Aggregates and entities in Microservices - System Design Exercise

Design: Order Management System

Design the domain model focusing on aggregates and entities for order management. Include microservice boundaries and data consistency approach. Out of scope: payment processing, shipping logistics.

Functional Requirements

FR1: Allow customers to create and manage orders

FR2: Each order contains multiple items with quantity and price

FR3: Support updating order status (e.g., pending, shipped, delivered)

FR4: Ensure data consistency within orders and their items

FR5: Allow querying orders by customer and status

Non-Functional Requirements

NFR1: Handle 1000 concurrent order creations per minute

NFR2: API response time p99 < 300ms

NFR3: System availability 99.9%

NFR4: Data consistency within an order must be strong

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Order Service

Order Item Entity

Customer Service (external)

Database with transaction support

API Gateway

Design Patterns

Aggregate pattern for grouping entities

Domain-Driven Design (DDD) principles

Eventual consistency vs strong consistency

Transactional boundaries within aggregates

Reference Architecture

Client
  |
  v
API Gateway
  |
  v
Order Service (Aggregate Root: Order)
  |-- Order Items (Entities)
  |
Database (Transactional)

External Customer Service (for customer data)

Components

Order Service

Spring Boot microservice with REST API

Manages orders as aggregate roots, handles business logic and consistency

Order Item Entity

JPA Entities within Order Service

Represents individual items within an order, part of the aggregate

Database

PostgreSQL with ACID transactions

Stores orders and items ensuring strong consistency within aggregates

API Gateway

Kong or AWS API Gateway

Routes client requests to appropriate microservices

Customer Service

External microservice

Manages customer data, queried by Order Service as needed

Request Flow

1. Client sends create order request to API Gateway

2. API Gateway forwards request to Order Service

3. Order Service validates request and creates Order aggregate with Order Items

4. Order Service saves aggregate in database within a transaction

5. Order Service returns confirmation to client

6. Client queries order status via API Gateway to Order Service

7. Order Service fetches order and items from database and returns data

Database Schema

Entities: - Order (Aggregate Root): order_id (PK), customer_id, status, created_at, updated_at - OrderItem: item_id (PK), order_id (FK), product_id, quantity, price Relationships: - One Order has many OrderItems (1:N) - Order is aggregate root; OrderItems cannot exist without Order - Customer data referenced by customer_id but stored externally

Scaling Discussion

Bottlenecks

Database write contention on orders during high concurrency

Order Service CPU/memory limits under load

API Gateway throughput limits

Solutions

Use database connection pooling and optimize indexes for order queries

Scale Order Service horizontally with stateless instances behind load balancer

Use API Gateway autoscaling and caching for read-heavy endpoints

Interview Tips

Time: 10 minutes for requirements and clarifying questions, 15 minutes for design and data modeling, 10 minutes for scaling and trade-offs discussion

Explain aggregate concept as consistency boundary

Justify why Order is aggregate root and Order Items are entities inside it

Discuss transactional consistency within aggregate

Mention microservice boundaries and external customer service

Describe scaling strategies and bottleneck mitigation

Practice

1. In microservices, what is the main role of an aggregate root entity?

easy

A. It acts as a database for all microservices.

B. It stores unrelated data from different services.

C. It handles user interface rendering.

D. It controls all changes within the aggregate to keep data consistent.

Solution

1. Step 1: Understand aggregate root responsibility

   The aggregate root is the main entity that manages all changes inside its aggregate to ensure consistency.

2. Step 2: Eliminate unrelated options

   Options A, B, and D describe roles unrelated to aggregate roots in microservices.

