejected with 403 ✓
  - Test: Credentials disabled by default ✓
  - **File**: backend/tests/cors_test.rs (9 integration tests)
  - **Complexity**: Medium | **Uncertainty**: Low
  - **Tests Written**: 9 comprehensive integration tests covering all CORS scenarios

**Checkpoint**: CORS configuration complete, production-ready security with environment-specific settings

---

## Phase 2: Domain Layer - Type-Driven Development (1 day)

**Purpose**: Build domain types with 100% test coverage, bottom-to-top

**⚠️ TDD CRITICAL**: Write failing tests FIRST for every type, then implement

- [x] **T017** [US1] [TDD] Write tests for RelayId newtype
  - Test: RelayId::new(1) → Ok(RelayId(1))
  - Test: RelayId::new(8) → Ok(RelayId(8))
  - Test: RelayId::new(0) → Err(InvalidRelayId)
  - Test: RelayId::new(9) → Err(InvalidRelayId)
  - Test: RelayId::as_u8() returns inner value
  - **File**: src/domain/relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T018** [US1] [TDD] Implement RelayId newtype with validation
  - #[repr(transparent)] newtype wrapping u8
  - Constructor validates 1..=8 range
  - Implement Display, Debug, Clone, Copy, PartialEq, Eq
  - **File**: src/domain/relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T019** [P] [US1] [TDD] Write tests for RelayState enum
  - Test: RelayState::On → serializes to "on"
  - Test: RelayState::Off → serializes to "off"
  - Test: Parse "on"/"off" from strings
  - **File**: src/domain/relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T020** [P] [US1] [TDD] Implement RelayState enum
  - Enum: On, Off
  - Implement Display, Debug, Clone, Copy, PartialEq, Eq, serde::Serialize/Deserialize
  - **File**: src/domain/relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T021** [US1] [TDD] Write tests for Relay aggregate
  - Test: Relay::new(RelayId(1), RelayState::Off, None) creates relay
  - Test: relay.toggle() flips state
  - Test: relay.turn_on() sets state to On
  - Test: relay.turn_off() sets state to Off
  - **File**: src/domain/relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T022** [US1] [TDD] Implement Relay aggregate
  - Struct: Relay { id: RelayId, state: RelayState, label: Option<RelayLabel> }
  - Methods: new(), toggle(), turn_on(), turn_off(), state(), label()
  - **File**: src/domain/relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T023** [P] [US4] [TDD] Write tests for RelayLabel newtype
  - Test: RelayLabel::new("Pump") → Ok
  - Test: RelayLabel::new("A".repeat(50)) → Ok
  - Test: RelayLabel::new("") → Err(EmptyLabel)
  - Test: RelayLabel::new("A".repeat(51)) → Err(LabelTooLong)
  - **File**: src/domain/relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T024** [P] [US4] [TDD] Implement RelayLabel newtype
  - #[repr(transparent)] newtype wrapping String
  - Constructor validates 1..=50 length
  - Implement Display, Debug, Clone, PartialEq, Eq
  - **File**: src/domain/relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T025** [US1] [TDD] Write tests for ModbusAddress type
  - Test: ModbusAddress::from(RelayId(1)) → ModbusAddress(0)
  - Test: ModbusAddress::from(RelayId(8)) → ModbusAddress(7)
  - **File**: src/domain/modbus.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T026** [US1] [TDD] Implement ModbusAddress type with From<RelayId>
  - #[repr(transparent)] newtype wrapping u16
  - Implement From<RelayId> with offset: user 1-8 → Modbus 0-7
  - **File**: src/domain/modbus.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T027** [US3] [TDD] Write tests and implement HealthStatus enum
  - Enum: Healthy, Degraded { consecutive_errors: u32 }, Unhealthy { reason: String }
  - Test transitions between states
  - **File**: src/domain/health.rs
  - **Complexity**: Medium | **Uncertainty**: Low

**Checkpoint**: Domain types complete with 100% test coverage

---

## Phase 3: Infrastructure Layer (2 days)

**Purpose**: Implement Modbus client, mocks, and persistence

- [ ] **T028** [P] [US1] [TDD] Write tests for MockRelayController
  - Test: read_state() returns mocked state
  - Test: write_state() updates mocked state
  - Test: read_all() returns 8 relays in known state
  - **File**: src/infrastructure/modbus/mock_controller.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T029** [P] [US1] [TDD] Implement MockRelayController
  - Struct with Arc<Mutex<HashMap<RelayId, RelayState>>>
  - Implement RelayController trait with in-memory state
  - **File**: src/infrastructure/modbus/mock_controller.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T030** [US1] [TDD] Define RelayController trait
  - async fn read_state(&self, id: RelayId) → Result<RelayState, ControllerError>
  - async fn write_state(&self, id: RelayId, state: RelayState) → Result<(), ControllerError>
  - async fn read_all(&self) → Result<Vec<(RelayId, RelayState)>, ControllerError>
  - async fn write_all(&self, state: RelayState) → Result<(), ControllerError>
  - **File**: src/infrastructure/modbus/controller.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T031** [P] [US1] [TDD] Define ControllerError enum
  - Variants: ConnectionError(String), Timeout(u64), ModbusException(String), InvalidRelayId(u8)
  - Implement std::error::Error, Display, Debug
  - Use thiserror derive macros
  - **File**: src/infrastructure/modbus/error.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T032** [US1] [TDD] Write tests for ModbusRelayController
  - **REQUIRES HARDWARE/MOCK**: Integration test with tokio_modbus::test utilities
  - Test: Connection succeeds with valid config (Modbus TCP on port 502)
  - Test: read_state() returns correct coil value
  - Test: write_state() sends correct Modbus TCP command (no CRC needed)
  - **File**: src/infrastructure/modbus/modbus_controller.rs
  - **Complexity**: High → DECOMPOSED below
  - **Uncertainty**: High

