Why each service owns its data in Microservices - The Real Reasons

The Big Idea

What if your whole app stopped because one part waited on another's data?

The Scenario

Imagine a team building a big app where many parts share one giant database. Every time someone changes data, everyone else waits and checks if the data is still correct.

It's like a group of friends trying to write a story together on one notebook, but they keep bumping into each other's writing and erasing mistakes.

The Problem

This shared database slows everything down because everyone must wait for others to finish. Mistakes happen when two parts change the same data at once. Fixing these errors takes a lot of time and causes confusion.

It's hard to grow or change one part without breaking others. The whole system becomes fragile and slow.

The Solution

When each service owns its own data, it's like each friend having their own notebook. They write their part freely without waiting or breaking others.

Services talk by sending messages, not by sharing data directly. This keeps each part independent, faster, and easier to fix or improve.

Before vs After

✗ Before

SELECT * FROM shared_database WHERE service='A';
UPDATE shared_database SET value=10 WHERE id=5;

✓ After

serviceA.getData()
serviceA.updateData(5, 10);

What It Enables

This approach lets teams build, test, and scale parts independently, making the whole system more reliable and faster to improve.

Real Life Example

Think of an online store where the payment service owns payment data, and the shipping service owns shipping data. They don't share one big database but communicate through messages, so each can work without blocking the other.

Key Takeaways

Sharing one database causes slowdowns and errors.

Each service owning its data keeps parts independent and faster.

Independent data ownership helps teams build and scale easily.

Practice

1. Why should each microservice own its own data instead of sharing a common database?

easy

A. To ensure services are independent and can evolve separately

B. To reduce the total amount of data stored in the system

C. To make it easier to write SQL queries across services

D. To allow all services to access data faster by sharing it

Solution

1. Step 1: Understand service independence

   Each microservice owning its data means it can change its database without affecting others.

2. Step 2: Recognize benefits of separate data ownership

   This independence improves scalability and reduces tight coupling between services.

3. Final Answer:

   To ensure services are independent and can evolve separately -> Option A

4. Quick Check:

   Service independence = D [OK]

Hint: Think about service independence and avoiding tight coupling [OK]

Common Mistakes:

• Assuming shared databases improve performance
• Believing data sharing reduces storage needs
• Thinking SQL queries are easier with shared data

2. Which of the following is the correct way for microservices to access data owned by another service?

easy

A. Directly querying the other service's database

B. Sharing a common database schema

C. Using APIs or messaging to request data

D. Copying the entire database locally

Solution

1. Step 1: Identify proper data access method

   Microservices should not access each other's databases directly to avoid tight coupling.

2. Step 2: Recognize communication via APIs or messages

   Services communicate data through APIs or messaging systems to maintain independence.

3. Final Answer:

   Using APIs or messaging to request data -> Option C

4. Quick Check:

   Data access via APIs/messages = B [OK]

Hint: Remember: no direct DB access, use APIs or messages [OK]

Common Mistakes:

• Trying to query another service's database directly
• Assuming shared schema is best practice
• Copying entire databases unnecessarily

3. Consider two microservices: Service A owns customer data, and Service B owns order data. Service B needs customer info to process orders. Which approach correctly respects data ownership?

medium

A. Service B queries Service A's database directly for customer info

B. Service B calls Service A's API to get customer info

C. Service B duplicates customer data in its own database

D. Service B uses a shared database for both customer and order data

Solution

1. Step 1: Identify correct data access respecting ownership

   Service B should not access Service A's database directly or share databases.

2. Step 2: Use API calls for data retrieval

   Calling Service A's API allows Service B to get needed data without breaking ownership rules.

3. Final Answer:

   Service B calls Service A's API to get customer info -> Option B

4. Quick Check:

   API calls respect ownership = A [OK]

Hint: Use APIs to get data from other services, not direct DB access [OK]

Common Mistakes:

• Direct DB queries across services
• Duplicating data causing inconsistency
• Using shared databases breaking independence

4. A team notices that two microservices share a database schema and directly query each other's tables. What is the main problem with this design?

medium

A. It causes tight coupling and reduces service independence

B. It improves scalability by sharing data

C. It simplifies API design between services

D. It reduces the need for data synchronization

Solution

1. Step 1: Analyze impact of shared database schema

   Sharing schema and direct queries create tight coupling between services.

2. Step 2: Understand consequences on independence

   Tight coupling reduces the ability to change or scale services independently.

3. Final Answer:

   It causes tight coupling and reduces service independence -> Option A

4. Quick Check:

   Tight coupling problem = C [OK]

Hint: Shared DB means tight coupling, which is bad for microservices [OK]

Common Mistakes:

• Thinking shared DB improves scalability
• Assuming it simplifies API design
• Believing it removes sync needs

5. You are designing a microservices system with three services: User, Inventory, and Order. Each service owns its data. How should you handle a scenario where the Order service needs to confirm inventory availability before placing an order?

hard

A. All services share a single database to simplify data access

B. Order service queries Inventory service's database directly to check stock

C. Order service duplicates inventory data locally and updates it periodically

D. Order service calls Inventory service's API to check stock availability

Solution

1. Step 1: Respect data ownership in design

   Each service must own and manage its own data; direct DB queries or shared DB break this.

2. Step 2: Use API calls for inter-service communication

   Order service should call Inventory service's API to get real-time stock info, ensuring data consistency and independence.

3. Final Answer:

   Order service calls Inventory service's API to check stock availability -> Option D

4. Quick Check:

   API communication respects ownership = A [OK]

Hint: Always use APIs for cross-service data, never direct DB access [OK]

Common Mistakes:

• Direct DB queries breaking independence
• Duplicating data causing stale info
• Using shared DB increasing coupling