Webhook System

public class WebhookSubscription : AuditableEntity
{
    public Guid Id { get; set; }
    
    [Required] [MaxLength(500)]
    public string Url { get; set; } = string.Empty;       // Allowed HTTPS destination URL
    
    [Required] [MaxLength(100)]
    public string Secret { get; set; } = string.Empty;    // HMAC signature key
    
    [MaxLength(100)]
    public string? PreviousSecret { get; set; }           // Temporary old key
    public DateTime? PreviousSecretExpiresAt { get; set; } // 24-hour grace period threshold
    
    public bool IsActive { get; set; } = true;            // Toggle state (disabled by circuit breaker)
    public string EventsJson { get; set; } = "[]";        // Subscribed event types JSON array
    public WebhookScope Scope { get; set; }               // Scope filter settings
    
    public int MaxRetries { get; set; } = 4;              // Max retry attempts
    public int MaxConsecutiveFailures { get; set; } = 10; // Circuit breaker threshold
    public int ConsecutiveFailures { get; set; } = 0;     // Consecutive failure counter
    
    public Guid? TenantId { get; set; }                   // Owning tenant ID context
    public virtual Tenant? Tenant { get; set; }
}

// Server-side: Sign payload with timestamp prefix
long timestamp = DateTimeOffset.UtcNow.ToUnixTimeSeconds();
string signatureInput = $"{timestamp}.{payloadJson}";
using var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(subscription.Secret));
byte[] hashBytes = hmac.ComputeHash(Encoding.UTF8.GetBytes(signatureInput));
string signature = $"sha256={Convert.ToHexString(hashBytes).ToLowerInvariant()}";

// Dispatched HTTP Headers:
// X-Webhook-Signature: sha256={hash}
// X-Webhook-Signature-Old: sha256={oldHash} (only during secret rotation)
// X-Webhook-Timestamp: {timestamp}
// X-Webhook-Event: {eventType}
// X-Webhook-Delivery: {eventDeliveryId} (same for all retries)
// X-Webhook-Attempt: {attemptNumber}

// Receiver: Reconstruct signature input & verify
string receivedSignature = request.Headers["X-Webhook-Signature"];
string receivedTimestamp = request.Headers["X-Webhook-Timestamp"];
string requestBody = await ReadBodyAsStringAsync(request);

string computedInput = $"{receivedTimestamp}.{requestBody}";
using var verifierHmac = new HMACSHA256(Encoding.UTF8.GetBytes(mySecret));
byte[] computedHash = verifierHmac.ComputeHash(Encoding.UTF8.GetBytes(computedInput));
string computedSignature = $"sha256={Convert.ToHexString(computedHash).ToLowerInvariant()}";

bool isValid = CryptographicOperations.FixedTimeEquals(
    Encoding.UTF8.GetBytes(computedSignature),
    Encoding.UTF8.GetBytes(receivedSignature)
);

public class WebhookDeliveryLog
{
    public Guid Id { get; set; }
    public Guid SubscriptionId { get; set; }
    public Guid EventDeliveryId { get; set; }     // Matches across all retry attempts for de-duplication
    public Guid? EventTenantId { get; set; }       // Tenant context of origin event
    public string EventType { get; set; } = "";
    public string PayloadJson { get; set; } = "";
    
    public string RequestUrl { get; set; } = "";
    public string? RequestHeaders { get; set; }    // JSON string of headers (excluding secrets)
    public int HttpStatusCode { get; set; }        // HTTP response status code (0 for connection issues)
    public string? ResponseBody { get; set; }      // Snapped response snippet (max 2048 chars)
    
    public int AttemptNumber { get; set; }         // Attempt index: 1, 2, 3, etc.
    public double LatencyMs { get; set; }          // Network latency timing
    public bool IsSuccess { get; set; }            // Status flag: HTTP status 2xx
    public string? ErrorMessage { get; set; }      // Network error snippet (if any)
    public DeliveryStatus Status { get; set; }     // State enum: Delivered, Retrying, DeadLettered
    public DateTime CreatedAt { get; set; }
}