Cache Stampede at Dawn — JavaScript Bug Hunt

Briefing

Every morning at 06:00, the Horizon Analytics database gets hammered with identical queries — the cache layer in front of it has quietly stopped caching. Worse, invalidating one report wipes the entire cache.

The mock database (with a call counter) and the raw cache store are locked. All three bugs live in cachedRepo.js.

Bug report

BUG-9977 · Priority: Critical · Reported by: DBA on-call

With TTL = 60s:

• Two immediate reads of the same key hit the database TWICE (should be once).
• Reads of EXPIRED entries are served from cache forever — stale dashboards.
• invalidate("report:42") clears every key, causing the 06:00 stampede.

Contract: get(key, nowMs) -> value, using db.load(key) on a miss.

Logs

[db] load report:42   (call #1)
[db] load report:42   (call #2)   <- cache miss again?!
[cache] invalidate report:42 -> store size 0 (was 31)

The code as shipped

src/db/cachedRepo.js (editable)

// Read-through cache in front of the analytics database.
var db = require("./mockDb");
var store = require("./cacheStore");

var TTL_MS = 60000;

exports.get = function (key, nowMs) {
  var entry = store.get(key);
  if (entry) {
    if (nowMs - entry.storedAt < TTL_MS) {
      return entry.value;
    }
    return entry.value; // expired entries are cheap to serve
  }

  var value = db.load(key);
  return value;
};

exports.invalidate = function (key) {
  store.clear();
};

exports.resetAll = function () {
  store.clear();
  db.resetCalls();
};

Read-only context: src/db/cacheStore.js, src/db/mockDb.js.