Predictalytics/src/Predictalytics.Hosting/WatchdogHeartbeatService.cs

using System.Text;
using System.Text.Json;
using Serilog;

namespace Predictalytics.Hosting;

/// <summary>
/// Sends periodic dead-man's-switch heartbeats to the external Watchdog server
/// (POST /api/heartbeat) so an outage of this app — or the whole machine — raises
/// an alarm. A Watchdog outage must never impact the app: every call is best effort.
/// </summary>
public sealed class WatchdogHeartbeatService : IDisposable
{
    private readonly HttpClient _http = new() { Timeout = TimeSpan.FromSeconds(10) };
    private readonly string _baseUrl;
    private readonly string _apiKey;
    private readonly string _source;
    private readonly string _instance;
    private readonly int _intervalSeconds;
    private readonly Func<object?>? _metadataProvider;
    private readonly DateTime _startedUtc = DateTime.UtcNow;

    private System.Threading.Timer? _timer;
    private bool _lastSendFailed;

    public WatchdogHeartbeatService(
        string baseUrl,
        string apiKey,
        string source,
        string instance,
        int intervalSeconds,
        Func<object?>? metadataProvider = null)
    {
        _baseUrl = baseUrl.TrimEnd('/');
        _apiKey = apiKey;
        _source = source;
        _instance = string.IsNullOrWhiteSpace(instance) ? "default" : instance;
        _intervalSeconds = Math.Max(15, intervalSeconds);
        _metadataProvider = metadataProvider;
    }

    public void Start()
    {
        _timer?.Dispose();
        _timer = new System.Threading.Timer(
            async _ => await SendHeartbeatAsync("ok"),
            null, TimeSpan.Zero, TimeSpan.FromSeconds(_intervalSeconds));
        Log.Information("🐕 Watchdog heartbeat started → {Url} (source={Source}, every {Interval}s)",
            _baseUrl, _source, _intervalSeconds);
    }

    /// <summary>
    /// Reports a planned shutdown so it is not alarmed as a crash.
    /// Must be "stopped_graceful": the server's event_log.kind ENUM has no "stopping"
    /// value, and that variant fails the insert with HTTP 500 after the state update.
    /// </summary>
    public void NotifyStopping()
    {
        try
        {
            var payload = new
            {
                source = _source,
                instance = _instance,
                kind = "stopped_graceful",
                severity = "info",
                message = "Predictalytics wird planmäßig beendet."
            };
            // Synchronous with a short cap: the form is closing and must not hang.
            PostAsync("/api/event", payload).Wait(TimeSpan.FromSeconds(4));
        }
        catch
        {
            // Best effort only.
        }
    }

    private async Task SendHeartbeatAsync(string status)
    {
        try
        {
            var payload = new
            {
                source = _source,
                instance = _instance,
                type = "heartbeat",
                status,
                message = (string?)null,
                metrics = new
                {
                    uptimeSec = (long)(DateTime.UtcNow - _startedUtc).TotalSeconds,
                    app = _metadataProvider?.Invoke()
                },
                group = "C# Applications",
                interval = _intervalSeconds
            };

            await PostAsync("/api/heartbeat", payload);

            if (_lastSendFailed)
            {
                _lastSendFailed = false;
                Log.Information("🐕 Watchdog heartbeat wieder erfolgreich zugestellt.");
            }
        }
        catch (Exception ex)
        {
            // Log the first failure as warning, subsequent ones quietly (no log flood).
            if (!_lastSendFailed)
            {
                _lastSendFailed = true;
                Log.Warning("🐕 Watchdog heartbeat fehlgeschlagen (weitere Fehler werden unterdrückt): {Error}", ex.Message);
            }
            else
            {
                Log.Debug(ex, "Watchdog heartbeat failed");
            }
        }
    }

    private async Task PostAsync(string path, object payload)
    {
        using var request = new HttpRequestMessage(HttpMethod.Post, _baseUrl + path);
        request.Headers.Add("X-Watchdog-Key", _apiKey);
        request.Content = new StringContent(JsonSerializer.Serialize(payload), Encoding.UTF8, "application/json");

        using var response = await _http.SendAsync(request);
        if (!response.IsSuccessStatusCode)
        {
            var body = await response.Content.ReadAsStringAsync();
            throw new HttpRequestException($"Watchdog API HTTP {(int)response.StatusCode}: {body}");
        }
    }

    public void Dispose()
    {
        _timer?.Dispose();
        _timer = null;
        _http.Dispose();
    }
}