Service Boundaries

1. What Are Service Boundaries?

A service boundary defines the scope of responsibility for a software service—what it owns, what it exposes, and what lies outside its control. In simple terms: it’s the fence that keeps a service’s data, logic, and contracts clearly separated from others.

2. Why It Matters

Without clear boundaries, systems turn into a tangled mess where services overlap, share databases, and break each other constantly. With boundaries in place:

• Teams know exactly what they own.
• Systems evolve safely and independently.
• Scaling, debugging, and compliance become far easier.

3. Core Principles

A good service boundary is built on these pillars:

1. Single Responsibility — the service solves one coherent problem.
2. Own Your Data — each service is the source of truth for its domain.
3. Clear Contracts — APIs/events are the only way others interact with it.
4. Independence — it can be developed, deployed, and scaled separately.
5. Alignment with Business Domains — boundaries reflect real-world workflows, not arbitrary code splits.

4. How to Apply It (Step by Step)

1. Map the business domains → identify areas like billing, orders, accounts.
2. Draw ownership lines → decide which team/service “owns” which data.
3. Define contracts → design APIs or event streams for communication.
4. Isolate storage → give each service its own schema or database.
5. Test autonomy → ask: can this service be changed or scaled without breaking others?

5. Practical Examples & Analogies

Example 1 — E-commerce System

• Orders Service manages orders and order items.
• Payments Service handles payment processing.
• The boundary: payments never query the orders table directly; it listens for an OrderCreated event instead.

Example 2 — Real-World Analogy
Think of a restaurant:

• The kitchen owns recipes and ingredients.
• The waitstaff owns the customer interaction.
• They communicate only through the order slip (the “API”).
  Crossing boundaries (a waiter cooking without training, or a chef seating customers) leads to chaos.

6. Common Mistakes to Avoid

• Shared databases: two services writing to the same table.
• Leaky APIs: exposing internal fields just because another service “needs them.”
• Over-fragmentation: too many microservices with unclear boundaries.
• Ignoring business logic: splitting services by technology (e.g., frontend vs backend) instead of by domain.

7. Quick Checklist

• Does each service have one clear responsibility?
• Is it the only owner of its data?
• Do others interact only through APIs/events?
• Can it evolve independently?
• Is the boundary aligned with a real business domain?

8. Actionable Takeaways

1. Treat service boundaries as contracts of responsibility, not just code splits.
2. Always tie boundaries to business domains, not technical layers.
3. Protect autonomy: own your data, avoid shared databases.
4. Keep contracts clear and stable; version when necessary.
5. Remember: boundaries aren’t about microservices—they apply equally in monoliths, monorepos, and distributed systems.