Mediumsystem-designdatabasereplication

Replicas for the feed

Acceptance: —Solved by: 0

Statement

A social feed: 4000 requests/s, 90 % reads. A cache is off-limits in this product (data changes every second), so the only way to take reads off the database is read replicas. A replica serves reads only and must be linked to the primary; the app server splits reads evenly across live replicas. Writes go to the primary only. Evenings bring 1.3× the load.

Constraints

p95 ≤ 300 ms, errors ≤ 0.1 %, budget $4500/month, ≤ 5 instances per node.

Requirements

SimulationBuild from scratch

p95

≤ 300 ms

Errors

≤ 0.10 %

Budget

$4,500/mo

Users

500,000

Your architecture costs$0 / $4,500

★ within budget · ★★ ≤ 75 % · ★★★ ≤ 50 % of the budget

Scenarios (checked in order)

1Daytime feedLoad
4000 requests/s for 60 seconds: 90 % reads, 5 % writes, 5 % static.
60 s · 4,000 rps · R 90 / W 5 / S 5 %
2Evening peakSpike
From second 20, traffic grows 1.3× for 60 seconds (5200 requests/s).
100 s · 4,000 rps · R 90 / W 5 / S 5 % · ×1.3 from s 20 for 60 s

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