[
  {
    "id": 1,
    "path": "lessons/01-the-last-seat.md",
    "title": "Two successful transactions, one impossible result",
    "prerequisites": [],
    "sources": [
      "components/21-database-isolation-levels-acid-and-concurrency-anomalies.md"
    ],
    "problem": "Two buyers both reserve the last seat.",
    "prototype": true
  },
  {
    "id": 2,
    "path": "lessons/02-the-promise-in-ok.md",
    "title": "The write that survived the process but not the machine",
    "prerequisites": [
      1
    ],
    "sources": [
      "components/07-write-ahead-log-and-lsm-trees.md",
      "loop-primitives/01-write-ahead-log-fsync-and-group-commit.md"
    ],
    "problem": "A successful write disappears after power loss.",
    "prototype": false
  },
  {
    "id": 3,
    "path": "lessons/03-recovering-the-right-history.md",
    "title": "The file exists, but recovery must ignore it",
    "prerequisites": [
      2
    ],
    "sources": [
      "components/07-write-ahead-log-and-lsm-trees.md",
      "loop-primitives/01-write-ahead-log-fsync-and-group-commit.md",
      "loop-primitives/02-lsm-trees-and-compaction.md"
    ],
    "problem": "A crash leaves an SSTable outside the durable manifest.",
    "prototype": false
  },
  {
    "id": 4,
    "path": "lessons/04-the-compaction-debt.md",
    "title": "Fast writes, then a completely frozen database",
    "prerequisites": [
      3
    ],
    "sources": [
      "components/07-write-ahead-log-and-lsm-trees.md",
      "loop-primitives/02-lsm-trees-and-compaction.md"
    ],
    "problem": "Background merging cannot keep up with foreground writes.",
    "prototype": false
  },
  {
    "id": 5,
    "path": "lessons/05-asking-a-cheaper-question.md",
    "title": "Can we prove the key is absent without reading it?",
    "prerequisites": [
      4
    ],
    "sources": [
      "components/03-bloom-filters-and-counting-filters.md"
    ],
    "problem": "Negative lookups spend most of the disk budget.",
    "prototype": false
  },
  {
    "id": 6,
    "path": "lessons/06-the-timeout-that-did-not-cancel.md",
    "title": "The client gave up; the server kept working",
    "prerequisites": [
      1
    ],
    "sources": [
      "components/22-circuit-breaker-bulkhead-and-fault-tolerance-patterns.md",
      "loop-primitives/10-retries-timeouts-backpressure-and-load-shedding.md"
    ],
    "problem": "Retries multiply work whose outcomes remain unknown.",
    "prototype": false
  },
  {
    "id": 7,
    "path": "lessons/07-saving-the-healthy-half.md",
    "title": "An optional widget exhausts the checkout pool",
    "prerequisites": [
      6
    ],
    "sources": [
      "components/14-api-gateway-and-reverse-proxy.md",
      "components/22-circuit-breaker-bulkhead-and-fault-tolerance-patterns.md",
      "loop-primitives/10-retries-timeouts-backpressure-and-load-shedding.md"
    ],
    "problem": "A slow dependency consumes resources needed by healthy routes.",
    "prototype": false
  },
  {
    "id": 8,
    "path": "lessons/08-one-bucket-one-decision.md",
    "title": "The limiter that allows twice its advertised limit",
    "prerequisites": [
      1
    ],
    "sources": [
      "components/02-distributed-rate-limiting.md",
      "loop-primitives/14-rate-limiting-algorithms.md"
    ],
    "problem": "A window boundary permits an unexpected burst.",
    "prototype": true
  },
  {
    "id": 9,
    "path": "lessons/09-sharing-a-budget.md",
    "title": "Twenty correct limiters enforce the wrong limit",
    "prerequisites": [
      8,
      7
    ],
    "sources": [
      "components/02-distributed-rate-limiting.md",
      "loop-primitives/14-rate-limiting-algorithms.md"
    ],
    "problem": "Local enforcement multiplies a tenant budget by fleet size.",
    "prototype": false
  },
  {
    "id": 10,
    "path": "lessons/10-the-price-that-came-back.md",
    "title": "Deleting the cache resurrected the old price",
    "prerequisites": [
      1,
      6
    ],
    "sources": [