3. Final Answer:

   It controls all changes within the aggregate to keep data consistent. -> Option D

4. Quick Check:

   Aggregate root controls changes = C [OK]

Hint: Aggregate root manages changes inside its group [OK]

Common Mistakes:

• Confusing aggregate root with database or UI component
• Thinking aggregate root stores unrelated data
• Assuming aggregate root handles external service data

2. Which of the following correctly represents an aggregate in a microservice domain model?

easy

A. Order (root) -> OrderItems (entities) -> PaymentDetails (entity)

B. OrderItems (root) -> Order -> PaymentDetails

C. PaymentDetails (root) -> Order -> OrderItems

D. Order -> PaymentDetails -> OrderItems (all roots)

Solution

1. Step 1: Identify the aggregate root

   In an order system, the Order is the root entity controlling related entities like OrderItems and PaymentDetails.

2. Step 2: Check the hierarchy correctness

   Order (root) -> OrderItems (entities) -> PaymentDetails (entity) shows Order as root with related entities under it, which is correct. Other options misplace roots or treat all as roots.

3. Final Answer:

   Order (root) -> OrderItems (entities) -> PaymentDetails (entity) -> Option A

4. Quick Check:

   Root entity is Order controlling others = A [OK]

Hint: Root entity leads related entities in aggregate [OK]

Common Mistakes:

• Assigning wrong entity as root
• Treating all entities as roots
• Ignoring aggregate boundaries

3. Given the aggregate root Customer with entities Address and Order, which operation should only be performed through Customer?

medium

A. Deleting Order independently from Customer

B. Directly updating an Order without Customer involvement

C. Adding a new Address via the Customer aggregate root

D. Querying Order data directly from the database

Solution

1. Step 1: Understand aggregate root control

   The aggregate root Customer controls all changes to its entities like Address and Order to maintain consistency.

2. Step 2: Identify allowed operations

   Adding a new Address should go through Customer. Direct updates or deletes bypassing root break consistency.

3. Final Answer:

   Adding a new Address via the Customer aggregate root -> Option C

4. Quick Check:

   Changes go through root entity = A [OK]

Hint: All changes pass through aggregate root only [OK]

Common Mistakes:

• Updating entities directly without root
• Deleting entities independently
• Confusing querying with updating

4. You have a microservice with an aggregate root Invoice and entities LineItem. The code allows direct modification of LineItem without Invoice. What is the main problem?

medium

A. Performance will improve due to direct access.

B. Data consistency may break because changes bypass the aggregate root.

C. It will reduce network calls between services.

D. It simplifies the codebase without side effects.

Solution

1. Step 1: Identify aggregate root role in consistency

   The aggregate root Invoice ensures all changes to LineItem are consistent and valid.

2. Step 2: Analyze direct modification impact

   Directly modifying LineItem bypasses Invoice, risking inconsistent or invalid data.

3. Final Answer:

   Data consistency may break because changes bypass the aggregate root. -> Option B

4. Quick Check:

   Bypassing root risks consistency = B [OK]

Hint: Bypass root risks data consistency [OK]

Common Mistakes:

• Assuming direct access improves design
• Ignoring consistency importance
• Confusing performance with correctness

5. You design a microservice for a shopping cart system. The cart is an aggregate root with entities like CartItem and Discount. Which design choice best ensures data consistency and scalability?

hard

A. Make Cart the aggregate root controlling all CartItem and Discount changes.

B. Use a single database table for Cart, CartItem, and Discount without aggregates.

C. Store CartItem and Discount in separate microservices with no coordination.

D. Allow CartItem and Discount to be updated independently without Cart involvement.

Solution

1. Step 1: Apply aggregate root principle for consistency

   Cart as aggregate root should control all changes to CartItem and Discount to keep data consistent.

2. Step 2: Consider scalability and design best practices

   Centralizing changes through Cart allows easier management and scaling of the microservice without data conflicts.

3. Step 3: Evaluate other options

   Options A and C risk inconsistency; B ignores aggregate design and can cause complexity.

4. Final Answer:

   Make Cart the aggregate root controlling all CartItem and Discount changes. -> Option A

5. Quick Check:

   Aggregate root controls changes for consistency and scale = D [OK]

Hint: Aggregate root controls related entities for consistency and scale [OK]

Common Mistakes:

• Allowing independent updates breaking consistency
• Splitting tightly coupled entities into separate services
• Ignoring aggregate design principles