---

### T025: ModbusRelayController Implementation (DECOMPOSED)

**Complexity**: High → Broken into 6 sub-tasks
**Uncertainty**: High
**Rationale**: Nested Result handling, Arc<Mutex> synchronization, timeout wrapping
**Protocol**: Native Modbus TCP (MBAP header, no CRC16 validation)

- [ ] **T025a** [US1] [TDD] Implement ModbusRelayController connection setup
  - Struct: ModbusRelayController { ctx: Arc<Mutex<Context>>, timeout_duration: Duration }
  - Constructor: new(host, port, slave_id, timeout_secs) → Result<Self, ControllerError>
  - Use tokio_modbus::client::tcp::connect_slave()
  - **File**: src/infrastructure/modbus/modbus_controller.rs
  - **Complexity**: Medium | **Uncertainty**: Medium

  **Pseudocode**:
  ```rust
  pub struct ModbusRelayController {
      ctx: Arc<Mutex<tokio_modbus::client::Context>>,
      timeout_duration: Duration,
  }

  impl ModbusRelayController {
      pub async fn new(host: &str, port: u16, slave_id: u8, timeout_secs: u64)
          -> Result<Self, ControllerError>
      {
          use tokio_modbus::prelude::*;

          // Connect using native Modbus TCP protocol (port 502)
          let socket_addr = format!("{}:{}", host, port)
              .parse()
              .map_err(|e| ControllerError::ConnectionError(format!("Invalid address: {}", e)))?;

          let ctx = tcp::connect_slave(socket_addr, Slave(slave_id))
              .await
              .map_err(|e| ControllerError::ConnectionError(e.to_string()))?;

          Ok(Self {
              ctx: Arc::new(Mutex::new(ctx)),
              timeout_duration: Duration::from_secs(timeout_secs),
          })
      }
  }
  ```

  **TDD Checklist** (write these tests FIRST):
  - [ ] Test: new() with valid config connects successfully
  - [ ] Test: new() with invalid host returns ConnectionError
  - [ ] Test: new() stores correct timeout_duration

- [ ] **T025b** [US1] [TDD] Implement timeout-wrapped read_coils helper
  - Private method: read_coils_with_timeout(addr: u16, count: u16) → Result<Vec<bool>, ControllerError>
  - Wrap ctx.read_coils() with tokio::time::timeout()
  - Handle nested Result: timeout → io::Error → Modbus Exception
  - **Note**: Modbus TCP uses MBAP header (no CRC validation needed)
  - **File**: src/infrastructure/modbus/modbus_controller.rs
  - **Complexity**: Medium | **Uncertainty**: Medium

  **Pseudocode** (CRITICAL PATTERN):
  ```rust
  async fn read_coils_with_timeout(&self, addr: u16, count: u16)
      -> Result<Vec<bool>, ControllerError>
  {
      use tokio::time::timeout;

      let ctx = self.ctx.lock().await;

      // tokio-modbus returns nested Results: Result<Result<T, Exception>, io::Error>
      // We must unwrap 3 layers: timeout → io::Error → Modbus Exception

      let result = timeout(self.timeout_duration, ctx.read_coils(addr, count))
          .await  // Result<Result<Result<Vec<bool>, Exception>, io::Error>, Elapsed>
          .map_err(|_| ControllerError::Timeout(self.timeout_duration.as_secs()))?  // Handle timeout
          .map_err(|e| ControllerError::ConnectionError(e.to_string()))?  // Handle io::Error
          .map_err(|e| ControllerError::ModbusException(format!("{:?}", e)))?;  // Handle Exception

      Ok(result)
  }
  ```

  **TDD Checklist**:
  - [ ] Test: read_coils_with_timeout() returns coil values on success
  - [ ] Test: read_coils_with_timeout() returns Timeout error when operation exceeds timeout
  - [ ] Test: read_coils_with_timeout() returns ConnectionError on io::Error
  - [ ] Test: read_coils_with_timeout() returns ModbusException on protocol error

- [ ] **T025c** [US1] [TDD] Implement timeout-wrapped write_single_coil helper
  - Private method: write_single_coil_with_timeout(addr: u16, value: bool) → Result<(), ControllerError>
  - Similar nested Result handling as T025b
  - **File**: src/infrastructure/modbus/modbus_controller.rs
  - **Complexity**: Low | **Uncertainty**: Low