      "components/04-distributed-cache-patterns-and-eviction.md",
      "loop-primitives/11-caching-and-invalidation.md"
    ],
    "problem": "A slow refill races a committed update.",
    "prototype": false
  },
  {
    "id": 11,
    "path": "lessons/11-when-the-cache-disappears.md",
    "title": "The database cannot survive a successful cache",
    "prerequisites": [
      10,
      7,
      5
    ],
    "sources": [
      "components/04-distributed-cache-patterns-and-eviction.md",
      "loop-primitives/11-caching-and-invalidation.md"
    ],
    "problem": "A cold cache turns nearly every request into origin work.",
    "prototype": false
  },
  {
    "id": 12,
    "path": "lessons/12-where-a-key-lives.md",
    "title": "Adding a server moved almost everything",
    "prerequisites": [
      1
    ],
    "sources": [
      "components/01-consistent-hashing.md",
      "components/08-database-sharding-and-partition-keys.md",
      "loop-primitives/08-sharding-hot-keys-and-rebalancing.md"
    ],
    "problem": "Modulo routing remaps data during expansion.",
    "prototype": false
  },
  {
    "id": 13,
    "path": "lessons/13-the-hot-key.md",
    "title": "The cluster is idle and one customer is throttled",
    "prerequisites": [
      12,
      7,
      11
    ],
    "sources": [
      "components/01-consistent-hashing.md",
      "components/08-database-sharding-and-partition-keys.md",
      "loop-primitives/08-sharding-hot-keys-and-rebalancing.md"
    ],
    "problem": "One logical key exceeds the capacity of its owner.",
    "prototype": false
  },
  {
    "id": 14,
    "path": "lessons/14-copies-that-disagree.md",
    "title": "The database acknowledged a write the replica cannot read",
    "prerequisites": [
      2
    ],
    "sources": [
      "components/01-consistent-hashing.md",
      "components/09-distributed-consensus-raft-and-paxos.md",
      "loop-primitives/06-replication-quorums-and-read-your-writes.md"
    ],
    "problem": "Durable, committed, and applied positions differ.",
    "prototype": false
  },
  {
    "id": 15,
    "path": "lessons/15-who-may-commit.md",
    "title": "Two machines both say they are the leader",
    "prerequisites": [
      14
    ],
    "sources": [
      "components/09-distributed-consensus-raft-and-paxos.md",
      "loop-primitives/06-replication-quorums-and-read-your-writes.md"
    ],
    "problem": "An isolated former leader continues to receive requests.",
    "prototype": false
  },
  {
    "id": 16,
    "path": "lessons/16-the-worker-that-returned.md",
    "title": "The lease expired; the old worker did not",
    "prerequisites": [
      15,
      1
    ],
    "sources": [
      "components/06-distributed-locks-and-leases.md",
      "loop-primitives/03-leases-fencing-tokens-and-distributed-locks.md"
    ],
    "problem": "A paused worker resumes after a replacement takes ownership.",
    "prototype": true
  },
  {
    "id": 17,
    "path": "lessons/17-moving-live-data.md",
    "title": "The copy finished, then the old owner accepted a write",
    "prerequisites": [
      12,
      13,
      14,
      16
    ],
    "sources": [
      "components/01-consistent-hashing.md",
      "components/08-database-sharding-and-partition-keys.md",
      "loop-primitives/08-sharding-hot-keys-and-rebalancing.md"
    ],
    "problem": "A live migration races stale routers and ongoing writes.",
    "prototype": false
  },
  {
    "id": 18,
    "path": "lessons/18-one-intent-many-attempts.md",
    "title": "The payment succeeded twice after one timeout",
    "prerequisites": [
      1,
      6
    ],
    "sources": [
      "components/10-two-phase-commit-and-saga-orchestration.md",
      "loop-primitives/04-idempotency-and-effectively-once-processing.md"
    ],
    "problem": "An ambiguous external effect is retried as a new operation.",
    "prototype": false
  },
  {
    "id": 19,
    "path": "lessons/19-the-message-that-will-not-leave.md",
    "title": "The queue is healthy; the customer is still waiting",
    "prerequisites": [
