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Load Balancer vs API Gateway - System Design Interview Guide
Table of Contents
- ? Load Balancer vs API Gateway (Interview Master Sheet)
- 1?? Core Idea (1-liner difference)
- 2?? How They Work (Architecture Level)
- ?? Load Balancer (Architecture)
- ?? API Gateway (Architecture)
- 3?? Key Trade-offs (Must Say)
- 4?? Signals / Hints (Interviewer Gold ?)
- ?? Choose Load Balancer if:
- ?? Choose API Gateway if:
- 5?? Can We Use Both? (VERY IMPORTANT ??)
- ? YES � Standard FAANG Architecture
- ?? Typical Setup:
- Why?
- 6?? Advanced Architectures
- ?? Pattern 1: LB ? API Gateway (Most Common)
- ?? Pattern 2: API Gateway ? LB
- ?? Pattern 3: LB ? Gateway ? LB
- 7?? FAANG-Level Interview Questions + Answers
- ? Q1: Why not use only API Gateway?
- ? Q2: Can Load Balancer do routing like API Gateway?
- ? Q3: Where to place authentication?
- ? Q4: What is bottleneck risk?
- ? Q5: API Gateway vs Reverse Proxy?
- 8?? Quick Examples (Must Remember)
- 9?? 30-Second Revision (Final Script)
- ?? Final FAANG Tip
? Load Balancer vs API Gateway (Interview Master Sheet)
1?? Core Idea (1-liner difference)
- Load Balancer ? Distributes traffic across servers
- API Gateway ? Manages, secures, and routes APIs
?? Script:
�Load balancer handles traffic distribution for scalability, while API Gateway handles API management, security, and orchestration.�
2?? How They Work (Architecture Level)
?? Load Balancer (Architecture)
Flow:
- Client ? Load Balancer
- LB ? Chooses backend (round-robin, least connections)
- Server ? Response
Types:
- L4 (TCP/UDP)
- L7 (HTTP)
Key Responsibilities:
- Traffic distribution
- Health checks
- Failover
- High availability
Examples:
- NGINX
- HAProxy
?? Script:
�Load balancer ensures no single server is overloaded and improves availability via health checks and failover.�
?? API Gateway (Architecture)
Flow:
-
Client ? API Gateway
-
Gateway:
- Authenticates
- Routes
- Aggregates
-
Calls multiple services
-
Returns unified response
Key Responsibilities:
- Authentication & Authorization
- Rate limiting
- Request/response transformation
- API aggregation
Examples:
- Kong
- Amazon API Gateway
?? Script:
�API Gateway acts as a smart entry point that handles security, routing, and aggregation for microservices.�
3?? Key Trade-offs (Must Say)
| Factor | Load Balancer | API Gateway |
|---|---|---|
| Purpose | Traffic distribution | API management |
| Complexity | ? Simple | ? Complex |
| Features | Limited | Rich (auth, rate limit) |
| Latency | ? Low | ? Slight overhead |
| Scope | Infrastructure | Application layer |
?? Script:
�Load balancer is lightweight and fast, while API Gateway adds intelligence but also latency and complexity.�
4?? Signals / Hints (Interviewer Gold ?)
?? Choose Load Balancer if:
- Scaling servers
- High traffic distribution
- Need high availability
- No API logic required
?? Example:
- Serving web traffic
?? Script:
�If the problem is about scaling servers and distributing traffic, I will use a load balancer.�
?? Choose API Gateway if:
- Microservices architecture
- Multiple backend services
- Need auth, rate limiting
- Need API aggregation
?? Example:
- Mobile app backend
?? Script:
�If I need a unified API layer with security and orchestration, I will use an API Gateway.�
5?? Can We Use Both? (VERY IMPORTANT ??)
? YES � Standard FAANG Architecture
?? Typical Setup:
Client
?
Load Balancer
?
API Gateway (multiple instances)
?
Microservices
Why?
- LB ? Scalability + availability
- Gateway ? Logic + security
?? Script:
�In production, I use load balancer before API Gateway to scale gateway instances and avoid bottlenecks.�
6?? Advanced Architectures
?? Pattern 1: LB ? API Gateway (Most Common)
- Distribute traffic across gateway instances
?? Pattern 2: API Gateway ? LB
- Gateway routes ? internal LB ? services
?? Pattern 3: LB ? Gateway ? LB
- External LB + internal service-level LB
?? Script:
�Large systems often use load balancers at multiple layers for both gateway scaling and service-level distribution.�
7?? FAANG-Level Interview Questions + Answers
? Q1: Why not use only API Gateway?
? Answer:
�API Gateway is not optimized for traffic distribution at scale. Load balancer is needed for efficient traffic handling and failover.�
? Q2: Can Load Balancer do routing like API Gateway?
? Answer:
�Yes at L7 level, but it lacks advanced features like authentication, rate limiting, and aggregation.�
? Q3: Where to place authentication?
? Answer:
�At API Gateway, since it�s the centralized entry point.�
? Q4: What is bottleneck risk?
? Answer:
�API Gateway can become a bottleneck, so we scale it behind a load balancer.�
? Q5: API Gateway vs Reverse Proxy?
? Answer:
�API Gateway is an advanced reverse proxy with added features like security, rate limiting, and orchestration.�
8?? Quick Examples (Must Remember)
- Load Balancer ? Distribute traffic across servers
- API Gateway ? Handle login, route APIs, combine responses
9?? 30-Second Revision (Final Script)
?? Script:
�Load balancer distributes traffic across servers to improve scalability and availability, while API Gateway provides a centralized API layer handling authentication, routing, and aggregation. In real systems, both are used together�load balancer in front of API Gateway to scale and avoid bottlenecks.�
?? Final FAANG Tip
?? Always say:
�Load balancer solves scaling, API Gateway solves complexity.