5 systems are free to run end-to-end. The other 26 unlock with Pro. Hit play, watch a request flow, click any box, trigger a failure, then get quizzed.
The smallest real system on Reqflow. Hit play, follow the request through cache and DB, then click every box.
Fan-out on write vs read, ranking, CDN.
Persistent connections, message queues, delivery.
Room-based pub/sub fanout across gateway instances.
64-bit time-sortable IDs: timestamp + worker id + sequence, minted locally with no per-request coordination.
Geo-indexing, dispatch, real-time location streams.
CDN-first, adaptive bitrate, recommendation.
Idempotency, fraud, async webhooks, ledger.
Chunking, content-addressed dedup, multi-device sync.
Multi-channel async delivery, user preferences, retries.
Distributed counters, consistent hashing, fail-open vs fail-closed.
Consistent hashing ring, replication, hot key handling.
Video pipeline, ML-driven recommendation, infinite scroll.
Web crawl, inverted index, scatter-gather query, ranking.
Geohashing, nearby search, review writes, hot-query caching.
Seat holds, payment timeouts, virtual queue for hot events.
Hybrid fan-out: push for most users, pull for celebrities, merged at read.
Partitioned, replicated log. Brokers, ISR, consumer groups, leader failover.
Multi-AZ erasure coding, sharded metadata, strong read-after-write.
Operational Transforms, per-document op log, snapshot + delta replay.
Single-threaded per-symbol matching, in-memory order book, microsecond latency.
Geo-search, atomic seat-holds, authorize-then-capture payments, async fan-out.
Federated social: per-user PDS, global firehose Relay, plug-in AppViews and custom feeds.
Redis sorted sets for O(log n) rank queries; durable points in MySQL; top-N, your rank, players around you.
URL frontier, politeness delays, Bloom-filter dedup, distributed fetching at billions of pages.
Precomputed trie cache in Redis, debounced prefix queries, batch + streaming frequency pipeline.
Cron at scale: leader election, CAS dispatch, at-least-once delivery, exactly-once via dedup, catch-up execution.
Leaderless replication, vector clocks, sloppy quorums, hinted handoff, the architecture behind DynamoDB and Cassandra.
Time-series ingestion at millions of points/sec, Prometheus scraping, TSDB columnar storage, alerting pipeline.
Exactly-once stream aggregation, Lambda vs Kappa architecture, time-windowed counts, fraud detection.
Cross-wallet transfers, double-entry ledger, saga compensations, event sourcing + CQRS.