Notification Flow in Microservices

 In a microservices architecture, services are decoupled. So if you want to notify a user about an event (like "order processed"), you usually use an asynchronous messaging system rather than calling services directly.

Flow Overview

1. Event happens in a microservice
   Example: OrderService processes an order.

   var orderProcessedEvent = new OrderProcessedEvent { OrderId = 123, UserId = 456 };
   

2. Publish Event to Message Broker

   • Microservice publishes the event to Azure Service Bus topic/queue.

   • Topics allow multiple subscribers, queues deliver to one consumer.

3. Notification Service Subscribes

   • A dedicated Notification Microservice subscribes to the topic/queue.

   • When a message arrives, it triggers notification logic (email, SMS, push).

4. Send Notification to Users

   • Notification Service decides which users should get notified.

   • It uses user preferences/configurations stored in a database.

5. Delivery

   • Email: via SMTP, SendGrid, or Azure Communication Services.

   • Push: via Firebase or Azure Notification Hub.

   • SMS: via Twilio, Azure Communication Services, etc.

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2. Azure Service Bus Setup

• Queues: point-to-point (one consumer).

• Topics & Subscriptions: publish-subscribe pattern (multiple consumers can get the same event).

Example:

• OrderProcessedTopic

  • Subscription 1 → NotificationService

  • Subscription 2 → AnalyticsService

// Publishing
var client = new ServiceBusClient(connectionString);
var sender = client.CreateSender("OrderProcessedTopic");
await sender.SendMessageAsync(new ServiceBusMessage(JsonSerializer.Serialize(orderProcessedEvent)));

// Subscribing in Notification Service
var processor = client.CreateProcessor("OrderProcessedTopic", "NotificationSubscription");
processor.ProcessMessageAsync += async args => {
    var body = args.Message.Body.ToString();
    var orderEvent = JsonSerializer.Deserialize<OrderProcessedEvent>(body);

    // Send notification logic
    await SendEmailNotification(orderEvent.UserId, "Order Processed");
};

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3. Email Notification Service

Components:

1. Email Templates – HTML/Plain text templates.

2. SMTP/Email Provider Configuration – SMTP settings (host, port, username, password) or a service like SendGrid/Azure Communication Services.

3. Email Sending Logic – Called by the Notification Service when a message is received.

Sample code using SMTP:

public async Task SendEmailNotification(int userId, string message)
{
    var user = await dbContext.Users.FindAsync(userId);
    if (user == null || string.IsNullOrEmpty(user.Email)) return;

    var smtp = new SmtpClient("smtp.yourmail.com")
    {
        Port = 587,
        Credentials = new NetworkCredential("username", "password"),
        EnableSsl = true,
    };

    var mail = new MailMessage("noreply@yourapp.com", user.Email, "Order Update", message);
    await smtp.SendMailAsync(mail);
}

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4. Configuring Users for Notifications

1. User Preferences Table

   • Table: UserNotifications

   • Columns: UserId, NotificationType, IsEnabled, Email, PushToken, PhoneNumber

   UserId | NotificationType | IsEnabled | Email           | PushToken | PhoneNumber
   456    | OrderProcessed   | 1         | user@example.com | xyz       | 9999999999
   

2. Check Before Sending

   • Before sending, check if the user wants that type of notification:

   var config = dbContext.UserNotifications
                 .Where(u => u.UserId == userId && u.NotificationType == "OrderProcessed" && u.IsEnabled)
                 .FirstOrDefault();
   if(config != null) SendEmail(...);
   

3. Optional UI – Let users manage preferences in Settings → Notification Preferences.

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5. Overall Flow Diagram

[OrderService] --(event)--> [Azure Service Bus Topic] --> [NotificationService]
                                                        |--> Email/SMS/Push
                                                        |--> Uses User Preferences from DB

✅ Key Points:

• Use Service Bus for decoupling services.

• Use Notification Service as a central microservice.

• Use user preferences to decide who gets notified.

• Email provider can be SMTP, SendGrid, or Azure Communication Services.

• Optional: extend for SMS, push notifications, or mobile apps.