🏦 Fintech Traffic Architecture — Reference Design (Interview Level)

Let’s assume:

Public fintech platform
Web + mobile clients
Partner APIs
Microservices backend (EKS/ECS)
Lambda-based async flows
Strong security + audit requirements

✅ Layered Traffic Model (How I Explain in Interview)

I design traffic in layers, not tools:

Layer 1 — Global Edge Layer

CloudFront
WAF
DDoS protection
TLS edge termination
static asset caching

Layer 2 — API Control Layer

API Gateway (for external APIs)
throttling
auth
schema validation
partner control

Layer 3 — Application Routing Layer

ALB (HTTP microservices routing)
path/host routing
service fan-out

Layer 4 — High-Performance TCP Layer

NLB (when needed)
gRPC / TCP / static IP

Interviewers like layered thinking.

🌍 CloudFront — When to Use in Fintech Traffic

✅ Use CloudFront When:

public user traffic global
frontend assets
document downloads
statement PDFs
JS/CSS bundles
WAF + bot protection needed
geo restriction required
edge TLS termination
origin shielding needed

❌ Don’t Use CloudFront When:

internal APIs only
ultra low latency internal calls
highly dynamic non-cacheable APIs (sometimes still OK but limited gain)

⚠️ CloudFront Tradeoffs

cache invalidation cost
config complexity
header forwarding affects cache hit
not ideal for east-west traffic
origin timeout limits exist

🚪 API Gateway — When to Use in Fintech

✅ Use API Gateway When:

external partner APIs
fintech integrations
merchant APIs
webhook endpoints
Lambda backends
need throttling per client
usage plans needed
request validation needed
auth centralization needed

💪 API Gateway Strengths

built-in throttling
per-client limits
JWT/IAM auth
request validation
direct Lambda/SQS integration
no servers
audit visibility

⚠️ API Gateway Tradeoffs

per-request cost high at scale
payload limits
timeout limits
not ideal for streaming
adds latency vs ALB
mapping templates complexity

⚖️ ALB — When to Use

✅ Use ALB When:

microservices on EKS/ECS
path-based routing needed
multiple services behind one domain
HTTP/gRPC routing
WAF integration needed
container backends
need host/path rules

💪 ALB Strengths

L7 smart routing
multiple target groups
sticky sessions
WebSocket support
cheaper than API GW at high RPS
good for internal + external apps

⚠️ ALB Tradeoffs

no per-client throttling
no usage plans
less API governance
regional only
higher latency than NLB
HTTP only

⚡ NLB — When to Use

✅ Use NLB When:

TCP/UDP traffic
gRPC high performance
static IP required
partner IP allowlisting
TLS passthrough
ultra low latency
very high RPS

⚠️ NLB Tradeoffs

no path routing
no WAF
no auth layer
no header rules
L4 only

🧠 ALB Buffering & Request Handling — Interview Trap Topic

✅ Does ALB buffer requests?

Yes — ALB buffers client request body before sending to target (for HTTP). Implications:

protects backend from slow clients
adds memory buffering at LB
large uploads can stress LB
backend sees full request at once

⚠️ ALB buffering tradeoffs

large uploads → latency
streaming not ideal
timeout risk for slow upload
better to use S3 pre-signed upload for large files

Interview bonus if you mention this.

⏱️ Timeout & Limit Differences (Critical)

API Gateway timeout

shorter integration timeout
good for fast APIs only

ALB timeout

configurable idle timeout
better for longer HTTP calls

CloudFront timeout

origin timeout limits
retries configurable

Mismatch causes 502/504 errors — real production failure pattern.

🧾 Fintech Pattern — Typical Final Architecture

Public Web App

CloudFront → WAF → ALB → EKS services

Partner API

CloudFront (optional) → API Gateway → Lambda / ALB backend

High-Perf gRPC

Client → NLB → service

🧮 Decision Matrix — Interview Gold Answer

✅ When interviewer asks: “ALB vs API Gateway vs CloudFront?”