      18,
      7,
      16
    ],
    "sources": [
      "components/05-message-queues-vs-event-streams.md",
      "loop-primitives/12-queues-and-delivery-semantics.md"
    ],
    "problem": "Redelivery, poison records, and early acknowledgement break progress.",
    "prototype": false
  },
  {
    "id": 20,
    "path": "lessons/20-the-announcement-that-never-happened.md",
    "title": "The order committed, but nobody heard about it",
    "prerequisites": [
      18,
      19,
      3
    ],
    "sources": [
      "components/12-change-data-capture-and-outbox-pattern.md",
      "loop-primitives/05-change-streams-and-transactional-outbox.md"
    ],
    "problem": "A database commit and a publish cannot be made atomic by ordering calls.",
    "prototype": false
  },
  {
    "id": 21,
    "path": "lessons/21-counting-time-not-arrivals.md",
    "title": "Yesterday changed after the stream restarted",
    "prerequisites": [
      19,
      20,
      16,
      4
    ],
    "sources": [
      "components/05-message-queues-vs-event-streams.md",
      "loop-primitives/07-event-time-watermarks-and-checkpoints.md",
      "loop-primitives/04-idempotency-and-effectively-once-processing.md"
    ],
    "problem": "Late events and replay make a plausible counter incorrect.",
    "prototype": false
  },
  {
    "id": 22,
    "path": "lessons/22-the-average-that-lied.md",
    "title": "The dashboard recovered by hiding the incident",
    "prerequisites": [
      21,
      4
    ],
    "sources": [
      "components/07-write-ahead-log-and-lsm-trees.md",
      "loop-primitives/15-time-series-storage-and-roll-ups.md"
    ],
    "problem": "A roll-up discards the information needed to answer the question.",
    "prototype": false
  },
  {
    "id": 23,
    "path": "lessons/23-waiting-for-a-decision.md",
    "title": "All participants voted yes; no participant can finish",
    "prerequisites": [
      1,
      15
    ],
    "sources": [
      "components/10-two-phase-commit-and-saga-orchestration.md",
      "loop-primitives/13-distributed-transactions-sagas-vs-two-phase-commit.md"
    ],
    "problem": "Prepared transactions wait for a durable global decision.",
    "prototype": false
  },
  {
    "id": 24,
    "path": "lessons/24-undo-is-another-operation.md",
    "title": "The refund failed after the booking failed",
    "prerequisites": [
      18,
      20,
      23,
      16
    ],
    "sources": [
      "components/10-two-phase-commit-and-saga-orchestration.md",
      "loop-primitives/13-distributed-transactions-sagas-vs-two-phase-commit.md"
    ],
    "problem": "Independent commits require recoverable semantic compensation.",
    "prototype": false
  },
  {
    "id": 25,
    "path": "lessons/25-moving-the-writer.md",
    "title": "Traffic failed over before authority did",
    "prerequisites": [
      14,
      15,
      16,
      18,
      7
    ],
    "sources": [
      "components/24-cloud-disaster-recovery-and-multi-region-active-active.md",
      "loop-primitives/09-multi-region-failover.md"
    ],
    "problem": "Routing users to a standby does not make it a safe writer.",
    "prototype": false
  },
  {
    "id": 26,
    "path": "lessons/26-rebuilding-a-view.md",
    "title": "The current balance cannot explain its own history",
    "prerequisites": [
      18,
      20
    ],
    "sources": [
      "components/23-event-sourcing-and-cqrs-architecture.md"
    ],
    "problem": "A new query needs a replayable model without duplicating effects.",
    "prototype": false
  },
  {
    "id": 27,
    "path": "lessons/27-an-id-is-not-a-clock.md",
    "title": "The unique ID generator repeated itself after restart",
    "prerequisites": [
      16
    ],
    "sources": [
      "components/15-distributed-unique-id-generators.md"
    ],
    "problem": "Clock rollback and worker reuse violate assumed uniqueness.",
    "prototype": false
  },
  {
    "id": 28,
    "path": "lessons/28-suspected-is-not-dead.md",
    "title": "The failure detector removed a healthy machine",
