Design review patterns

Ambiguity you would resolve with the interviewer: scope, scale, who uses it, what counts as done.

• Skipped clarifications

  clarification

  Started designing without asking clarifying questions.

  clarification.skipped
• No scale question

  clarification

  Did not ask about scale, traffic, or growth before designing.

  clarification.missed-scale
• Unbounded scope

  clarification

  Did not narrow scope; tried to solve everything.

  clarification.missed-scope

Bullets, not prose. One capability per line.

• Vague requirement

  functional

  Functional requirements lack specificity or measurable outcome.

  functional.vague-requirement
• No prioritization

  functional

  Did not separate must-have from nice-to-have.

  functional.missing-priority

Latency, availability, consistency, durability, security targets.

• No latency target

  non-functional

  Did not specify p50/p99 latency targets.

  non-functional.missing-latency
• No availability target

  non-functional

  Did not specify SLO or availability target.

  non-functional.missing-availability
• No consistency model

  non-functional

  Did not commit to a consistency model.

  non-functional.missing-consistency
• No durability target

  non-functional

  Did not specify durability or RPO.

  non-functional.missing-durability

QPS, storage, bandwidth, growth. Order of magnitude before boxes.

• Skipped capacity

  capacity

  Did not estimate capacity before drawing components.

  capacity.skipped
• No write QPS

  capacity

  Did not estimate write QPS, only read.

  capacity.missing-write-qps
• No read QPS

  capacity

  Did not estimate read QPS.

  capacity.missing-read-qps
• No storage estimate

  capacity

  Did not estimate storage growth.

  capacity.missing-storage
• No bandwidth estimate

  capacity

  Did not estimate egress or bandwidth.

  capacity.missing-bandwidth
• Math error

  capacity

  Estimate is wrong by an order of magnitude.

  capacity.math-error
• Unjustified assumption

  capacity

  Pulled a number without grounding.

  capacity.unjustified-assumption

Endpoints, request/response shapes, protocols.

• No pagination

  api

  List endpoint without pagination.

  api.missing-pagination
• No idempotency

  api

  Mutation endpoint without idempotency key strategy.

  api.missing-idempotency
• No auth model

  api

  API design did not specify authn/authz.

  api.missing-auth
• Weak error model

  api

  No error codes, retry semantics, or partial-failure handling.

  api.weak-error-model
• No rate limiting

  api

  Public endpoint without rate limit.

  api.missing-rate-limit

Schema, indexes, partition keys, database choice — and why.

• DB without defense

  data-model

  Picked a database without defending the choice against access patterns.

  data-model.db-without-defense
• No partition strategy

  data-model

  Did not specify partition or shard key for a sharded store.

  data-model.no-partition-key
• No index strategy

  data-model

  Did not name secondary indexes for query patterns.

  data-model.missing-index-strategy
• Wrong DB family

  data-model

  Picked a DB family that does not match the access pattern.

  data-model.wrong-family

Walk one requirement at a time. Answer the hard question each one raises before you draw anything.

Components, data flow, technologies — placed by drag or by keyboard, then defended.

• No cache failure story

  hld

  Cache present but no plan for cache miss, cold start, or stampede.

  hld.no-cache-failure-story
• No replication

  hld

  Stateful component without replication or failover.

  hld.no-replication
• Single point of failure

  hld

  Critical component without redundancy.

  hld.spof
• No async boundary

  hld

  Slow or unreliable work done synchronously instead of via queue.

  hld.no-async-boundary
• Orphan component

  hld

  Component drawn but not connected to data flow.

  hld.orphan-component
• No rate limit at edge

  hld

  No rate limit at gateway or edge for public traffic.

  hld.missing-rate-limit
• No load balancer

  hld

  Multi-instance service without LB or service discovery.

  hld.missing-load-balancer

Problem-specific hard parts. Pick the worst failure modes.

• Shallow failure analysis

  deep-dives

  Failure-mode discussion is generic, not specific to this design.

  deep-dives.shallow-failure-mode
• No numbers

  deep-dives

  Deep dive lacks back-of-envelope numbers.

  deep-dives.no-back-of-envelope

The failure story per component, and what you gave up to get here.

• No 10× story

  tradeoffs

  Did not address what breaks at 10× scale.

  tradeoffs.missing-10x-story
• No cost discussion

  tradeoffs

  Did not discuss cost trade-offs.

  tradeoffs.no-cost-discussion
• No failure modes

  tradeoffs

  Did not enumerate failure modes per component.

  tradeoffs.no-failure-mode

Patterns that span multiple stages of the design discussion.

• Unjustified choice

  cross-stage

  A specific choice was made without naming the trade-off.

  cross-stage.unjustified-choice
• Contradicts requirements

  cross-stage

  Design choice contradicts a requirement the user stated earlier.

  cross-stage.contradicts-requirements
• Over-engineered

  cross-stage

  Added complexity that the requirements do not justify.

  cross-stage.over-engineered
• Capacity ignored

  cross-stage

  Stage-4 capacity numbers say a drawn component is overloaded, but the HLD wasn't scaled to match.

  cross-stage.capacity-ignored