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Multiple requests at the exact same millisecondClock skew / time jumps (system time moves backward?)Memory growth for inactive userslimit = 0 or window = 0Thread safety under same user, concurrent requests“How would you clean old users?”“What if this runs for weeks?”“What if allowRequest is called concurrently?”
Overwriting a key resets TTL?TTL = 0 or negativeConcurrent get/putMemory leak from expired keysClock precision
Confirm called twiceHold expires while confirmingHold ID is invalidSame user holds multiple seatsSeat released after expiry
Credit or debit with negative amountInteger overflow on balanceConcurrent debit callsTransaction list mutabilityOrdering of transactions
Discount makes price negativeOrder of rule executionTwo conflicting discountsAdding new discount without changing engineFloating-point precision
Partial success (Email fails, SMS succeeds)Infinite retry loopChannel order changeRetry count per channel or global?Blocking calls
Schedule same taskId twiceCancel while task is executingTask scheduled in the pastExecutor thread exhaustionMemory leak of completed tasks
Reserve more than availableRelease more than reservedNegative stock inputConcurrent reserve + releaseLock granularity per product vs global
Payment called twiceFulfillment before paymentRetry payment after failure?Order creation duplicate IDIdempotency
rolloutPercentage < 0 or > 100Feature recreated (overwrite?)Determinism across deploymentsHash collisionsUserId is null
Handler throws exceptionSlow handler blocking othersPublish with no subscribersSubscribe during publishOrdering guarantees?
Usage before customer addedReset during concurrent usageFloating-point roundingTier change mid-monthOverflow in counters
Rule execution orderRule state shared across usersRolling window memory cleanupTime boundary conditionsThread safety
Task succeeds after retries startedDuplicate task submissionJVM restart (persistence?)Backoff overflowThread leaks
Handle at least 2–3 hidden cases explicitlyMention others verbally
Code happy pathIgnore verbal edge casesSurface edge cases proactivelyEncode invariantsDefend design verbally
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Hidden Edge Cases Interviewers Expect (Day-wise)
Excellent choice.
These are the “spoken edge cases” interviewers don’t write, but expect you to ask about or handle. Missing these is what usually differentiates average SDE-2 from strong SDE-2 / SDE-3.
Below is a problem-by-problem hidden edge-case checklist you should mentally scan during interviews.
🔍 Hidden Edge Cases Interviewers Expect (Day-wise)
🟢 Day 1 — Foundations
1️⃣ Rate Limiter (Sliding Window)
Hidden Edge Cases
Verbal Follow-ups Interviewers Ask
Strong Signal
“I’d lazily clean expired timestamps and optionally evict inactive users.”
2️⃣ Key-Value Store with TTL
Hidden Edge Cases
Verbal Trap
“Should expired keys still occupy memory?”
Correct Answer
“No, expired keys should be removed lazily or via cleanup.”
🟡 Day 2 — State & Invariants
3️⃣ Seat Hold & Reservation
Hidden Edge Cases
Verbal Follow-up
“What if confirm comes exactly at expiry time?”
Strong Answer
“Define expiry strictly (≤ expiry invalid) and enforce consistently.”
4️⃣ Wallet & Transaction Ledger
Hidden Edge Cases
Verbal Trap
“What if two debits come at same time?”
Expected
Synchronization / atomicity explanation.
🟠 Day 3 — Extensibility & Failures
5️⃣ Discount Engine
Hidden Edge Cases
Verbal Follow-up
“What if tomorrow we add loyalty points discount?”
Correct Signal
“Just implement another DiscountRule — no engine change.”
6️⃣ Notification Dispatcher
Hidden Edge Cases
Verbal Trap
“What if all channels fail?”
Expected
Return false + logging + metrics.
🔵 Day 4 — Concurrency
7️⃣ Task Scheduler
Hidden Edge Cases
Verbal Follow-up
“What happens if schedule() is called with past timestamp?”
Good Answer
“Execute immediately or reject — define contract clearly.”
8️⃣ Inventory Manager
Hidden Edge Cases
Verbal Trap
“Why not just synchronize everything?”
Expected
Trade-off explanation (scalability vs simplicity).
🟣 Day 5 — Workflow & Config
9️⃣ Order Processing Pipeline
Hidden Edge Cases
Verbal Follow-up
“Can we retry payment?”
Strong Answer
“Yes, but only from CREATED, not terminal states.”
🔟 Feature Flag System
Hidden Edge Cases
Verbal Trap
“Why not random() < rollout?”
Correct
“Random breaks determinism — users would flicker.”
🔴 Day 6 — Async & Aggregation
1️⃣1️⃣ Event Bus
Hidden Edge Cases
Verbal Follow-up
“Should one bad handler stop others?”
Expected
“No — isolate handler failures.”
1️⃣2️⃣ Billing System
Hidden Edge Cases
Verbal Trap
“What if tier changes mid-cycle?”
Good Answer
“Either prorate or apply from next cycle — clarify.”
⚫ Day 7 — Interview Killers
1️⃣3️⃣ Payment Rule Engine
Hidden Edge Cases
Verbal Follow-up
“How would you add a new fraud rule?”
Correct Signal
“Implement Rule interface — engine unchanged.”
1️⃣4️⃣ Retry Queue + DLQ
Hidden Edge Cases
Verbal Trap
“What if task succeeds on retry #2?”
Expected
Stop retrying immediately, never DLQ.
🧠 How to Use This in Interviews (Very Important)
Before Coding (Say This)
“Let me clarify edge cases like concurrency, retries, and invalid inputs.”
While Coding
If Time Runs Out
“I’d add safeguards for X, Y, Z as discussed.”
That alone boosts evaluation.
🏁 Final Truth
Most candidates:
Strong candidates:
You’re now training like the top 10–15%.
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