Question 1

BFF (Backend for Frontend) vs API Gateway, are they the same?

Accepted Answer

No. BFF is a lightweight aggregator service that shapes data for a specific client (web, mobile, TV). API Gateway is the entry point enforcing rate limiting, auth, versioning, logging. Modern architecture: client → API Gateway → BFF → microservices. A single BFF talks to 5–10 backend services; API Gateway sits in front of all BFFs.

Question 2

How do I implement idempotency keys for safe retries?

Accepted Answer

Client generates a UUID (idempotency key). Server stores {key → response} in cache (Redis, Memcached) for 24 hours. On duplicate request, return cached response instead of re-executing. Implement as middleware; decorate all mutation endpoints (POST, PUT, DELETE). Critical for payments, refunds, and account changes.

Question 3

Polling vs webhooks vs SSE vs WebSocket, which pattern for what?

Accepted Answer

Polling: client asks every N seconds (cheapest for sparse data, ~30s+). Webhooks: server POSTs to client on event (fire-and-forget, idempotency required, backoff needed). SSE: one-way server→client stream over HTTP (good for live updates, auto-reconnect). WebSocket: bidirectional, stateful (real-time chat, multiplayer games). For notifications: webhooks. For feeds: SSE. For two-way: WebSocket. For batch jobs: polling.

Question 4

CQRS over APIs, separate read and write endpoints?

Accepted Answer

Yes. /api/v1/users/{id} (read) lives on a read replica or cache. /api/v1/users/{id}/update (write) hits primary DB + event bus. Allows different scaling, caching, and retry strategies per operation. Not required for small systems; essential for >1M QPS.

Question 5

Schema evolution without breaking clients, how?

Accepted Answer

Add-only: new optional fields never break old clients. Deprecation headers: `Deprecation: true`, `Sunset: 2026-12-31T00:00:00Z`. Versioning: /v2/ endpoint runs in parallel for 6 months. Coexist strategies: clients read extra fields without error, ignore unknown fields. Always default missing fields on read, never require.

Question 6

Saga pattern for distributed transactions, orchestration vs choreography?

Accepted Answer

Orchestration: central service (saga coordinator) calls services A→B→C, compensates on failure. Choreography: services publish events; downstream services listen and act, compensate by publishing reverse events. Orchestration is easier to debug. Choreography scales better and is loosely coupled. For <5 services: orchestration. For >10 with independent teams: choreography.

Question 7

API rate limiting, fixed window vs sliding window vs token bucket?

Accepted Answer

Fixed window (simplest): reset counts every minute (fairness issues at boundaries). Sliding window: track requests in a time window (expensive). Token bucket (recommended): refill N tokens/minute, burst allowed. For public APIs: token bucket at gateway level + per-user quotas at service level. IETF draft-6585: `RateLimit-Limit: 100`, `RateLimit-Remaining: 45`, `RateLimit-Reset: 1640995200`.

प्रदेश	ज्युनियर	मध्यम	सीनियर
USA	$125k	$160k	$220k
UK	£70k	£95k	£140k
EU	€75k	€105k	€150k
CANADA	C$130k	C$170k	C$235k

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