#+title: Implementation Tasks: Modbus Relay Control System
#+author: Lucien Cartier-Tilet
#+email: lucien@phundrak.com
#+options: ^:nil
#+LATEX_CLASS_OPTIONS: [a4paper,10pt]
#+LATEX_HEADER: \makeatletter \@ifpackageloaded{geometry}{\geometry{margin=2cm}}{\usepackage[margin=2cm]{geometry}} \makeatother
#+LATEX_HEADER: \setlength{\parindent}{0pt}
#+latex_header: \sloppy
#+latex_header: \usepackage[none]{hyphenat}
#+latex_header: \raggedright
#+todo: TODO(t) STARTED(s!) | DONE(d!)

#+begin_src emacs-lisp :exports none :results none
(defun org-summary-todo (n-done n-not-done)
  "Switch entry to DONE when all subentries are done, to TODO otherwise."
  (let (org-log-done org-todo-log-states)   ; turn off logging
    (org-todo (if (= n-not-done 0) "DONE" "TODO"))))

(add-hook 'org-after-todo-statistics-hook #'org-summary-todo)
#+end_src

- 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

--------------

* Development phases [0/0]
** DONE Phase 1: Setup & Foundation (0.5 days) [8/8]
*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

--------------

** DONE Phase 0.5: CORS Configuration & Production Security (0.5 days) [8/8]
*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*:

  #+begin_src 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
  }
  #+end_src

- [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

--------------

** DONE Phase 2: Domain Layer - Type-Driven Development (1 day) [11/11]
*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
- [X] *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
- [X] *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
- [X] *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
- [X] *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
- [X] *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
- [X] *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
- [X] *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
- [X] *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
- [X] *T026* [US1] [TDD] Implement =ModbusAddress= type with From
  - =#[repr(transparent)]= newtype wrapping =u16=
  - Implement From with offset: user 1-8 → Modbus 0-7
  - *File*: =src/domain/modbus.rs=
  - *Complexity*: Low | *Uncertainty*: Low
- [X] *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

--------------

** TODO Phase 3: Infrastructure Layer (2 days) [5/5]
*Purpose*: Implement Modbus client, mocks, and persistence

- [X] *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
- [X] *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
- [X] *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
- [X] *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
- [X] *T032* [US1] [TDD] Write tests for =MockRelayController=
  - Test: =read_relay_state()= returns mocked state ✓
  - Test: =write_relay_state()= updates mocked state ✓
  - Test: =read_all_states()= returns 8 relays in known state ✓
  - Test: =write_relay_state()= for all 8 relays independently ✓
  - Test: =read_relay_state()= with invalid relay ID (type system prevents) ✓
  - Test: concurrent access is thread-safe ✓
  - *File*: =src/infrastructure/modbus/mock_controller.rs=
  - *Complexity*: Low | *Uncertainty*: Low
  - *Tests Written*: 6 comprehensive tests covering all mock controller scenarios

--------------

*** STARTED T025: ModbusRelayController Implementation (DECOMPOSED) [9/13]
- 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)

- [X] *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*:

  #+begin_src 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),
          })
      }
  }
  #+end_src

  *TDD Checklist* (write these tests FIRST):

  - [X] Test: =new()= with valid config connects successfully
  - [X] Test: =new()= with invalid host returns =ConnectionError=
  - [X] Test: =new()= stores correct timeout_duration

- [X] *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):

  #+begin_src 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)
  }
  #+end_src

  *TDD Checklist*:

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

- [X] *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*:

  #+begin_src 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(())
  }
  #+end_src

  *TDD Checklist*:

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

- [X] *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*:

  #+begin_src 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 })
      }
  }
  #+end_src

  *TDD Checklist*:

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

- [X] *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*:

  #+begin_src 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
  }
  #+end_src

  *TDD Checklist*:

  - [X] Test: =write_state(RelayId(1), RelayState::On)= writes true to coil
  - [X] Test: =write_state(RelayId(1), RelayState::Off)= writes false to coil

- [X] *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*:

  #+begin_src 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(())
  }
  #+end_src

  *TDD Checklist*:

  - [X] Test: =read_all()= returns 8 relay states
  - [X] Test: =write_all(RelayState::On)= turns all relays on
  - [X] 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=
- [X] *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
- [X] *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
- [X] *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

--------------

** TODO Phase 4: US1 - Monitor & Toggle Relay States (MVP) (2 days) [0/5]
*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

*** TODO Application Layer [0/4]
- [ ] *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

*** TODO Presentation Layer (Backend API) [0/2]
- [ ] *T045* [US1] [TDD] Define =RelayDto= in presentation layer
  - Fields: =id= (=u8=), =state= ("on"/"off"), =label= (=Option=)
  - Implement =From= 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

--------------

*** TODO T039: Dependency Injection Setup (DECOMPOSED) [0/8]
- 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~
  - 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*:

  #+begin_src 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())
  }
  #+end_src

  *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 =RelayLabelRepositor=y factory

  - Factory function: ~create_label_repository(db_path, use_mock) => Arc~
  - If use_mock: return =MockLabelRepository=
  - Else: return =SQLiteLabelRepository= connected to =db_path=
  - *File*: =src/infrastructure/persistence/factory.rs=
  - *Complexity*: Low | *Uncertainty*: Low

  *Pseudocode*:

  #+begin_src 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)))
  }
  #+end_src

  *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*:

  #+begin_src 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 })
      }
  }
  #+end_src

  *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

*** TODO Frontend Implementation [0/2]
- [ ] *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=
  - *File*: =frontend/src/api/relayApi.ts=
  - *Complexity*: Low | *Uncertainty*: Low

--------------

*** TODO T046: HTTP Polling Composable (DECOMPOSED) [0/7]
*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*:

  #+begin_src 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,
    };
  }
  #+end_src

  *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*:

  #+begin_src 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;
    }
  };
  #+end_src

  *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*:

  #+begin_src 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();
  });
  #+end_src

  *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*:

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

  *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

--------------

** TODO Phase 5: US2 - Bulk Relay Controls (0.5 days) [0/9]
*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

--------------

** TODO Phase 6: US3 - Health Monitoring (1 day) [0/8]
*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

--------------

** TODO Phase 7: US4 - Relay Labeling (0.5 days) [0/6]
*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

--------------

** TODO Phase 8: Polish & Deployment (1 day) [0/10]
*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):
#+begin_src sh
# 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)"
#+end_src

--------------

* 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