Lenskart System Design Engineer Interview Preparation

A queue absorbs a temporary mismatch but cannot create processing capacity. If arrivals exceed sustained service capacity, waiting time and memory grow. Bound queue length and age, prioritize essential work and reject excess demand explicitly. Estimate both steady-state throughput and burst drain time.

Check your understanding: Keep arrivals above service rate and verify bounded waiting and memory.

Common trap: An unbounded queue presented as a scalability solution.

Concept reference: Google SRE: handling overload

Enforce the invariant where the durable state changes, using an atomic conditional write or an appropriate transaction. Caches can accelerate reads but are not automatically the authority for balance updates. Define conflict handling and reconcile retries with the committed operation identity.

Check your understanding: Interleave two debits against the last available balance.

Common trap: Checking the invariant only in a cache or application-local lock.

Concept reference: PostgreSQL: transaction isolation

Allocate an end-to-end deadline and bounded retry budget, then propagate remaining time to downstream calls. Use backoff with jitter where retrying is appropriate and stop admitting work that cannot finish usefully. Measure attempts per original request to expose amplification across layers.

Check your understanding: Simulate a slow dependency and compare original traffic with total attempts.

Common trap: Three retries at every layer with no aggregate limit.

Concept reference: Google SRE: handling overload

First, identify technical constraints and define measurable service objectives. Next, turn requirements into quantified capacity, state ownership and explicit failure boundaries. Contrast architectural trade-offs across simplicity, correctness, maintainability and scale, explicitly mitigate the risk of the new schema reaches only part of the fleet, and confirm system stability using a compatibility test matrix and a staged rollout metric.

Common trap: Reaching for a specific library or framework before defining constraints, failure envelopes, and automated verification criteria.

Prioritise the failure mode exhibiting the highest user blast radius and lowest observability. Formulate an explicit containment boundary, implement idempotent retries with jitter, and establish an automated rollback threshold. Verify resilience through lease-expiry tests, idempotency records and a restart recovery drill.

Common trap: Relying on passive monitoring dashboards without defining explicit error-budget alerts, rollback triggers, and verified recovery procedures.

Structure the response using Context-Decision-Tradeoff-Result: articulate the business and technical constraints, compare viable alternatives, explain the implementation (turn requirements into quantified capacity, state ownership and explicit failure boundaries), and document the accepted trade-off. Provide concrete proof: a project example, measured result and repeatable verification step.

Common trap: Speaking only in high-level abstractions or team accomplishments without detailing your direct implementation decisions, trade-offs, and measured results.

Formulate a falsifiable hypothesis from observable telemetry before altering configurations. Then compare service-level indicators, queue growth, dependency budgets and recovery-point objectives. Isolate the defect to the smallest reproducible boundary, validate root cause with evidence, and confirm full resolution using a concurrency test and an audit trail demonstrating conflict handling.

Common trap: Applying speculative fixes or restarting services blindly without establishing an observable signal connected to a falsifiable hypothesis.

Produce a capacity worksheet, architecture decision record and failure-recovery drill. Document baseline assumptions, technical mechanism (turn requirements into quantified capacity, state ownership and explicit failure boundaries), rejected alternatives, bounded failure envelopes, and deterministic pass criteria. Supply reproducible verification via a compatibility contract, expand-and-contract rollout and rollback rehearsal.

Common trap: Presenting architecture diagrams or slides lacking automated unit/integration tests, observable metrics, or automated rollback configurations.