  **Pseudocode**:
  ```rust
  async fn write_single_coil_with_timeout(&self, addr: u16, value: bool)
      -> Result<(), ControllerError>
  {
      use tokio::time::timeout;

      let ctx = self.ctx.lock().await;

      timeout(self.timeout_duration, ctx.write_single_coil(addr, value))
          .await
          .map_err(|_| ControllerError::Timeout(self.timeout_duration.as_secs()))?
          .map_err(|e| ControllerError::ConnectionError(e.to_string()))?
          .map_err(|e| ControllerError::ModbusException(format!("{:?}", e)))?;

      Ok(())
  }
  ```

  **TDD Checklist**:
  - [ ] Test: write_single_coil_with_timeout() succeeds for valid write
  - [ ] Test: write_single_coil_with_timeout() returns Timeout on slow device
  - [ ] Test: write_single_coil_with_timeout() returns appropriate error on failure

- [ ] **T025d** [US1] [TDD] Implement RelayController::read_state() using helpers
  - Convert RelayId → ModbusAddress (0-based)
  - Call read_coils_with_timeout(addr, 1)
  - Convert bool → RelayState
  - **File**: src/infrastructure/modbus/modbus_controller.rs
  - **Complexity**: Low | **Uncertainty**: Low

  **Pseudocode**:
  ```rust
  #[async_trait]
  impl RelayController for ModbusRelayController {
      async fn read_state(&self, id: RelayId) -> Result<RelayState, ControllerError> {
          let addr = ModbusAddress::from(id).as_u16();
          let coils = self.read_coils_with_timeout(addr, 1).await?;

          Ok(if coils[0] { RelayState::On } else { RelayState::Off })
      }
  }
  ```

  **TDD Checklist**:
  - [ ] Test: read_state(RelayId(1)) returns On when coil is true
  - [ ] Test: read_state(RelayId(1)) returns Off when coil is false
  - [ ] Test: read_state() propagates ControllerError from helper

- [ ] **T025e** [US1] [TDD] Implement RelayController::write_state() using helpers
  - Convert RelayId → ModbusAddress
  - Convert RelayState → bool (On=true, Off=false)
  - Call write_single_coil_with_timeout()
  - **File**: src/infrastructure/modbus/modbus_controller.rs
  - **Complexity**: Low | **Uncertainty**: Low

  **Pseudocode**:
  ```rust
  async fn write_state(&self, id: RelayId, state: RelayState) -> Result<(), ControllerError> {
      let addr = ModbusAddress::from(id).as_u16();
      let value = matches!(state, RelayState::On);
      self.write_single_coil_with_timeout(addr, value).await
  }
  ```

  **TDD Checklist**:
  - [ ] Test: write_state(RelayId(1), RelayState::On) writes true to coil
  - [ ] Test: write_state(RelayId(1), RelayState::Off) writes false to coil

- [ ] **T025f** [US1] [TDD] Implement RelayController::read_all() and write_all()
  - read_all(): Call read_coils_with_timeout(0, 8), map to Vec<(RelayId, RelayState)>
  - write_all(): Loop over RelayId 1-8, call write_state() for each
  - Add firmware_version() method (read holding register 0x9999, optional)
  - **File**: src/infrastructure/modbus/modbus_controller.rs
  - **Complexity**: Medium | **Uncertainty**: Low

  **Pseudocode**:
  ```rust
  async fn read_all(&self) -> Result<Vec<(RelayId, RelayState)>, ControllerError> {
      let coils = self.read_coils_with_timeout(0, 8).await?;

      let mut relays = Vec::new();
      for (idx, &coil_value) in coils.iter().enumerate() {
          let relay_id = RelayId::new((idx + 1) as u8)?;
          let state = if coil_value { RelayState::On } else { RelayState::Off };
          relays.push((relay_id, state));
      }
      Ok(relays)
  }

  async fn write_all(&self, state: RelayState) -> Result<(), ControllerError> {
      for i in 1..=8 {
          let relay_id = RelayId::new(i)?;
          self.write_state(relay_id, state).await?;
      }
      Ok(())
  }
  ```

  **TDD Checklist**:
  - [ ] Test: read_all() returns 8 relay states
  - [ ] Test: write_all(RelayState::On) turns all relays on
  - [ ] Test: write_all(RelayState::Off) turns all relays off

---

- [ ] **T034** [US1] [TDD] Integration test with real hardware (optional)
  - **REQUIRES PHYSICAL DEVICE**: Test against actual Modbus relay at configured IP
  - Skip if device unavailable, rely on MockRelayController for CI
  - **File**: tests/integration/modbus_hardware_test.rs
  - **Complexity**: Medium | **Uncertainty**: High
  - **Note**: Use #[ignore] attribute, run with cargo test -- --ignored

- [ ] **T035** [P] [US4] [TDD] Write tests for RelayLabelRepository trait
  - Test: get_label(RelayId(1)) → Option<RelayLabel>
  - Test: set_label(RelayId(1), label) → Result<(), RepositoryError>
  - Test: delete_label(RelayId(1)) → Result<(), RepositoryError>
  - **File**: src/infrastructure/persistence/label_repository.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T036** [P] [US4] [TDD] Implement SQLite RelayLabelRepository
  - Implement get_label(), set_label(), delete_label() using SQLx
  - Use sqlx::query! macros for compile-time SQL verification
  - **File**: src/infrastructure/persistence/sqlite_label_repository.rs
  - **Complexity**: Medium | **Uncertainty**: Low

