Architecture & Tech Leadership interview questions and answers

Rounds specific to senior, staff, principal, and architect titles, where the subject is judgement and organisational impact rather than code.

21 published across 7 topics.

Architecture & Tech LeadershipA 22-minute read on how the topic fits together, before the questions.Architecture and tech leadership fundamentals56 short answers on one page, for revising rather than studying.

masterclass guide

Working With Legacy Systems: A Masterclass on Modernisation InterviewsA complete treatment of the legacy modernisation interview for engineers and architects: what makes a system legacy, characterisation tests and seams, the strangler fig and where it fails, rewrite versus incremental replacement, data migration and cutover, parallel running and reconciliation, anti-corruption layers, sequencing a multi-year programme, and the signal an interviewer grades behind every question.98 min

Architecture Trade-offs

4 questions

Defending a decision under challenge, articulating what you gave up, and reasoning about reversibility.

hardDesignScenario

You are asked to design something distributed and your honest answer is one Postgres box. When is that the strongest answer in the room, and when does it end the interview?

One PostgreSQL primary with a replica and tested backups is the strongest answer when a capacity estimate built from the prompt fits it with headroom and the core invariants need transactions. It ends the interview when the prompt rules it out, or when you use it to decline the distributed design you were asked for.

5 minsenior, staff, lead

hardDesignScenario

How do you decide between a modular monolith and microservices?

The choice turns on team topology and independent deployability, whether domain boundaries are stable, data ownership and distributed-transaction cost, and real operational maturity. Default to a modular monolith with enforced internal boundaries and extract services only on evidence.

4 minsenior, staff, lead

hardDesignScenario

Two teams need the same customer data. Do you give them one shared database, or two copies that are allowed to disagree?

Name the single writer first, then choose which coupling you would rather own: a shared schema couples the two teams' release calendars, while a replicated copy couples them to a staleness window and obliges someone to detect and repair divergence. Both are permanent, so pick the one your organisation can pay for.

6 minmid, senior, staff

mediumDesignConcept

Service A needs something from service B. When should that be a synchronous call and when should it be an event?

A synchronous call couples availability and latency but keeps the outcome knowable at the point of decision; an event decouples both but converts every failure into something the caller will never learn about. Choose by asking whether the caller needs the answer to proceed, not by which style is fashionable.

4 minmid, senior, staff

All 4 in Architecture Trade-offs

Quality Attributes & NFRs

3 questions

Eliciting and prioritising non-functional requirements, and turning them into testable targets.

mediumDesignScenario

The business says the system must be highly available. How do you turn that into a number, and what does that number cost?

Convert the phrase into an error budget over a stated window, derived from what an outage costs the business rather than from a round number of nines. Each additional nine roughly multiplies cost, because it forces redundancy, cross-region failover and operational maturity that the previous nine did not need.

4 minmid, senior, staff, lead

hardDesignScenarioCase Study

A stakeholder tells you the system must be scalable. How do you turn that into a requirement you can design and test against?

Rewrite it as a quality-attribute scenario: a named stimulus arriving at a stated rate, hitting a stated system state, with a response measured at a percentile over a defined window. A number with a load figure, a duration and a named sacrifice is designable and testable; a list of maxima is not.

7 minsenior, staff, lead

hardScenarioDesign

Two quality attributes are in direct conflict and both stakeholders say theirs is non-negotiable. How do you resolve that?

Express both attributes as measurable scenarios, then lay out the intermediate designs so the choice is between numbers rather than adjectives. Where the conflict is genuinely irreducible, escalate to whoever owns both consequences and record what was sacrificed, with a trigger for revisiting.

7 minsenior, staff, lead

All 3 in Quality Attributes & NFRs

Documentation & ADRs

3 questions

Recording decisions so they survive turnover, and communicating architecture to mixed audiences.

mediumConceptScenario

Every architecture document you have inherited is out of date. How do you write documentation that survives contact with a changing system?

Stale documentation is a symptom of documenting things that change faster than anyone will maintain them. Write down decisions and their reasons, which are immutable once made, generate anything derivable from the code, and let the rest go undocumented on purpose.

4 minmid, senior, staff, lead

hardScenarioConcept

You want to reverse a decision recorded in an ADR two years ago, and everyone who made it has left. How do you go about it?

Reconstruct the original context first and test whether it has genuinely changed or whether you simply inherited a cost you dislike, then write a new record that supersedes rather than edits the old one, price the half-reversed state explicitly, and get agreement from the teams who now carry the consequences.

6 minsenior, staff, lead

mediumConceptDesign

What belongs in an architecture decision record, and how do you decide a decision deserves one?

An ADR captures the context that forced a choice, the options considered with the reason each was rejected, the decision, and the consequences including the costs you accept. Write one when the decision is expensive to reverse or would surprise a future reader, and supersede it rather than edit it.

