HardExperience level: 8-15 years1 min read

What does an API gateway do, and why put one in front of your services?

Reviewed by Gurusankar M.

Asked inAmazonMicrosoftGoogleDeloitte
#api gateway#cross-cutting#auth#rate limiting#routing
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Short Answer

A gateway is a single entry point that handles cross-cutting concerns — authentication, rate limiting, routing/load balancing, TLS termination, request/response transformation, caching, and observability — so individual services don't each reimplement them. It also decouples clients from internal service topology.

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Coffee Chat Question

“What does an API gateway do, and why put one in front of your services?”

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Mind Map Answer

Cross-cutting
auth, rate limit, TLS, logging
Routing
path → service, load balance
Decouple
clients don't know internal topology
Risk
can become a bottleneck / SPOF

Think Beyond the Expected

What If?

What's the downside of putting business logic in the gateway?

It becomes a fat, shared bottleneck and a single point of failure, couples teams to one deploy, and is hard to test. Keep the gateway to generic cross-cutting concerns; business logic belongs in the services (and run the gateway redundantly).

Real World

API gateways (Kong, AWS API Gateway, Spring Cloud Gateway) centralize auth/rate-limit/routing in microservices; the anti-pattern is an 'ESB-like' gateway stuffed with business logic.

Interviewer's Expectation

Keywords they're listening for:

  • single entry point
  • cross-cutting concerns
  • routing/LB/TLS
  • decouples clients
  • avoid business logic / SPOF

Common Mistakes

  • Business logic in the gateway
  • Single non-redundant gateway (SPOF)
  • Skipping per-service authZ (trusting the gateway alone)

Best Practices

  • Gateway for generic cross-cutting only
  • Run it redundantly; still authorize in services
  • Consider BFF for client-specific needs

Follow-up Questions

  1. Gateway vs service mesh — how do they differ?
  2. How do you avoid the gateway being a SPOF?
  3. What is BFF (backend-for-frontend)?

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: Auth (REST APIs)
Interview question: "What does an API gateway do, and why put one in front of your services?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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