- [ ] **T037** [US4] [TDD] Write tests for in-memory mock LabelRepository
  - For testing without SQLite dependency
  - **File**: src/infrastructure/persistence/mock_label_repository.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T038** [US4] [TDD] Implement in-memory mock LabelRepository
  - HashMap-based implementation
  - **File**: src/infrastructure/persistence/mock_label_repository.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T039** [US3] [TDD] Write tests for HealthMonitor service
  - Test: track_success() transitions Degraded → Healthy
  - Test: track_failure() transitions Healthy → Degraded → Unhealthy
  - **File**: src/application/health_monitor.rs
  - **Complexity**: Medium | **Uncertainty**: Low

- [ ] **T040** [US3] [TDD] Implement HealthMonitor service
  - Track consecutive errors, transition states per FR-020, FR-021
  - **File**: src/application/health_monitor.rs
  - **Complexity**: Medium | **Uncertainty**: Low

**Checkpoint**: Infrastructure layer complete with trait abstractions

---

## Phase 4: US1 - Monitor & Toggle Relay States (MVP) (2 days)

**Goal**: View current state of all 8 relays + toggle individual relay on/off

**Independent Test**: GET /api/relays returns 8 relays, POST /api/relays/{id}/toggle changes state

### Application Layer

- [ ] **T041** [US1] [TDD] Write tests for ToggleRelayUseCase
  - Test: execute(RelayId(1)) toggles relay state via controller
  - Test: execute() returns error if controller fails
  - **File**: src/application/use_cases/toggle_relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T042** [US1] [TDD] Implement ToggleRelayUseCase
  - Orchestrate: read current state → toggle → write new state
  - **File**: src/application/use_cases/toggle_relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T043** [P] [US1] [TDD] Write tests for GetAllRelaysUseCase
  - Test: execute() returns all 8 relays with states
  - **File**: src/application/use_cases/get_all_relays.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T044** [P] [US1] [TDD] Implement GetAllRelaysUseCase
  - Call controller.read_all(), map to domain Relay objects
  - **File**: src/application/use_cases/get_all_relays.rs
  - **Complexity**: Low | **Uncertainty**: Low

### Presentation Layer (Backend API)

- [ ] **T045** [US1] [TDD] Define RelayDto in presentation layer
  - Fields: id (u8), state ("on"/"off"), label (Option<String>)
  - Implement From<Relay> for RelayDto
  - **File**: src/presentation/dto/relay_dto.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T046** [US1] [TDD] Define API error responses
  - ApiError enum with status codes and messages
  - Implement poem::error::ResponseError
  - **File**: src/presentation/error.rs
  - **Complexity**: Low | **Uncertainty**: Low

---

### T039: Dependency Injection Setup (DECOMPOSED)

**Complexity**: High → Broken into 4 sub-tasks
**Uncertainty**: Medium
**Rationale**: Graceful degradation (FR-023), conditional mock/real controller

- [ ] **T039a** [US1] [TDD] Create ModbusRelayController factory with retry and fallback
  - Factory function: create_relay_controller(settings, use_mock) → Arc<dyn RelayController>
  - Retry 3 times with 2s backoff on connection failure
  - Graceful degradation: fallback to MockRelayController if all retries fail (FR-023)
  - **File**: src/infrastructure/modbus/factory.rs
  - **Complexity**: Medium | **Uncertainty**: Medium

  **Pseudocode**:
  ```rust
  pub async fn create_relay_controller(
      settings: &ModbusSettings,
      use_mock: bool,
  ) -> Arc<dyn RelayController> {
      if use_mock {
          tracing::info!("Using MockRelayController (test mode)");
          return Arc::new(MockRelayController::new());
      }

      // Retry 3 times with 2s backoff
      for attempt in 1..=3 {
          match ModbusRelayController::new(
              &settings.host,
              settings.port,
              settings.slave_id,
              settings.timeout_secs,
          ).await {
              Ok(controller) => {
                  tracing::info!("Connected to Modbus device on attempt {}", attempt);
                  return Arc::new(controller);
              }
              Err(e) => {
                  tracing::warn!(
                      attempt,
                      error = %e,
                      "Failed to connect to Modbus device, retrying..."
                  );
                  if attempt < 3 {
                      tokio::time::sleep(Duration::from_secs(2)).await;
                  }
              }
          }
      }

      // Graceful degradation: fallback to MockRelayController
      tracing::error!(
          "Could not connect to Modbus device after 3 attempts, \
           using MockRelayController as fallback"
      );
      Arc::new(MockRelayController::new())
  }
  ```

  **TDD Checklist**:
  - [ ] Test: use_mock=true returns MockRelayController immediately
  - [ ] Test: Successful connection returns ModbusRelayController
  - [ ] Test: Connection failure after 3 retries returns MockRelayController
  - [ ] Test: Retry delays are 2 seconds between attempts
  - [ ] Test: Logs appropriate messages for each connection attempt

- [ ] **T039b** [US4] [TDD] Create RelayLabelRepository factory
  - Factory function: create_label_repository(db_path, use_mock) → Arc<dyn RelayLabelRepository>
  - If use_mock: return MockLabelRepository
  - Else: return SQLiteLabelRepository connected to db_path
  - **File**: src/infrastructure/persistence/factory.rs
  - **Complexity**: Low | **Uncertainty**: Low

