using ItWorx.KnowledgeHub.Application.Operations;

namespace ItWorx.KnowledgeHub.Api.BackgroundServices;

public sealed partial class OperationalTelemetryBackgroundService(
    IServiceScopeFactory scopeFactory, PlatformMetrics metrics, TimeProvider clock,
    ILogger<OperationalTelemetryBackgroundService> logger) : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            try
            {
                // Do not race initial database migration during host startup.
                await Task.Delay(TimeSpan.FromSeconds(30), clock, stoppingToken);
                using var scope = scopeFactory.CreateScope();
                var snapshot = await scope.ServiceProvider.GetRequiredService<IOperationalTelemetryReader>()
                    .ReadAsync(stoppingToken);
                var now = clock.GetUtcNow();
                metrics.SetQueueState(snapshot.QueuedJobs, snapshot.RunningJobs, snapshot.FailedJobs,
                    Age(snapshot.OldestQueuedJobAt, now));
                metrics.SetOperationalAges(Age(snapshot.OldestPendingIngestionAt, now),
                    Age(snapshot.LatestInventoryCaptureAt, now), Age(snapshot.LatestVerifiedBackupAt, now));
            }
            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested) { break; }
            catch (Exception)
            {
                // Omit stale gauges after a failed read, rather than exporting old success.
                metrics.ClearSnapshots();
                LogReadFailed();
            }
        }
    }

    private static double? Age(DateTimeOffset? value, DateTimeOffset now) =>
        value is { } time && time <= now ? (now - time).TotalSeconds : null;

    [LoggerMessage(Level = LogLevel.Warning, Message = "Operationele metriekmeting mislukt; waarden blijven onbekend tot de volgende geslaagde meting.")]
    private partial void LogReadFailed();
}