Event types (domain, integration, notification) in Microservices - Scalability & System Analysis

Scalability Analysis - Event types (domain, integration, notification)

Growth Table: Event Types in Microservices

Users / Events	Domain Events	Integration Events	Notification Events
100 users	Few domain events per second, simple event handling within services.	Low integration events, direct service calls mostly.	Notifications sent manually or via simple queues.
10,000 users	Hundreds of domain events/sec, event sourcing or CQRS may start.	Integration events increase, async messaging used for decoupling.	Notifications automated, batched, and queued.
1,000,000 users	Thousands of domain events/sec, event stores and replay needed.	High volume integration events, message brokers scale horizontally.	Notifications use push services, throttling, and personalization.
100,000,000 users	Millions of domain events/sec, partitioned event streams, multi-region replication.	Integration events require global event mesh, guaranteed delivery.	Notifications use CDN, multi-channel, and real-time delivery at scale.

First Bottleneck

The first bottleneck is usually the event broker or message queue system handling integration events.

At small scale, domain events are handled within services easily.

As user count grows, the broker's throughput and latency limits appear first because it must reliably deliver many events to multiple consumers.

Scaling Solutions

• Horizontal scaling: Add more broker nodes (e.g., Kafka partitions) to distribute event load.
• Partitioning: Split event streams by domain or customer to isolate traffic.
• Caching: Use caches for notification content to reduce recomputation.
• Event filtering: Consumers subscribe only to relevant events to reduce processing.
• Backpressure and throttling: Control event flow to avoid overload.
• Multi-region replication: For global scale, replicate event brokers closer to users.

Back-of-Envelope Cost Analysis

• At 1M users, assume 10 domain events/user/day -> ~115 events/sec.
• Integration events multiply by number of services; assume 10x domain events -> ~1,150 events/sec.
• Notification events depend on user activity; assume 5 notifications/user/day -> ~58 notifications/sec peak.
• Storage: Event logs grow by millions daily; need scalable storage like distributed logs.
• Bandwidth: Event payloads small (~1KB), so 1,150 events/sec ≈ 1.15 MB/s; notifications may require more bandwidth if multimedia.

Interview Tip

Start by defining each event type clearly: domain, integration, notification.

Explain how their scale differs and why their bottlenecks differ.

Discuss the event flow and how you would scale each component step-by-step.

Use real numbers to show understanding of throughput and storage.

Self Check

Your event broker handles 1000 QPS integration events. Traffic grows 10x. What do you do first?

Answer: Add partitions and broker nodes to horizontally scale the event broker, distributing load and maintaining throughput.

Key Result

Integration event brokers become the first bottleneck as event volume grows; horizontal scaling with partitioning is the primary solution.

Practice

1. Which type of event in microservices captures important business actions inside a single service?

easy

A. Domain event

B. Integration event

C. Notification event

D. System event

Solution

1. Step 1: Understand event types in microservices

   Domain events represent significant business actions occurring within a single service boundary.

2. Step 2: Differentiate from other event types

   Integration events share data between services, and notification events alert users or external systems, so they are not internal business actions.

3. Final Answer:

   Domain event -> Option A

4. Quick Check:

   Business action inside service = Domain event [OK]

Hint: Domain events are about internal business actions [OK]

Common Mistakes:

• Confusing integration events with domain events
• Thinking notification events capture business logic
• Assuming system event is a standard event type

2. Which of the following is the correct way to describe an integration event in microservices?

easy

A. An event that triggers UI updates within the same service

B. An event that shares information between different services

C. An event that sends alerts to users only

D. An event that logs errors internally

Solution

1. Step 1: Define integration events

   Integration events are designed to share information or changes between different microservices to keep them in sync.

2. Step 2: Eliminate incorrect options

   UI updates are usually local, alerts to users are notification events, and error logs are internal diagnostics, not integration events.

3. Final Answer:

   An event that shares information between different services -> Option B

4. Quick Check:

   Sharing info between services = Integration event [OK]

Hint: Integration events connect multiple services [OK]

Common Mistakes:

• Mixing notification events with integration events
• Thinking integration events only affect one service
• Confusing error logs with integration events

3. Consider this code snippet in a microservice:

publishEvent({ type: 'UserRegistered', payload: { userId: 123 } });

What type of event is this most likely representing?

medium

A. Notification event

B. System event

C. Integration event

D. Domain event

Solution

1. Step 1: Analyze the event name and context

   The event 'UserRegistered' indicates a business action inside the service, such as a user signing up.

2. Step 2: Match event type to definition

   Since it captures a business action inside the service, it is a domain event, not a notification or integration event.

3. Final Answer:

   Domain event -> Option D

4. Quick Check:

   Business action event = Domain event [OK]

Hint: Event names with business actions are domain events [OK]

Common Mistakes:

• Assuming all events are integration events
• Confusing notification events with domain events
• Ignoring event naming conventions

4. A microservice is sending an event to notify users about a password change. The event is mistakenly labeled as an integration event. What is the main issue here?

medium

A. Notification events should not be sent to users

B. Password change is a domain event, not a notification

C. Notification events should not be labeled as integration events

D. Integration events cannot carry user-related data

Solution

1. Step 1: Identify the event purpose

   The event is meant to notify users, which fits the notification event type.

2. Step 2: Understand event labeling importance

   Labeling a notification event as an integration event causes confusion and wrong handling in the system.

3. Final Answer:

   Notification events should not be labeled as integration events -> Option C

4. Quick Check:

   Correct event labeling avoids confusion [OK]

Hint: Match event label to its purpose carefully [OK]

Common Mistakes:

• Mixing notification and integration event roles
• Assuming integration events can't have user data
• Thinking notification events are internal only

5. You are designing a microservices system where a user registration triggers multiple actions: updating internal user stats, notifying other services, and sending a welcome email. Which event types should you use for these actions respectively?

hard

A. Domain event, integration event, notification event

B. Integration event, domain event, notification event

C. Notification event, domain event, integration event

D. Domain event, notification event, integration event

Solution

1. Step 1: Map actions to event types

   Updating internal user stats is a business action inside the service, so it is a domain event.

2. Step 2: Identify cross-service communication

   Notifying other services requires sharing information between services, so it is an integration event.

3. Step 3: Recognize user alerts

   Sending a welcome email is a message to the user, which fits notification events.

4. Final Answer:

   Domain event, integration event, notification event -> Option A

5. Quick Check:

   Internal action, cross-service, user alert = Domain, Integration, Notification [OK]

Hint: Match event type to action scope: internal, cross-service, user [OK]

Common Mistakes:

• Swapping integration and notification events
• Using domain events for cross-service communication
• Confusing notification with domain events