  **Pseudocode**:
  ```rust
  pub fn create_label_repository(
      db_path: &str,
      use_mock: bool,
  ) -> Result<Arc<dyn RelayLabelRepository>, RepositoryError> {
      if use_mock {
          tracing::info!("Using MockLabelRepository (test mode)");
          return Ok(Arc::new(MockLabelRepository::new()));
      }

      let db = Database::new(db_path)?;
      Ok(Arc::new(SQLiteLabelRepository::new(db)))
  }
  ```

  **TDD Checklist**:
  - [ ] Test: use_mock=true returns MockLabelRepository
  - [ ] Test: use_mock=false returns SQLiteLabelRepository
  - [ ] Test: Invalid db_path returns RepositoryError

- [ ] **T039c** [US1] [TDD] Wire dependencies in Application::build()
  - Determine test mode: cfg!(test) || env::var("CI").is_ok()
  - Call create_relay_controller() and create_label_repository()
  - Pass dependencies to RelayApi::new()
  - **File**: src/startup.rs
  - **Complexity**: Medium | **Uncertainty**: Low

  **Pseudocode**:
  ```rust
  impl Application {
      pub async fn build(settings: Settings) -> Result<Self, StartupError> {
          let use_mock = cfg!(test) || std::env::var("CI").is_ok();

          // Create dependencies
          let relay_controller = create_relay_controller(&settings.modbus, use_mock).await;
          let label_repository = create_label_repository(&settings.database.path, use_mock)?;

          // Create API with dependencies
          let relay_api = RelayApi::new(relay_controller, label_repository);

          // Build OpenAPI service
          let api_service = OpenApiService::new(relay_api, "STA API", "1.0.0")
              .server("http://localhost:8080");

          let ui = api_service.swagger_ui();
          let spec = api_service.spec();

          let app = Route::new()
              .nest("/api", api_service)
              .nest("/", ui)
              .at("/openapi.json", poem::endpoint::make_sync(move |_| spec.clone()));

          Ok(Self { app, settings })
      }
  }
  ```

  **TDD Checklist**:
  - [ ] Test: Application::build() succeeds in test mode
  - [ ] Test: Application::build() creates correct mock dependencies when CI=true
  - [ ] Test: Application::build() creates real dependencies when not in test mode

- [ ] **T039d** [US1] [TDD] Register RelayApi in route aggregator
  - Add RelayApi to OpenAPI service
  - Tag: "Relays"
  - **File**: src/startup.rs
  - **Complexity**: Low | **Uncertainty**: Low

  **TDD Checklist**:
  - [ ] Test: OpenAPI spec includes /api/relays endpoints
  - [ ] Test: Swagger UI renders Relays tag

---

- [ ] **T048** [US1] [TDD] Write contract tests for GET /api/relays
  - Test: Returns 200 with array of 8 RelayDto
  - Test: Each relay has id 1-8, state, and optional label
  - **File**: tests/contract/test_relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T049** [US1] [TDD] Implement GET /api/relays endpoint
  - #[oai(path = "/relays", method = "get")]
  - Call GetAllRelaysUseCase, map to RelayDto
  - **File**: src/presentation/api/relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T050** [US1] [TDD] Write contract tests for POST /api/relays/{id}/toggle
  - Test: Returns 200 with updated RelayDto
  - Test: Returns 404 for id < 1 or id > 8
  - Test: State actually changes in controller
  - **File**: tests/contract/test_relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T051** [US1] [TDD] Implement POST /api/relays/{id}/toggle endpoint
  - #[oai(path = "/relays/:id/toggle", method = "post")]
  - Parse id, call ToggleRelayUseCase, return updated state
  - **File**: src/presentation/api/relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

### Frontend Implementation

- [ ] **T052** [P] [US1] [TDD] Create RelayDto TypeScript interface
  - Generate from OpenAPI spec or manually define
  - **File**: frontend/src/types/relay.ts
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T053** [P] [US1] [TDD] Create API client service
  - getAllRelays(): Promise<RelayDto[]>
  - toggleRelay(id: number): Promise<RelayDto>
  - **File**: frontend/src/api/relayApi.ts
  - **Complexity**: Low | **Uncertainty**: Low

---

### T046: HTTP Polling Composable (DECOMPOSED)

**Complexity**: High → Broken into 4 sub-tasks
**Uncertainty**: Medium
**Rationale**: Vue 3 lifecycle hooks, polling management, memory leak prevention

- [ ] **T046a** [US1] [TDD] Create useRelayPolling composable structure
  - Setup reactive refs: relays, isLoading, error, lastFetchTime
  - Define interval variable and fetch function signature
  - **File**: frontend/src/composables/useRelayPolling.ts
  - **Complexity**: Low | **Uncertainty**: Low

  **Pseudocode**:
  ```typescript
  import { ref, Ref } from 'vue';
  import type { RelayDto } from '@/types/relay';

  export function useRelayPolling(intervalMs: number = 2000) {
    const relays: Ref<RelayDto[]> = ref([]);
    const isLoading = ref(true);
    const error: Ref<string | null> = ref(null);
    const lastFetchTime: Ref<Date | null> = ref(null);
    const isConnected = ref(false);

    let pollingInterval: number | null = null;

    // TODO: Implement fetchData, startPolling, stopPolling

    return {
      relays,
      isLoading,
      error,
      isConnected,
      lastFetchTime,
      refresh: fetchData,
      startPolling,
      stopPolling,
    };
  }
  ```

