Level 2 · Pastebin
CACHING

Stage 2 · Caching · Read-Heavy Workloads

Pastebin

Your paste-sharing site went viral on a forum. 90% of traffic is people reading the same popular pastes — and the database is buckling. The budget is too tight to just buy more database; make the reads cheaper instead.

Requirements

Functional

  • ·User submits a block of text and gets a paste URL
  • ·User visits a paste URL and reads its contents

Non-functional

  • ·~300 requests/sec
  • ·90% reads, concentrated on a handful of viral pastes
  • ·p99 latency ≤ 200ms
  • ·99% availability

Constraints

  • ·Budget: $7/hr or less — can't just scale the database vertically

Out of scope

  • ·Paste editing
  • ·User accounts
  • ·Setting visibility/custom links

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

~300 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.≤ 200ms
  • 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.≥ 250 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.≤ $7/hr
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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 DB
600 r/s · 30ms · $4/hr
Lvl 3
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