Real backend problems, reproduced as running services.
Each case takes a real problem, a race or a limit or a query that lies, implements it for real, and measures what happens. The status squares are honest: they come from each demo's health endpoint.
REGISTRY — 6 ENTRIEScolumns from /health, not claims
cache-invalidationCache invalidation: reads, writes, and the stale window
———failure-labRetry, backoff, and the breaker that stops the bleeding
———idempotent-paymentIdempotent payments: the same request, charged exactly once
———pagination-benchPagination at depth: offset vs keyset, measured
———security-labWho gets in: roles, tenants, and the audit trail
———inventory-lockingThe oversell race: locking, versions, and idempotency at checkout
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