6 minmid, senior, staff, lead

All 3 in Documentation & ADRs

Migration & Modernisation

2 questions

Strangler-fig sequencing, dual writes, data migration, and de-risking a legacy replacement.

hardScenarioCase Study

Both the old and the new system are running side by side. How do you decide the day you switch customers over, and what does rollback mean once you have?

Write the entry criteria before the parallel run begins so the decision is a checklist rather than a mood, ramp through a stepped population with an abort condition at each step, and recognise that rollback stops meaning redeploy once the new system takes its first authoritative write.

6 minsenior, staff, lead

hardDesignCase Study

How do you replace a system that cannot be switched off?

Put a routing facade in front of the old system, migrate one business capability at a time, and treat dual-write as a reconciliation problem rather than two inserts: name a writer of record at every moment, shadow-compare before flipping reads, and sequence slices so value lands early.

6 minsenior, staff, lead

Cost & Efficiency

3 questions

Cloud cost modelling, build-versus-buy reasoning, and defending spend to non-technical stakeholders.

hardScenarioDesign

Your observability bill is now bigger than your compute bill. What do you cut first, and what must you never lose?

Attribute observability cost by signal, service and pricing dimension. Then cut by value per byte: debug and duplicate logs at source, logs that should be counters, hot retention nobody queries. Protect audit and security logs, error events and SLO signals from sampling, and never cut by a flat percentage.

5 minsenior, staff, lead

hardDesignScenario

Your cloud bill is too high. Where does the money go, and how do you bring it down without degrading the service?

Attribute the bill before touching it, then work in order: fix the architectural decisions that generate spend, right-size against measured demand, and only commit to discounts once the shape is stable. Track a unit cost so a reduction can be told apart from a traffic drop.

6 minsenior, staff, lead

hardScenarioCase Study

Your largest customer costs more to serve than they pay you. What do you do about that architecturally?

Earn the right to the claim by attributing infrastructure cost per tenant, then separate cost caused by their usage shape from cost caused by your own design. Architecture can change the shape; the decision about who pays is commercial, and it needs the engineering evidence to happen at all.

6 minsenior, staff, lead

All 3 in Cost & Efficiency

Technical Strategy

3 questions

Setting direction, managing technical debt as a portfolio, and aligning architecture to business goals.

mediumDesignScenarioCase Study

How do you decide whether to build a capability in-house or buy it?

Build what your users pay you for and buy the rest, correcting that heuristic for switching cost and data gravity. The comparison people get wrong is licence fee versus zero, when it is really licence fee versus salaries, on-call and opportunity cost for as long as the system lives.

5 minsenior, staff, lead

hardScenarioCase Study

Feature delivery and paying down architectural debt are competing for the same engineers. How do you decide the split and defend it?

Stop treating debt as a backlog of tasks and express it as a rate — the interest you are paying in change lead time, incident load and onboarding cost — then fund only the remediation that measurably moves one of those numbers. A fixed percentage tax is easy to agree and the first thing cut under pressure.

6 minsenior, staff, lead

hardCase StudyBehavioural

How do you write a technical strategy that changes what teams do, and get it adopted without authority?

A strategy is a diagnosis, a small set of decisions already made, and the things you are choosing not to do — written so a team can tell whether next quarter's plan complies. Adoption comes from applying it to a live decision within weeks, wiring it into existing rituals, and making the compliant path the cheapest one.

6 minstaff, lead

All 3 in Technical Strategy

Architecture Scenarios

3 questions

Situations to reason through: a failing rewrite, a vendor lock-in, a security mandate, a merged codebase.

hardScenarioCase Study

A vendor whose product sits in your critical path has announced end of life in nine months. Walk me through the decision.

Establish what the product is load-bearing for and what the deadline really is once procurement, security review and change freezes are subtracted, then price four options against that calendar: pay for an extension, move to the vendor's successor, replace it, or absorb the capability.

6 minsenior, staff, lead

hardScenarioDesign

You discover two teams have independently built notification services that do roughly the same thing. What do you do?

Find out why it happened before deciding what to do about it, because duplication is usually a symptom of a discovery or ownership failure that consolidation alone will not fix. Then choose deliberately between merging, keeping both, and letting one wither, on the basis of migration cost rather than tidiness.

4 minsenior, staff, lead

hardScenarioCase Study

You have inherited a system nobody left behind any knowledge of, and you have two weeks to produce a plan. Walk me through it.

Spend the first week building an evidence map from money, traffic, incidents and deploys rather than from code, and interviewing the people who touch it with one fixed set of questions. Spend the second producing a risk-ordered plan of stabilisation before structure, with unknowns listed explicitly.

6 minsenior, staff, lead

All 3 in Architecture Scenarios