Level 7 · Service Tiers
SEPARATION OF CONCERNS

Stage 3 · Separation of Concerns · App Tier · Gateway vs Backend

Service Tiers

Your app grew from a script into a real service. Right now the gateway does everything — routing AND business logic AND database calls — and the team keeps stepping on each other. Split the concerns: let the gateway be the front door and move the application logic into its own tier behind it.

Requirements

Functional

  • ·Gateway handles routing/ingress only; business logic and data access move to a dedicated app tier

Non-functional

  • ·~150 requests/sec
  • ·80% reads
  • ·p99 latency ≤ 250ms
  • ·99% availability

Constraints

  • ·Budget: $12/hr or less
  • ·The gateway must never talk to the database directly — the app tier is a hard boundary, not a suggestion

Out of scope

  • ·Multiple app-tier services (this is one backend, not a microservice split)

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

~150 r/s · read-heavy
80% reads · 20% 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.≥ 130 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.≤ $12/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 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

QueueACKs writes instantly, drains them to storage — absorbs bursts, not sustained overload.
8000 r/s · 8ms · $1/hr

Security

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