Overview

The Notification Service is a supporting service within BookWorm’s microservices architecture, responsible for sending transactional emails to customers at critical points in the order lifecycle. Built using an event-driven approach, this service listens for integration events from other domains and transforms them into appropriate customer communications.

Technical Implementation

The Notification Service is implemented using modern cloud-native practices:

Email Providers

The service supports multiple email delivery mechanisms through a clean abstraction:

• Development Environment: Uses local SMTP server (MailPit) for testing
• Production Environment: Integrates with SendGrid for reliable email delivery
• Interface Abstraction: Implements ISmtpClient interface for provider flexibility

public interface ISmtpClient
{
    Task SendEmailAsync(MailMessage mailMessage, CancellationToken cancellationToken = default);
}

Resilience Patterns

The service implements robust resilience patterns to ensure message delivery:

services.AddResiliencePipeline(
    nameof(Notification),
    pipelineBuilder =>
    {
        pipelineBuilder
            .AddRetry(
                new()
                {
                    ShouldHandle = new PredicateBuilder().Handle<Exception>(),
                    BackoffType = DelayBackoffType.Exponential,
                    MaxRetryAttempts = 3,
                    Delay = TimeSpan.FromSeconds(2),
                    UseJitter = true,
                }
            )
            .AddTimeout(TimeSpan.FromSeconds(45));
    }
);

Event Handlers

The service consumes integration events through MassTransit and processes them with dedicated handlers:

Order Completion Handler

[AsyncApi]
public sealed class CompleteOrderCommandHandler : IConsumer<CompleteOrderCommand>
{
    [Channel("notification-complete-order")]
    public async Task Consume(ConsumeContext<CompleteOrderCommand> context)
    {
        var message = context.Message;

        // Email sending logic
        const string subject = "Your order has been completed";
        var body = $"Your order with ID {message.OrderId} has been completed successfully. Total amount: {message.TotalMoney:C}";

        // Send email using the configured provider
    }
}

Observability

The service integrates with OpenTelemetry for distributed tracing:

services.AddOpenTelemetry().WithTracing(t =>
    t.AddSource(TelemetryTags.ActivitySourceName));

Message Flow

The Notification Service processes the following commands:

Email Templates

Each notification type follows a standardized template structure:

• Order Confirmation: “Your order has been placed successfully.”
• Order Completion: “Your order has been completed successfully.”
• Order Cancellation: “Your order has been cancelled.”

Architecture diagram

Infrastructure

The Notification Service is deployed on Microsoft Azure, leveraging Azure Service Bus for message consumption and Azure Monitor for observability.

For Development environment, the service uses MailPit for email delivery.

For Production environment, the service uses SendGrid for email delivery.