ESIM/EsimLao/Esim.SendMail/Services/EmailService.cs

using System.Collections.Concurrent;
using MailKit.Net.Smtp;
using MailKit.Security;
using MimeKit;
using Microsoft.Extensions.Configuration;

namespace Esim.SendMail.Services;

/// <summary>
/// High-performance email service optimized for millions of messages
/// Uses MailKit with connection pooling and batch processing
/// </summary>
public interface IEmailService
{
    Task<bool> SendEmailAsync(string to, string subject, string body, bool isHtml = true);
    Task<int> SendBatchAsync(IEnumerable<EmailMessage> messages);
    void Dispose();
}

public class EmailMessage
{
    public int Id { get; set; }
    public string To { get; set; } = null!;
    public string Subject { get; set; } = null!;
    public string Body { get; set; } = null!;
    public bool IsHtml { get; set; } = true;
}

public class EmailResult
{
    public int MessageId { get; set; }
    public bool Success { get; set; }
    public string? ErrorMessage { get; set; }
}

public class HighPerformanceEmailService : IEmailService, IDisposable
{
    private static readonly log4net.ILog log = log4net.LogManager.GetLogger(typeof(HighPerformanceEmailService));

    private readonly IConfiguration _configuration;
    private readonly string _smtpServer;
    private readonly int _smtpPort;
    private readonly string _senderEmail;
    private readonly string _senderName;
    private readonly string _senderPassword;
    private readonly bool _enableSsl;
    private readonly int _connectionPoolSize;
    private readonly int _maxConcurrentSends;
    private readonly int _retryDelayMs;

    // Connection pool for SMTP clients
    private readonly ConcurrentBag<SmtpClient> _connectionPool;
    private readonly SemaphoreSlim _poolSemaphore;
    private bool _disposed = false;

    public HighPerformanceEmailService(IConfiguration configuration)
    {
        _configuration = configuration;
        _smtpServer = _configuration["Email:SmtpServer"] ?? "smtp.gmail.com";
        _smtpPort = int.Parse(_configuration["Email:SmtpPort"] ?? "587");
        _senderEmail = _configuration["Email:SenderEmail"] ?? "";
        _senderName = _configuration["Email:SenderName"] ?? "EsimLao";
        _senderPassword = _configuration["Email:SenderPassword"] ?? "";
        _enableSsl = bool.Parse(_configuration["Email:EnableSsl"] ?? "true");
        _connectionPoolSize = int.Parse(_configuration["Email:ConnectionPoolSize"] ?? "5");
        _maxConcurrentSends = int.Parse(_configuration["Email:MaxConcurrentSends"] ?? "10");
        _retryDelayMs = int.Parse(_configuration["Email:RetryDelayMs"] ?? "1000");

        _connectionPool = new ConcurrentBag<SmtpClient>();
        _poolSemaphore = new SemaphoreSlim(_connectionPoolSize, _connectionPoolSize);

        // Pre-initialize connection pool
        InitializeConnectionPool();
    }

    private void InitializeConnectionPool()
    {
        log.Info($"Initializing SMTP connection pool with {_connectionPoolSize} connections");
        
        for (int i = 0; i < _connectionPoolSize; i++)
        {
            try
            {
                var client = CreateConnectedClient();
                if (client != null)
                {
                    _connectionPool.Add(client);
                }
            }
            catch (Exception ex)
            {
                log.Warn($"Failed to create initial SMTP connection {i + 1}: {ex.Message}");
            }
        }
        
        log.Info($"Connection pool initialized with {_connectionPool.Count} connections");
    }

    private SmtpClient? CreateConnectedClient()
    {
        try
        {
            var client = new SmtpClient();
            
