# Phase 4: Application Layer + Background Workers

**Status:** ⬜ Not Started
**Parent plan:** [PLAN.md](./PLAN.md)
**Domain:** backend
**Depends on:** Phase 1 (Domain), Phase 2 (Storage), Phase 3 (Scraping)

## Objective

Wire scraping + storage together via use-case orchestrators in the Application layer
and background services that execute pollers on configurable intervals.

## Tasks

- [ ] Implement use cases in `Marathon.Application/UseCases/`:
  - `PullUpcomingEventsUseCase(IOddsScraper, IEventRepository, ISnapshotRepository)`
    - `ExecuteAsync(CancellationToken)` → fetch upcoming events, persist new ones,
      capture initial pre-match snapshots for each
  - `PullLiveOddsUseCase(IOddsScraper, IEventRepository, ISnapshotRepository)`
    - `ExecuteAsync(CancellationToken)` → for each currently-live event, fetch a
      fresh snapshot, persist it
  - `PullResultsUseCase(IOddsScraper, IEventRepository, IResultRepository)`
    - `ExecuteAsync(DateRange range, IReadOnlyList<EventId>? selection, CancellationToken)`
      → fetch results for completed events (all or selected)
  - `ExportToExcelUseCase(IExcelExporter, IEventRepository)`
    - `ExecuteAsync(DateRange, ExportKind, CancellationToken)`
- [ ] Implement background services in `Marathon.Infrastructure/Workers/`:
  - `UpcomingEventsPoller : BackgroundService` — runs `PullUpcomingEventsUseCase` on
    a configurable cron-like schedule (default: every 6 hours)
  - `LiveOddsPoller : BackgroundService` — runs `PullLiveOddsUseCase` every
    `Scraping:PollingIntervalSeconds` seconds
  - Both honor `CancellationToken`, log via `ILogger<T>`, and skip cycles gracefully
    on errors (don't crash the host)
- [ ] Add `WorkerOptions` POCO bound to `Workers:*` config:
  ```csharp
  public sealed class WorkerOptions {
    public string UpcomingScheduleCron { get; init; } = "0 0 */6 * * *"; // every 6h
    public bool LivePollerEnabled { get; init; } = true;
    public bool UpcomingPollerEnabled { get; init; } = true;
  }
  ```
  Use `Cronos` package or simple TimeSpan for upcoming schedule.
- [ ] Add DI extension `AddMarathonApplication(IServiceCollection, IConfiguration)`
  in `Marathon.Application/DependencyInjection.cs`:
  - Registers all use cases
- [ ] Update `Marathon.Infrastructure/DependencyInjection.cs` to also register
  `BackgroundService`s under `services.AddHostedService<T>()`.
- [ ] Tests in `Marathon.Application.Tests`:
  - Mock `IOddsScraper` + repos with NSubstitute
  - Test: `PullUpcomingEventsUseCase` persists new events, skips duplicates
  - Test: `PullLiveOddsUseCase` writes a snapshot per live event
  - Test: `PullResultsUseCase` respects `selection` filter (when null, fetches all)
  - Test: `ExportToExcelUseCase` invokes `IExcelExporter.ExportAsync` with correct
    date range
- [ ] Tests in `Marathon.Infrastructure.Tests/Workers/`:
  - Test: `LiveOddsPoller` invokes use case at configured interval (use FakeTimeProvider)
  - Test: poller continues after a use-case exception (logs, doesn't propagate)

## Files to Modify/Create

- `src/Marathon.Application/UseCases/*.cs`
- `src/Marathon.Application/DependencyInjection.cs`
- `src/Marathon.Infrastructure/Workers/UpcomingEventsPoller.cs`
- `src/Marathon.Infrastructure/Workers/LiveOddsPoller.cs`
- `src/Marathon.Infrastructure/Configuration/WorkerOptions.cs`
- `tests/Marathon.Application.Tests/UseCases/**`
- `tests/Marathon.Infrastructure.Tests/Workers/**`

## Acceptance Criteria

- Compiles (Big Bang).
- Use cases depend only on Application abstractions (no Infrastructure refs).
- Workers honor cancellation and don't crash on transient errors.
- All variable timing/enabling is configurable.

## Notes

- Use `IHostedService` from `Microsoft.Extensions.Hosting` — works in WPF host via
  `Host.CreateApplicationBuilder()` pattern (Phase 5 will expose this).
- For the cron-style upcoming poller, prefer the `Cronos` package (small, mature)
  over hand-rolled scheduling.
- Big Bang: compile-only smoke check.

## Review Checklist

- [ ] Use cases have no Infrastructure dependencies
- [ ] Both pollers configurable (interval, enable/disable)
- [ ] Cancellation propagated correctly
- [ ] Errors logged, not propagated out of `ExecuteAsync`

## Handoff to Next Phase

<!-- Filled by Phase 4 implementer. Phase 5 needs to know how to start the host
     including these BackgroundServices. -->