  **TDD Checklist**:
  - [ ] Test: Composable returns correct reactive refs
  - [ ] Test: Initial state is loading=true, relays=[], error=null

- [ ] **T046b** [US1] [TDD] Implement fetchData with parallel requests
  - Fetch relays and health status in parallel using Promise.all
  - Update reactive state on success
  - Handle errors gracefully, set isConnected based on success
  - **File**: frontend/src/composables/useRelayPolling.ts
  - **Complexity**: Medium | **Uncertainty**: Low

  **Pseudocode**:
  ```typescript
  const fetchData = async () => {
    try {
      const [relayData, healthData] = await Promise.all([
        apiClient.getAllRelays(),
        apiClient.getHealth(),
      ]);

      relays.value = relayData.relays;
      isConnected.value = healthData.status === 'healthy';
      lastFetchTime.value = new Date();
      error.value = null;
    } catch (err: any) {
      error.value = err.message || 'Failed to fetch relay data';
      isConnected.value = false;
      console.error('Polling error:', err);
    } finally {
      isLoading.value = false;
    }
  };
  ```

  **TDD Checklist**:
  - [ ] Test: fetchData() updates relays on success
  - [ ] Test: fetchData() sets error on API failure
  - [ ] Test: fetchData() sets isLoading=false after completion
  - [ ] Test: fetchData() updates lastFetchTime

- [ ] **T046c** [US1] [TDD] Implement polling lifecycle with cleanup
  - startPolling(): Fetch immediately, then setInterval
  - stopPolling(): clearInterval and cleanup
  - Use onMounted/onUnmounted for automatic lifecycle management
  - **File**: frontend/src/composables/useRelayPolling.ts
  - **Complexity**: Medium | **Uncertainty**: Low

  **Pseudocode**:
  ```typescript
  import { onMounted, onUnmounted } from 'vue';

  const startPolling = () => {
    if (pollingInterval !== null) return; // Already polling

    fetchData(); // Immediate first fetch
    pollingInterval = window.setInterval(fetchData, intervalMs);
  };

  const stopPolling = () => {
    if (pollingInterval !== null) {
      clearInterval(pollingInterval);
      pollingInterval = null;
    }
  };

  // CRITICAL: Lifecycle cleanup to prevent memory leaks
  onMounted(() => {
    startPolling();
  });

  onUnmounted(() => {
    stopPolling();
  });
  ```

  **TDD Checklist**:
  - [ ] Test: startPolling() triggers immediate fetch
  - [ ] Test: startPolling() sets interval for subsequent fetches
  - [ ] Test: stopPolling() clears interval
  - [ ] Test: onUnmounted hook calls stopPolling()

- [ ] **T046d** [US1] [TDD] Add connection status tracking
  - Track isConnected based on fetch success/failure
  - Display connection status in UI
  - **File**: frontend/src/composables/useRelayPolling.ts
  - **Complexity**: Low | **Uncertainty**: Low

  **Pseudocode**:
  ```typescript
  // Already implemented in T046b, just ensure it's exposed
  return {
    relays,
    isLoading,
    error,
    isConnected,  // ← Connection status indicator
    lastFetchTime,
    refresh: fetchData,
    startPolling,
    stopPolling,
  };
  ```

  **TDD Checklist**:
  - [ ] Test: isConnected is true after successful fetch
  - [ ] Test: isConnected is false after failed fetch

---

- [ ] **T055** [US1] [TDD] Create RelayCard component
  - Props: relay (RelayDto)
  - Display relay ID, state, label
  - Emit toggle event on button click
  - **File**: frontend/src/components/RelayCard.vue
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T056** [US1] [TDD] Create RelayGrid component
  - Use useRelayPolling composable
  - Render 8 RelayCard components
  - Handle toggle events by calling API
  - Display loading/error states
  - **File**: frontend/src/components/RelayGrid.vue
  - **Complexity**: Medium | **Uncertainty**: Low

- [ ] **T057** [US1] [TDD] Integration test for US1
  - End-to-end test: Load page → see 8 relays → toggle relay 1 → verify state change
  - Use Playwright or Cypress
  - **File**: frontend/tests/e2e/relay-control.spec.ts
  - **Complexity**: Medium | **Uncertainty**: Medium