            // Connect with timeout
            client.Timeout = 30000; // 30 seconds
            client.Connect(_smtpServer, _smtpPort, _enableSsl ? SecureSocketOptions.StartTls : SecureSocketOptions.None);
            client.Authenticate(_senderEmail, _senderPassword);
            
            return client;
        }
        catch (Exception ex)
        {
            log.Error($"Failed to create SMTP connection: {ex.Message}");
            return null;
        }
    }

    private async Task<SmtpClient?> GetClientFromPoolAsync()
    {
        await _poolSemaphore.WaitAsync();

        if (_connectionPool.TryTake(out var client))
        {
            // Check if connection is still alive
            if (client.IsConnected)
            {
                return client;
            }
            
            // Connection dead, try to reconnect
            try
            {
                client.Dispose();
            }
            catch { }
        }

        // Create new connection
        return CreateConnectedClient();
    }

    private void ReturnClientToPool(SmtpClient? client)
    {
        if (client != null && client.IsConnected)
        {
            _connectionPool.Add(client);
        }
        else
        {
            // Try to create a new connection
            try
            {
                var newClient = CreateConnectedClient();
                if (newClient != null)
                {
                    _connectionPool.Add(newClient);
                }
            }
            catch { }
        }
        
        _poolSemaphore.Release();
    }

    public async Task<bool> SendEmailAsync(string to, string subject, string body, bool isHtml = true)
    {
        SmtpClient? client = null;
        
        try
        {
            client = await GetClientFromPoolAsync();
            if (client == null)
            {
                log.Error("Failed to get SMTP client from pool");
                return false;
            }

            var message = CreateMimeMessage(to, subject, body, isHtml);
            await client.SendAsync(message);
            
            log.Debug($"Email sent to {to}");
            return true;
        }
        catch (Exception ex)
        {
            log.Error($"Failed to send email to {to}: {ex.Message}");
            
            // Force reconnect on error
            if (client != null)
            {
                try { client.Disconnect(true); } catch { }
                try { client.Dispose(); } catch { }
                client = null;
            }
            
            return false;
        }
        finally
        {
            ReturnClientToPool(client);
        }
    }

    /// <summary>
    /// Send batch of emails with parallel processing
    /// Returns number of successfully sent emails
    /// </summary>
    public async Task<int> SendBatchAsync(IEnumerable<EmailMessage> messages)
    {
        var messageList = messages.ToList();
        if (!messageList.Any()) return 0;

        log.Info($"Sending batch of {messageList.Count} emails");
        
        int successCount = 0;
        var semaphore = new SemaphoreSlim(_maxConcurrentSends);
        var tasks = new List<Task<bool>>();

        foreach (var msg in messageList)
        {
            await semaphore.WaitAsync();
            
            var task = Task.Run(async () =>
            {
                try
                {
                    return await SendEmailAsync(msg.To, msg.Subject, msg.Body, msg.IsHtml);
                }
                finally
                {
                    semaphore.Release();
                }
            });
            
            tasks.Add(task);
        }

        var results = await Task.WhenAll(tasks);
        successCount = results.Count(r => r);
        
        log.Info($"Batch complete: {successCount}/{messageList.Count} successful");
        return successCount;
    }

    private MimeMessage CreateMimeMessage(string to, string subject, string body, bool isHtml)
    {
        var message = new MimeMessage();
        message.From.Add(new MailboxAddress(_senderName, _senderEmail));
        message.To.Add(MailboxAddress.Parse(to));
        message.Subject = subject;

        var bodyBuilder = new BodyBuilder();
        if (isHtml)
        {
            bodyBuilder.HtmlBody = body;
        }
        else
        {
            bodyBuilder.TextBody = body;
        }
        message.Body = bodyBuilder.ToMessageBody();

        return message;
    }

    public void Dispose()
    {
        if (_disposed) return;
        _disposed = true;

        log.Info("Disposing email service...");

        while (_connectionPool.TryTake(out var client))
        {
            try
            {
                if (client.IsConnected)
                {
                    client.Disconnect(true);
                }
                client.Dispose();
            }
            catch { }
        }

        _poolSemaphore.Dispose();
        log.Info("Email service disposed");
    }
}