Book Seller · Sources · Exercise Hub

Prompt

Defines customer requests, maximum price, registered sellers with varying APIs, synchronous and asynchronous processing, and 10,000 queries per second.

Submission A · Staff worker design

CombinedOliveTyrannosaurus163 · Staff ↗

MySQL, read replicas, Redis seller cache, SQS workers, seller API gateway, Stripe
DB-first intake, pending-order scanner, Redis order lock, visibility timeout
Ten-minute seller hold and idempotency keys
Main review concerns: all-seller fan-out, author partitioning, token storage, payment boundary

Submission B · Async purchase workers

Rohitashwa Nigam · Senior ↗

Purchase Service, Postgres, outbox, hosted queue, multi-step workers
Client-side Stripe card entry, per-seller circuit breakers and rate limits
Redis offer cache and worker thread-pool seller calls
Main review concerns: one-winner commit, compensations, lock semantics, broad fan-out

Submission C · Inventory stream

SunnyCoralWoodpecker578 · Staff ↗

Inventory ingestion, per-book price stream, partitioned Matching Service
Postgres inventory and order stores, saga, payment processor, third-party order service
HMAC seller updates and CDC notifications
Main review concerns: two inventory writers, missing hold lifecycle, replay ordering, external-call idempotency

Submission D · Kafka and OCC

DominantScarletBoa987 · Senior Manager ↗

Seller Registration Service, DynamoDB, Kafka, Book Service, Payment Processor
Per-item ordering and optimistic concurrency for inventory transitions
Strong community discussion of PCI scope, hot partitions, external inventory, and fairness

Cross-source matrix

Topic	A	B	C	D
Durable order store	MySQL	Postgres	Orders DB	DynamoDB
Async processing	SQS	Hosted queue	Book stream	Kafka
Seller protection	Gateway + rate limit	Rate limit + circuit	Signed ingestion	Gateway throttle
Inventory control	Seller hold	Seller attempt	OCC reservation	OCC + ordered stream
Payment	Stripe	Stripe PaymentIntent	Processor saga	Processor
Single winner	Redis lock	Review adds DB CAS	Partitioned matcher	Kafka ordering + OCC
Price strategy	Live seller calls	Cache then calls	Ingested updates	Internal listings

Evidence rules

Tier 1: prompt requirements plus architecture elements recurring across the submissions.
Tier 2: implementation choices with strengths and weaknesses stated by submissions or their review threads.
Tier 3: peripheral issues raised in comments that materially affect correctness, security, scale, or interview discussion.
AI-generated comments are review evidence and can contain errors. They inform criticism and follow-up questions; they carry lower authority than the submitted diagram and description.
Human comments from the Hello Interview administrator and community remain attributed as reviewer observations.

Claim mapping

Claim	Evidence
Persist before asynchronous fulfillment	A description; B diagram/outbox; C and D durable order stores
Temporary reservation precedes completion	A hold endpoint; C saga reservation; D inventory pending state; reviewer threads
Every external effect needs idempotency	A description; reviews on A, B, and C
Live all-seller fan-out risks scale	A review; B review; catalog alternatives in C and D
One writer or atomic winner is required	B review; C review; A workflow discussion
Client-side tokenization reduces PCI exposure	A and D comments; B diagram and review
Book-key ordering trades fairness for hot partitions	C diagram and review; D diagram and administrator thread

Verification record

Reviewed: 2026-09-18
Method: JavaScript-rendered pages opened in the in-app browser; full visible text and comments inspected; canvases fitted and zoomed for diagram review.
Diagram policy: two SVG reproductions on the diagrams page, each credited to its source submission.
Synthesis policy: the recall sheet and flow page state their cross-submission status explicitly.
Source pages: requirements, decisions, review notes, and source records contain only claims present in the reviewed prompt, diagrams, descriptions, or comments.