Q355: Architecture Interview Simulation Migrating a Legacy React Application

What Interviewers Want To Evaluate

Architecture migration interviews test whether you can improve a system while people are still using it.

The interviewer wants to know whether you understand risk, sequencing, compatibility, rollout, testing, ownership, communication, and how to avoid turning migration into an endless rewrite.

Short Interview Answer

For a legacy React migration, I would first understand the business goals, pain points, architecture constraints, release process, and risk areas. Then I would choose an incremental migration strategy with clear boundaries, compatibility layers, test coverage, feature flags, observability, and measurable milestones. I would avoid a big-bang rewrite unless the system is small enough and the risk is justified.

Simulation Prompt

A company has a large legacy React app.

Problems:

old routing patterns
mixed state management
large bundles
weak TypeScript coverage
fragile tests
slow releases
duplicated components
unclear ownership

They ask you to modernize it.

Clarifying Questions

Ask:

What business goal drives the migration?
Which parts of the app change most often?
Which flows are highest risk?
What is the current release process?
How much test coverage exists?
What team capacity is available?
Are there deadlines?
What would failure look like?
Which metrics should improve?

Migration without goals becomes endless cleanup.

Migration Strategy

Prefer incremental strategy:

map system
identify boundaries
choose pilot area
add test coverage
create compatibility layer
migrate one slice
measure impact
repeat
deprecate old patterns

This reduces risk.

It also builds confidence.

Boundary Selection

Good migration boundaries:

route group
feature area
component family
state domain
API client layer
design system layer
build tooling layer

Do not migrate randomly.

Choose boundaries that can be owned and validated.

Risk Register

Track:

critical user flows
payment or account areas
permission logic
analytics events
SEO pages
legacy browser support
third-party dependencies
release rollback

Senior migration planning makes risk visible.

Technical Plan

Possible improvements:

upgrade React and framework version
standardize routing
introduce stricter TypeScript
split bundles
replace duplicated components
improve data fetching boundaries
add integration tests
improve observability
document ownership

Do not do everything at once.

Sequence by value and risk.

Rollout Plan

Use:

feature flags
shadow routes
canary users
internal dogfooding
progressive traffic
rollback plan
release notes
monitoring dashboards

Migration should be reversible where possible.

Communication

Communicate:

why migration matters
what is in scope
what is out of scope
which teams are affected
how teams adopt new patterns
where docs live
what the timeline looks like
how progress is measured

Architecture work fails quietly when communication is weak.

Success Metrics

Measure:

bundle size reduction
release frequency
test reliability
TypeScript coverage
defect rate
developer satisfaction
build time
performance metrics
adoption of new patterns

Metrics keep migration tied to outcomes.

Common Mistakes

• Starting with a rewrite.
• Not defining business goals.
• Migrating low-value code first.
• Ignoring tests.
• No rollback plan.
• Breaking analytics or SEO.
• Underestimating team coordination.
• Letting both old and new systems live forever.

Final Mental Model

Legacy migration success means:

clear reason
bounded scope
incremental rollout
visible risk
measured improvement
strong communication
planned deprecation

A senior engineer modernizes systems by reducing risk while increasing capability.