EasyExperience level: 3-5 years · 8-15 years1 min read

Why does each microservice own its database, and what problem does that create?

Reviewed by Gurusankar M.

Asked inAmazonMicrosoftDeloitte
#database per service#data ownership#coupling#joins#consistency
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Short Answer

Each service owns its data so it can evolve its schema and scale independently — no other service may touch its DB directly (loose coupling). The cost: you can no longer do cross-service JOINs or ACID transactions, so you need API composition, events, and eventual consistency instead.

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Concept Made Simple

Coffee Chat Question

“Why does each microservice own its database, and what problem does that create?”

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Mind Map Answer

Why
independent schema + scaling, loose coupling
Rule
no service touches another's DB directly
Cost
no cross-service JOIN or ACID txn
Replace with
API composition / events / saga

Think Beyond the Expected

What If?

A report needs data from 4 services that each own their DB — how do you build it?

You can't JOIN across their databases. Options: API composition (query each service and join in memory), or maintain a read model / materialized view fed by their events (CQRS). For heavy reporting, the event-fed read model scales better than fan-out API calls.

Real World

The 'shared database' anti-pattern silently recouples services (one schema change breaks many); database-per-service is what makes independent deployment real, but it forces you to solve cross-service data via composition/events.

Interviewer's Expectation

Keywords they're listening for:

  • data ownership/loose coupling
  • no direct DB access by others
  • lose cross-service JOIN/ACID
  • API composition / events / CQRS
  • shared-DB anti-pattern

Common Mistakes

  • Sharing one database across services
  • Doing cross-service JOINs via direct DB access
  • Expecting ACID across services

Best Practices

  • One database per service; access only via its API
  • Use API composition or event-fed read models
  • Accept eventual consistency for cross-service data

Follow-up Questions

  1. Why is a shared database an anti-pattern here?
  2. API composition vs CQRS read model for queries?
  3. How do you keep a read model in sync?

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: Data Consistency (Microservices)
Interview question: "Why does each microservice own its database, and what problem does that create?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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