    "prerequisites": [
      15
    ],
    "sources": [
      "components/16-gossip-protocol-and-failure-detection.md"
    ],
    "problem": "A missing heartbeat cannot distinguish failure from delay.",
    "prototype": false
  },
  {
    "id": 29,
    "path": "lessons/29-following-one-request.md",
    "title": "More connections did not fix the slow page",
    "prerequisites": [
      6,
      7
    ],
    "sources": [
      "components/14-api-gateway-and-reverse-proxy.md",
      "components/19-api-protocols-http1-http2-http3-quic-grpc.md"
    ],
    "problem": "Transport, proxy, and dependency queues create different bottlenecks.",
    "prototype": false
  },
  {
    "id": 30,
    "path": "lessons/30-reconnecting-without-forgetting.md",
    "title": "The socket reconnected; the messages did not",
    "prerequisites": [
      19,
      29
    ],
    "sources": [
      "components/18-websocket-sse-and-long-polling.md"
    ],
    "problem": "A live transport supplies neither history nor replay correctness.",
    "prototype": false
  },
  {
    "id": 31,
    "path": "lessons/31-a-valid-signature-is-not-permission.md",
    "title": "The token is genuine and the request is forbidden",
    "prerequisites": [
      5,
      6,
      20,
      29
    ],
    "sources": [
      "components/20-oauth2-oidc-and-distributed-token-authentication.md"
    ],
    "problem": "Authentication, authorization, and revocation have different boundaries.",
    "prototype": false
  },
  {
    "id": 32,
    "path": "lessons/32-the-prefix-that-scanned-everything.md",
    "title": "Autocomplete found the prefix and still timed out",
    "prerequisites": [
      4,
      20
    ],
    "sources": [
      "components/13-trie-data-structure-and-inverted-index.md"
    ],
    "problem": "Finding a prefix does not find its best completions.",
    "prototype": false
  },
  {
    "id": 33,
    "path": "lessons/33-across-the-cell-boundary.md",
    "title": "The closest driver is invisible to the index",
    "prerequisites": [
      12
    ],
    "sources": [
      "components/11-geospatial-geohash-quadtree-s2.md"
    ],
    "problem": "Spatial cells omit nearby points unless coverage is complete.",
    "prototype": false
  },
  {
    "id": 34,
    "path": "lessons/34-nearest-among-whom.md",
    "title": "The vector index returns ten excellent unauthorized matches",
    "prerequisites": [
      31,
      32
    ],
    "sources": [
      "components/17-vector-databases-and-approximate-nn.md"
    ],
    "problem": "Approximate retrieval and access filters interact.",
    "prototype": false
  },
  {
    "id": 35,
    "path": "lessons/35-cheap-accounts-expensive-consequences.md",
    "title": "Every account obeyed the limit; the attack still worked",
    "prerequisites": [
      9,
      31
    ],
    "sources": [
      "components/25-bot-defense-sybil-resistance-and-registration-abuse.md",
      "components/26-honeypot-fields-and-canary-traps.md",
      "components/27-disposable-email-and-domain-blocklists.md"
    ],
    "problem": "Many cheap identities evade per-identity controls.",
    "prototype": false
  },
  {
    "id": 36,
    "path": "lessons/36-the-proof-that-was-replayed.md",
    "title": "The browser passed the challenge twice",
    "prerequisites": [
      35,
      18,
      7
    ],
    "sources": [
      "components/28-cloudflare-turnstile-managed-challenges.md",
      "components/29-google-recaptcha-v2-and-enterprise-risk-analysis.md"
    ],
    "problem": "A challenge token is accepted without a correct server-side lifecycle.",
    "prototype": false
  },
  {
    "id": 37,
    "path": "lessons/37-paying-for-each-attempt.md",
    "title": "The puzzle was hard once and free thereafter",
    "prerequisites": [
      35,
      18
    ],
    "sources": [
      "components/30-hashcash-and-client-side-proof-of-work.md"
    ],
    "problem": "A reusable proof makes client work irrelevant to repeated abuse.",
    "prototype": false
  }
]