Answer like this:

Use CloudFront when:

global edge delivery
caching
WAF edge protection
static or semi-static
user-facing web

Use API Gateway when:

external APIs
partner integrations
per-client throttling
auth + validation needed
serverless backend

Use ALB when:

container microservices
internal/external app routing
path routing
high RPS HTTP

Use NLB when:

TCP/gRPC
static IP needed
ultra performance
non-HTTP

🔥 Interview Power Move Answer (One-liner)

If interviewer asks:

“How do you choose between them?”

Say:

I choose based on layer — CloudFront for edge delivery and protection, API Gateway for API governance and throttling, ALB for microservice HTTP routing, and NLB for high-performance L4 traffic. I don’t treat them as competitors — I layer them.

That’s a senior-level answer.

Perfect — now we move into the Traffic Architecture Scenario Round — 20 Questions. This is exactly how senior DevOps / platform interviews probe depth: they give a traffic pattern and ask you to choose the right AWS traffic component + justify tradeoffs + limits.

Rules for this round:

Each scenario = realistic fintech / prod case
You answer with design + reasoning
I give you model interview-grade answer
Focus = CloudFront vs API Gateway vs ALB vs NLB decisions

We’ll do Questions 1–10 in this message (so it stays readable). Then next 10 after.

🧠 Traffic Architecture Scenario Round — Q1–Q10

✅ Scenario 1 — Global fintech web app with heavy static + dynamic API

Question: What traffic stack do you choose?

Answer: I put CloudFront at the edge for global acceleration, TLS, WAF, and static asset caching. Static content goes to S3 origin with long TTL. Dynamic API paths route to ALB or API Gateway depending on API type. This reduces origin load and latency while keeping API control separate.

✅ Scenario 2 — Partner payment API with strict per-client rate limits

Answer: API Gateway is the right front door because it supports usage plans, API keys, and per-client throttling. ALB cannot enforce per-client rate limits. I’d attach WAF and JWT/Lambda authorizer. Backend can be Lambda or ALB/EKS.

✅ Scenario 3 — Internal microservice-to-microservice traffic inside EKS

Answer: None of these — use ClusterIP + service mesh or internal service networking. Using ALB/API Gateway for east-west traffic adds latency and cost. Interviewers like when you say “don’t use edge tools internally.”

✅ Scenario 4 — Ultra-low latency gRPC trading service

Answer: NLB. It supports TCP/HTTP2/gRPC efficiently with low latency. ALB adds L7 overhead and may cause protocol issues. If static IP needed for exchange allowlist — NLB with Elastic IP.

✅ Scenario 5 — Public REST API with Lambda backend and moderate traffic

Answer: HTTP API Gateway — lower cost and latency than REST API type. Native Lambda integration, built-in auth and throttling. No need for ALB here unless traffic becomes very high and steady.

✅ Scenario 6 — 2GB file upload from clients

Answer: Direct S3 pre-signed upload — not ALB or API Gateway. Both have payload and buffering limits. LB buffering makes this inefficient and failure-prone. Edge can still use CloudFront → S3 if needed.

✅ Scenario 7 — Need WAF + bot filtering + geo block for web + APIs

Answer: CloudFront + WAF at edge. Then route to ALB or API Gateway origins. Edge WAF is cheaper and blocks earlier. Centralizes protection.

✅ Scenario 8 — Legacy TCP payment switch integration

Answer: NLB — TCP passthrough, stable IP, high connection scale. ALB/API Gateway are wrong protocol layer.

✅ Scenario 9 — 150 microservices HTTP platform, want single public endpoint

Answer: CloudFront → ALB with path/host routing to many target groups. One ALB per env is cost-efficient and operationally simpler. API Gateway would be expensive and rule-heavy at that scale.

✅ Scenario 10 — Need request/response transformation at gateway layer

Answer: API Gateway REST API (not HTTP API) because it supports mapping templates and transformation. ALB cannot transform payloads. This is a feature-based choice.

Rds Devsecops