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PR · idempotency-middleware
idempotency.go2 missed· 3 caught
23func (m *Middleware) Handle(...) {
24 key := r.Header.Get("Idempotency-Key")
25 if cached, ok := m.store[key]; ok {
26 writeCached(w, cached)
27 return
28 }
29 result := psp.Charge(...)
30 m.store[key] = result
31 writeCached(w, result)
32}
···
45func (m *Middleware) lock() {
46 m.mu.Lock()
47 defer m.mu.Unlock()
0
StrongSenior rubricscore / 100
S5correctness

Race window: cache check isn't atomic with PSP call.

S4correctness

In-memory store loses idempotency state on restart.

S4correctness

Float for money — precision drift on refunds.

S3observability

Missing trace ID on retry path.

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bug workspace · payment-service/tests
payment_test.go2 failures· 1 passing
=== RUN TestChargeIdempotency
    --- FAIL: duplicate charge on retry (2.31s)
    want: 1 charge created
    got:  2 charges (txn-4821, txn-4822)
=== RUN TestConcurrentRequests
    --- FAIL: race condition detected (1.84s)
    concurrent writes to m.store[key]
    DATA RACE: write at 0x00c000118040
=== RUN TestBasicCharge
    --- PASS: charge succeeds (0.12s)
FAIL   payment-service/tests  4.27s
ROOT CAUSE
S5correctness

Store mutation is not atomic — no check-before-insert. Concurrent requests bypass the idempotency guard.

HYPOTHESES RULED OUT
✗PSP deduplicated on its end
✗Network retry stripped the key
✓Race in m.store write (confirmed)
CANONICAL FIX
m.mu.Lock()
defer m.mu.Unlock()
// check store inside lock
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00—Build It · Low-level design

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lld workspace · rate-limiter · Go
Rate Limiter
Stage 3 / 4Senior filter

Design a distributed sliding window rate limiter. Enforce 100 requests per minute per user ID across a horizontally-scaled backend.

Constraints
→Redis backend, Lua for atomicity
→At-least-once delivery guarantee
→Sub-5ms overhead per request
→Prove conservation: allowed + rejected = total requests
Stage progress
Stage 1 · Fixed window✓ passed
Stage 2 · Sliding window✓ passed
Stage 3 · Distributedin progress
Stage 4 · Multi-regionlocked
Conservation invariant
allowed: 9,845
rejected: 153
total:  9,998 ≠ 10,000
✗ invariant violated — 2 lost
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Topics commonly tested in senior backend and system-design interviews, including Kafka, idempotency, Raft, distributed locks, rate limiting, and consistent hashing. 3 free, 13 total.

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deep dives · idempotency
13 Topics
Idempotencyreading
Saga · Outbox · CDC✓
Kafka internals—
Distributed cache—
Rate limiting—
Consistent hashing—
Distributed locks—
Two-phase commit—
Raft & leader election—
Chat at scale—
Push notifications—
Matching engine—
URL shortener—
Deep Dive 01

Idempotency

The most common correctness failure in distributed payment systems is the duplicate charge. It happens when a client retries a failed request that actually succeeded — the PSP charged, but the response was lost in transit.

The fix is an idempotency key: a client-generated UUID attached to each request.

// In prod: use Redis Lua for atomicity.
ifcached, ok := store.Get(key); ok {
return cached // idempotent response
}
result := psp.Charge(amount)
store.Set(key, result)

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Is the backend-heavy content still useful if my loop isn't payments?+

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