Lesson 2: Event-Driven Architecture

Synchronous vs. Asynchronous

• Synchronous (Request/Response): Client waits for the server to respond (e.g., HTTP REST).
• Asynchronous (Event-Driven): Client sends a message and continues work. The receiver processes it later.

Core Concepts

Message Queues (Point-to-Point)

A message is sent to a queue and processed by exactly one consumer.

• Use Case: Background jobs (e.g., image resizing).

Pub/Sub (Publish/Subscribe)

A message (event) is published to a topic. Multiple subscribers can receive a copy.

• Use Case: notifying multiple services (e.g., “UserSignedUp” -> EmailService, AnalyticsService).

---

🛠️ Sruja Perspective: Modeling Events

Sruja supports queue as a first-class citizen to model asynchronous communication.

import { * } from 'sruja.ai/stdlib'


User = person "End User"

Notifications = system "Notification System" {
    AuthService = container "Auth Service" {
        technology "Node.js"
        description "Handles user authentication and publishes events"
    }

    // Define a queue or topic
    UserEvents = queue "User Events Topic" {
        technology "Kafka"
        description "Events related to user lifecycle (signup, login, profile updates)."
    }

    EmailService = container "Email Service" {
        technology "Python"
        description "Sends transactional emails"
    }

    AnalyticsService = container "Analytics Service" {
        technology "Spark"
        description "Processes user events for analytics"
    }

    NotificationDB = database "Notification Database" {
        technology "PostgreSQL"
        description "Stores notification preferences and history"
    }

    // Pub/Sub flow
    User -> AuthService "Signs up"
    AuthService -> UserEvents "Publishes 'UserSignedUp' event"
    UserEvents -> EmailService "Consumes - sends welcome email"
    UserEvents -> AnalyticsService "Consumes - tracks signup"
    EmailService -> NotificationDB "Logs email sent"
}

// Model the event flow as a scenario
UserSignupFlow = scenario "User Signup Event Flow" {
    User -> AuthService "Submits registration"
    AuthService -> UserEvents "Publishes UserSignedUp"
    UserEvents -> EmailService "Triggers welcome email"
    UserEvents -> AnalyticsService "Tracks signup event"
    EmailService -> User "Sends welcome email"
}

// Data flow for analytics processing
flow AnalyticsPipeline "Analytics Data Pipeline" {
    UserEvents -> AnalyticsService "Streams events"
    AnalyticsService -> AnalyticsService "Processes batch"
    AnalyticsService -> AnalyticsService "Generates reports"
}

view index {
title "Notification System Overview"
include *
}

// Event flow view: Focus on async communication
view eventflow {
title "Event Flow View"
include Notifications.AuthService
include Notifications.UserEvents
include Notifications.EmailService
include Notifications.AnalyticsService
exclude User Notifications.NotificationDB
}

// Data view: Focus on data storage
view data {
title "Data Storage View"
include Notifications.EmailService
include Notifications.NotificationDB
include Notifications.AnalyticsService
exclude Notifications.AuthService Notifications.UserEvents
}

Key Concepts

1. Scenarios model behavioral flows (user journeys, use cases)
2. Flows model data pipelines (ETL, streaming, batch processing)
3. Views let you focus on different aspects (events vs data vs user experience)