Ticketmaster
The virtual waiting room, atomic seat holds in Redis, expiry via delayed queues, bot defence, seat-map reads at 500k QPS, and payment failures that must release inventory.
Online Auction
Actor-per-auction for linearizable bids, proxy bidding correctness, soft-close anti-sniping, broadcasting price to 200k watchers, and timers that must fire exactly once.
Payment System
Idempotency under unknown outcomes, immutable double-entry ledgers in integer cents, saga orchestration with compensations, bank-file reconciliation, PCI scope, and multi-currency.
Robinhood
A limit order book in memory, LMAX-style single-threaded matching, market data fan-out to millions, buying power under T+1 settlement, and the audit trail regulators require.
Ad Click Aggregator
Exactly-once aggregation with Flink two-phase commit, click fraud detection in the stream, watermarks for late mobile data, budget enforcement that cannot overspend, and attribution joins.