Microsoft·Software Engineer·Technical Phone Screen·Senior

Senior

Jul 2026

Summary

Microsoft software engineer round that skipped the leetcode grind entirely and went straight into backend design territory. Two meaty questions back to back, both requiring you to actually think about how software gets built and shipped in the real world.

Questions Asked (2)

You need to add new business logic to an existing backend service using object-oriented design. How do you identify the core domain objects, assign responsibilities, separate orchestration from business rules, and keep the code testable and extensible?

System DesignTechnical Trade-offs

Author's notes

This one took me a minute to get traction on.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the business requirement and identifying the core domain concepts using domain-driven design. Then walk through how you would assign responsibilities to objects, separate orchestration from business rules, and ensure testability and extensibility through patterns like dependency injection and hexagonal architecture.

Pro tip: Emphasize that you begin with the domain model and ubiquitous language, not the technical layers. Show how you would validate the design with unit tests and refactor as needed, demonstrating a test-driven and iterative approach.

1. Understand the Domain

Collaborate with domain experts to define the ubiquitous language and identify key business concepts, invariants, and workflows.

2. Identify Core Domain Objects

Extract entities, value objects, and aggregates that encapsulate business rules and state, ensuring high cohesion and low coupling.

3. Assign Responsibilities

Apply principles like SOLID and GRASP to assign behavior to the right objects, keeping business logic within the domain model.

4. Separate Orchestration from Business Rules

Use application services or use case classes to coordinate workflows, while domain objects handle the core logic, often via dependency inversion.

5. Ensure Testability and Extensibility

Design with interfaces, dependency injection, and patterns like strategy or factory to allow easy testing and future extension without modifying existing code.

Key Points to Mention

Domain-Driven Design (DDD) concepts: entities, value objects, aggregates, repositories, and ubiquitous language.
SOLID principles, especially Single Responsibility and Dependency Inversion, to guide responsibility assignment.
Separation of concerns: application services for orchestration vs. domain services for business logic.
Design patterns: strategy, factory, repository, and dependency injection for extensibility and testability.
Test-driven development (TDD) and unit testing to validate domain logic and enable safe refactoring.
Hexagonal architecture (ports and adapters) to isolate the domain from infrastructure and frameworks.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

You need to change a public API that existing clients already depend on. Walk through how you'd ship that change safely, covering backward compatibility, versioning strategy, testing, rollout approach, monitoring, and client communication.

API & IntegrationsTechnical Trade-offs

Author's notes

Probably the stronger of my two answers.

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AI HintsAI Generated

Suggested Approach

Structure your answer around a phased, backward-compatible rollout: first assess impact and design the new API, then implement with versioning and feature flags, test thoroughly, deploy gradually with monitoring, and communicate clearly with clients. Emphasize minimizing disruption and having a rollback plan.

Pro tip: Proactively mention that you'd deprecate the old version only after confirming zero usage via telemetry, and offer migration support like SDK updates and documentation to reduce client friction.

1. Assess Impact and Design

Identify all clients and usage patterns, then design the new API with backward compatibility in mind, deciding on additive vs. breaking changes.

2. Implement with Versioning and Flags

Introduce the new version alongside the old (e.g., /v2), use feature flags to control exposure, and maintain the old version during transition.

3. Test Thoroughly

Write contract tests, integration tests, and backward-compatibility tests; use canary testing with a subset of clients to validate behavior.

4. Rollout Gradually with Monitoring

Deploy incrementally (e.g., 1% → 10% → 100%), monitor key metrics (error rates, latency, usage), and be ready to roll back.

5. Communicate and Deprecate

Notify clients early via changelogs, emails, and docs; provide migration guides and support; deprecate old version only after zero usage.

Key Points to Mention

Backward compatibility strategies: additive changes, default values, and avoiding breaking changes
Versioning approaches: URI versioning, header versioning, or content negotiation
Feature flags and canary deployments for controlled rollout
Comprehensive testing: contract tests, integration tests, and client compatibility tests
Monitoring and observability: tracking usage, errors, and performance per version
Client communication: deprecation policy, migration guides, and support channels

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.