**Checkpoint**: US1 MVP complete - users can view and toggle individual relays

---

## Phase 5: US2 - Bulk Relay Controls (0.5 days)

**Goal**: Turn all relays on/off with single action

**Independent Test**: POST /api/relays/all/on turns all 8 relays on

- [ ] **T058** [US2] [TDD] Write tests for BulkControlUseCase
  - Test: execute(BulkOperation::AllOn) turns all relays on
  - Test: execute(BulkOperation::AllOff) turns all relays off
  - **File**: src/application/use_cases/bulk_control.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T059** [US2] [TDD] Implement BulkControlUseCase
  - Call controller.write_all(state)
  - **File**: src/application/use_cases/bulk_control.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T060** [US2] [TDD] Define BulkOperation enum
  - Variants: AllOn, AllOff
  - **File**: src/domain/relay.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T061** [US2] [TDD] Write contract tests for POST /api/relays/all/on
  - Test: Returns 200, all relays turn on
  - **File**: tests/contract/test_relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T062** [US2] [TDD] Implement POST /api/relays/all/on endpoint
  - Call BulkControlUseCase with AllOn
  - **File**: src/presentation/api/relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T063** [P] [US2] [TDD] Write contract tests for POST /api/relays/all/off
  - Test: Returns 200, all relays turn off
  - **File**: tests/contract/test_relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T064** [P] [US2] [TDD] Implement POST /api/relays/all/off endpoint
  - Call BulkControlUseCase with AllOff
  - **File**: src/presentation/api/relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T065** [US2] [TDD] Add bulk control buttons to frontend
  - Add "All On" and "All Off" buttons to RelayGrid component
  - Call API endpoints and refresh relay states
  - **File**: frontend/src/components/RelayGrid.vue
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T066** [US2] [TDD] Integration test for US2
  - Click "All On" → verify all 8 relays turn on
  - Click "All Off" → verify all 8 relays turn off
  - **File**: frontend/tests/e2e/bulk-control.spec.ts
  - **Complexity**: Low | **Uncertainty**: Low

**Checkpoint**: US2 complete - bulk controls functional

---

## Phase 6: US3 - Health Monitoring (1 day)

**Goal**: Display connection status and device health

**Independent Test**: GET /api/health returns health status

- [ ] **T067** [US3] [TDD] Write tests for GetHealthUseCase
  - Test: Returns Healthy when controller is responsive
  - Test: Returns Degraded after 3 consecutive errors
  - Test: Returns Unhealthy after 10 consecutive errors
  - **File**: src/application/use_cases/get_health.rs
  - **Complexity**: Medium | **Uncertainty**: Low

- [ ] **T068** [US3] [TDD] Implement GetHealthUseCase
  - Use HealthMonitor to track controller status
  - Return current HealthStatus
  - **File**: src/application/use_cases/get_health.rs
  - **Complexity**: Medium | **Uncertainty**: Low

- [ ] **T069** [US3] [TDD] Define HealthDto
  - Fields: status ("healthy"/"degraded"/"unhealthy"), consecutive_errors (optional), reason (optional)
  - **File**: src/presentation/dto/health_dto.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T070** [US3] [TDD] Write contract tests for GET /api/health
  - Test: Returns 200 with HealthDto
  - **File**: tests/contract/test_health_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T071** [US3] [TDD] Implement GET /api/health endpoint
  - Call GetHealthUseCase, map to HealthDto
  - **File**: src/presentation/api/health_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T072** [P] [US3] [TDD] Add firmware version display (optional)
  - If controller supports firmware_version(), display in UI
  - **File**: frontend/src/components/DeviceInfo.vue
  - **Complexity**: Low | **Uncertainty**: Medium
  - **Note**: Device may not support this feature

- [ ] **T073** [US3] [TDD] Create HealthIndicator component
  - Display connection status with color-coded indicator
  - Show firmware version if available
  - **File**: frontend/src/components/HealthIndicator.vue
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T074** [US3] [TDD] Integrate HealthIndicator in RelayGrid
  - Fetch health status in useRelayPolling composable
  - Pass to HealthIndicator component
  - **File**: frontend/src/components/RelayGrid.vue
  - **Complexity**: Low | **Uncertainty**: Low

**Checkpoint**: US3 complete - health monitoring visible

---

## Phase 7: US4 - Relay Labeling (0.5 days)

**Goal**: Set custom labels for each relay

**Independent Test**: PUT /api/relays/{id}/label sets label, GET /api/relays returns label

- [ ] **T075** [US4] [TDD] Write tests for SetLabelUseCase
  - Test: execute(RelayId(1), "Pump") sets label
  - Test: execute with empty label returns error
  - Test: execute with 51-char label returns error
  - **File**: src/application/use_cases/set_label.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T076** [US4] [TDD] Implement SetLabelUseCase
  - Validate label with RelayLabel::new()
  - Call label_repository.set_label()
  - **File**: src/application/use_cases/set_label.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T077** [US4] [TDD] Write contract tests for PUT /api/relays/{id}/label
  - Test: Returns 200, label is persisted
  - Test: Returns 400 for invalid label
  - **File**: tests/contract/test_relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T078** [US4] [TDD] Implement PUT /api/relays/{id}/label endpoint
  - Parse id and label, call SetLabelUseCase
  - **File**: src/presentation/api/relay_api.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T079** [US4] [TDD] Add label editing to RelayCard component
  - Click label → show input field
  - Submit → call PUT /api/relays/{id}/label
  - **File**: frontend/src/components/RelayCard.vue
  - **Complexity**: Medium | **Uncertainty**: Low

- [ ] **T080** [US4] [TDD] Integration test for US4
  - Set label for relay 1 → refresh → verify label persists
  - **File**: frontend/tests/e2e/relay-labeling.spec.ts
  - **Complexity**: Low | **Uncertainty**: Low

