Your billing system is relational for a reason — the finance team will not sign off on a NoSQL migration. But read traffic has outgrown what the primary can vertically scale to, no matter how much you're willing to spend on it. Scale the reads without touching the write path.
Requirements
Functional
·Scale read capacity for a relational billing system without changing the write path
Non-functional
·~700 requests/sec
·95% reads
·p99 latency ≤ 200ms
·99% availability
Constraints
·Budget: $21/hr or less
·Must stay relational — no NoSQL migration
·The primary's vertical ceiling can't be bought past, no matter the budget
Out of scope
·Multi-primary/multi-region writes
TrafficRequests per second — the rate of incoming traffic this level throws at your system.
~700 r/s · read-heavy
95% reads · 5% 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.≥ 650 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.≤ $21/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
BackendA second compute tier for heavier processing.
350 r/s · 25ms · $2/hr
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 ReplicaRead-only copy of the primary — scales reads past the primary's own ceiling.
200 r/s · 55ms · $3/hr
DB Standby
200 r/s · 50ms · $3/hr
Lvl 13
Networking
CDNEdge read-cache — serves content near the user.
50000 r/s · 5ms · $1/hr
Load BalancerManaged, premium front door — highest ceiling, lowest latency, costs more.
10000 r/s · 2ms · $2/hr
Reverse ProxyCheaper self-hosted front door — lower ceiling, the smart buy under 8,000 r/s.
8000 r/s · 3ms · $1/hr
Messaging
QueueACKs writes instantly, drains them to storage — absorbs bursts, not sustained overload.