Level 4 · Flash Sale
TRAFFIC SPIKES

Stage 3 · Traffic Spikes · Headroom · Elasticity

Flash Sale

An e-commerce store is running a flash sale. Normal traffic is fine — but when the sale drops, demand spikes 5×. Build a system that doesn't fall over the moment everyone shows up at once.

Requirements

Functional

  • ·Shoppers browse products and check out

Non-functional

  • ·~200 requests/sec baseline, 90% reads
  • ·must survive a 5× spike (~1,000 r/s) at ≥ 95% availability
  • ·p99 latency ≤ 250ms at baseline

Constraints

  • ·Budget: $16/hr or less — headroom for the spike still has to fit the budget

Out of scope

  • ·Payment processing details
  • ·Inventory locking

TrafficRequests per second — the rate of incoming traffic this level throws at your system.

~200 r/s · read-heavy
90% reads · 10% writesWhat fraction of requests are reads (fetches) vs writes (updates) — this decides which components sit on the hot path.

Win conditions

  • p99 latency99% of requests finish at or under this time — a stricter tail-latency bar than p95 that catches worst-case slowness.≤ 250ms
  • availabilityThe share of legitimate requests that succeeded, out of everything sent your way. 99% still means 1 in 100 users saw an error.≥ 99%
  • throughputHow many requests per second your system actually completed — not requested, completed.≥ 180 r/s
  • error rateThe share of requests that failed outright: timeouts, drops, or capacity overload.≤ 2%
  • costWhat your architecture costs to run per hour, based on the instances and components you've wired up.≤ $16/hr

Scenarios

  • traffic spike 5×≥ 95% up
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Components

Client

ClientWhere traffic originates.

Compute

API GatewayFront door — the app tier requests pass through.
500 r/s · 20ms · $2/hr
Backend
350 r/s · 25ms · $2/hr
Lvl 7

Storage

SQL DatabaseRelational store. Durable, but the lowest throughput.
200 r/s · 50ms · $3/hr
NoSQL DB3× the throughput of SQL — at a premium. Pay for it only when the load demands it.
600 r/s · 30ms · $4/hr
Redis CacheIn-memory read-cache — absorbs repeated reads. Useless for writes.
5000 r/s · 3ms · $2/hr
Read Replica
200 r/s · 55ms · $3/hr
Lvl 12
DB Standby
200 r/s · 50ms · $3/hr
Lvl 13

Networking

CDN
50000 r/s · 5ms · $1/hr
Lvl 8
Load Balancer
10000 r/s · 2ms · $2/hr
Lvl 9
Reverse Proxy
8000 r/s · 3ms · $1/hr
Lvl 9

Messaging

Queue
8000 r/s · 8ms · $1/hr
Lvl 6

Security

WAF
20000 r/s · 3ms · $1/hr
Lvl 14
Rate Limiter
20000 r/s · 1ms · $1/hr
Lvl 14