**Checkpoint**: US4 complete - relay labeling functional

---

## Phase 8: Polish & Deployment (1 day)

**Purpose**: Testing, documentation, and production readiness

- [ ] **T081** [P] Add comprehensive logging at all architectural boundaries
  - Log all API requests/responses
  - Log all Modbus operations
  - Log health status transitions
  - **Files**: All API and infrastructure modules
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T082** [P] Add OpenAPI documentation for all endpoints
  - Document request/response schemas
  - Add example values
  - Tag endpoints appropriately
  - **File**: src/presentation/api/*.rs
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T083** [P] Run cargo clippy and fix all warnings
  - Ensure compliance with strict linting
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T084** [P] Run cargo fmt and format all code
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T085** Generate test coverage report
  - Run: just coverage
  - Ensure > 80% coverage for domain and application layers
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T086** [P] Run cargo audit for dependency vulnerabilities
  - Fix any high/critical vulnerabilities
  - **Complexity**: Low | **Uncertainty**: Medium

- [ ] **T087** [P] Update README.md with deployment instructions
  - Document environment variables
  - Document Modbus device configuration
  - Add quickstart guide
  - **File**: README.md
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T088** [P] Create Docker image for backend
  - Multi-stage build with Rust
  - Include SQLite database setup
  - **File**: Dockerfile
  - **Complexity**: Medium | **Uncertainty**: Low

- [ ] **T089** [P] Create production settings/production.yaml
  - Configure for actual device IP
  - Set appropriate timeouts and retry settings
  - **File**: settings/production.yaml
  - **Complexity**: Low | **Uncertainty**: Low

- [ ] **T090** Deploy to production environment
  - Test with actual Modbus relay device
  - Verify all user stories work end-to-end
  - **Complexity**: Medium | **Uncertainty**: High

**Checkpoint**: Production ready, all user stories validated

---

## Dependencies & Execution Order

### Phase Dependencies

1. **Phase 1 (Setup)**: No dependencies - start immediately
2. **Phase 2 (Domain TyDD)**: Depends on Phase 1 module structure
3. **Phase 3 (Infrastructure)**: Depends on Phase 2 domain types
4. **Phase 4 (US1 MVP)**: Depends on Phase 3 infrastructure
5. **Phase 5 (US2)**: Depends on Phase 4 backend API complete
6. **Phase 6 (US3)**: Depends on Phase 4 backend API complete (can parallelize with US2)
7. **Phase 7 (US4)**: Depends on Phase 4 backend API complete (can parallelize with US2/US3)
8. **Phase 8 (Polish)**: Depends on all desired user stories complete

### User Story Independence

- **US1**: No dependencies on other stories
- **US2**: Reuses US1 backend infrastructure, but independently testable
- **US3**: Reuses US1 backend infrastructure, but independently testable
- **US4**: Reuses US1 backend infrastructure, adds new persistence layer

### Critical Path

**MVP (US1 only)**: Phase 1 → Phase 2 → Phase 3 → Phase 4 (5 days)
**Full Feature**: MVP + Phase 5 + Phase 6 + Phase 7 + Phase 8 (7 days)

### Parallel Opportunities

- **Phase 1**: T002, T003, T004, T005, T006, T007, T008 can run in parallel
- **Phase 2**: T011, T015 can run in parallel
- **Phase 3**: T020, T027, T028, T029, T030 can run in parallel after T022 complete
- **Phase 4**: T035, T044, T045 can run in parallel
- **After Phase 4**: US2, US3, US4 can be developed in parallel by different developers
- **Phase 8**: T073, T074, T075, T076, T078, T079, T080, T081 can run in parallel

**Total Parallelizable Tasks**: 35 tasks marked `[P]`

---

## Test-Driven Development Workflow

**CRITICAL**: For every task marked `[TDD]`, follow this exact sequence:

1. **Write failing test FIRST** (red)
2. **Verify test fails** for the right reason
3. **Implement minimum code** to pass test (green)
4. **Refactor** while keeping tests green
5. **Commit** after each task or logical group

### Example TDD Workflow (T010):

```bash
# 1. Write failing test for RelayId::new() validation
# In src/domain/relay.rs:
#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_relay_id_valid_range() {
        assert!(RelayId::new(1).is_ok());
        assert!(RelayId::new(8).is_ok());
    }

    #[test]
    fn test_relay_id_invalid_range() {
        assert!(RelayId::new(0).is_err());
        assert!(RelayId::new(9).is_err());
    }
}

# 2. Run test → VERIFY IT FAILS
cargo test test_relay_id

# 3. Implement RelayId to make test pass
# 4. Run test again → VERIFY IT PASSES
# 5. Refactor if needed, keep tests green
# 6. Commit
jj describe -m "feat: implement RelayId with validation (T010)"
```

---

## Notes

- **[P]** = Parallelizable (different files, no dependencies)
- **[US1/US2/US3/US4]** = User story mapping for traceability
- **[TDD]** = Test-Driven Development required
- **Complexity**: Low (< 1 hour) | Medium (1-3 hours) | High (> 3 hours or decomposed)
- **Uncertainty**: Low (clear path) | Medium (some unknowns) | High (requires research/spike)
- Commit after each task or logical group using `jj describe` or `jj commit`
- MVP delivery at task T049 (end of Phase 4)
- Stop at any checkpoint to independently validate user story