What You Will Learn

• What is API Gateway?
• Why API Gateway is Needed
• Microservice Communication Problems
• Request Routing
• Authentication & Authorization
• Rate Limiting
• Load Balancing
• Service Discovery
• API Aggregation
• Spring Cloud Gateway
• Enterprise Use Cases
• Interview Questions

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Introduction

Modern enterprise applications contain multiple microservices.

Example:

User Service

Order Service

Payment Service

Inventory Service

Notification Service

Without API Gateway:

Client

↓

Calls Every Service Directly

Problems:

Too Many Endpoints

Security Complexity

Routing Complexity

Tight Client Coupling

Difficult Monitoring

API Gateway solves these problems.

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What is API Gateway?

API Gateway acts as a single entry point for all client requests.

Instead of:

Client → Multiple Services

Use:

Client

↓

API Gateway

↓

Microservices

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Purpose of API Gateway

Primary Goal:

Single Entry Point

For All Requests

---

Without API Gateway

flowchart LR

A[Mobile App]

B[User Service]

C[Order Service]

D[Payment Service]

E[Inventory Service]

A --> B
A --> C
A --> D
A --> E

Client talks to every service.

---

Problems Without API Gateway

Multiple URLs

Complex Security

Repeated Authentication

Versioning Problems

Difficult Monitoring

---

With API Gateway

flowchart LR

A[Client]

B[API Gateway]

C[User Service]

D[Order Service]

E[Payment Service]

F[Inventory Service]

A --> B

B --> C
B --> D
B --> E
B --> F

Single entry point.

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Real World Analogy

Airport Security.

Passengers:

Mobile App

Web App

Partner App

All requests pass through:

Airport Security Gate

before reaching destinations.

API Gateway acts like airport security.

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API Gateway Responsibilities

Routing

Authentication

Authorization

Rate Limiting

Load Balancing

Monitoring

Logging

Response Aggregation

---

API Gateway Architecture

flowchart LR

A[Client]

B[API Gateway]

C[Authentication]

D[Routing]

E[Microservices]

A --> B

B --> C

C --> D

D --> E

---

Request Flow

sequenceDiagram

Client->>API Gateway: Request

API Gateway->>User Service: Route

User Service-->>API Gateway: Response

API Gateway-->>Client: Response

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Request Routing

One of the most important responsibilities.

Example:

/api/users

/api/orders

/api/payments

Gateway decides:

Where Request Should Go

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Routing Flow

flowchart LR
    A[Users API]
    B[Gateway]
    C[User Service]

    A --> B
    B --> C

---

Authentication

Without Gateway:

Every service must authenticate.

User Service

Order Service

Payment Service

All perform authentication.

---

With Gateway

Authentication happens once.

flowchart LR

A[Client]

B[API Gateway]

C[JWT Validation]

D[Services]

A --> B
B --> C
C --> D

---

Authorization

Gateway verifies:

Roles

Permissions

Scopes

Examples:

ADMIN

CUSTOMER

MANAGER

---

Security Flow

flowchart LR

A[Request]

B[JWT Validation]

C[Authorization]

D[Service]

A --> B
B --> C
C --> D

---

Rate Limiting

Protects backend services.

Example:

Maximum 100 Requests

Per Minute

---

Rate Limiting Flow

flowchart LR

A[Client]

B[Rate Limiter]

C[API Gateway]

D[Service]

A --> B
B --> C
C --> D

---

Benefits

Prevent Abuse

Prevent DDoS

Protect Resources

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Load Balancing

Gateway distributes traffic.

Example:

User Service Instance 1

User Service Instance 2

User Service Instance 3

---

Load Balancing Flow

flowchart LR

A[Gateway]

B[User Service 1]

C[User Service 2]

D[User Service 3]

A --> B
A --> C
A --> D

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Service Discovery

In Kubernetes:

Pods Change Frequently

Gateway uses:

Eureka

Consul

Kubernetes DNS

to locate services.

---

Service Discovery Flow

flowchart LR

A[Gateway]

B[Service Registry]

C[Microservice]

A --> B
B --> C

---

Response Aggregation

One request may require:

Customer Data

Order Data

Payment Data

Gateway combines responses.

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Aggregation Example

flowchart LR

A[Gateway]

B[Customer Service]

C[Order Service]

D[Payment Service]

A --> B
A --> C
A --> D

Single response returned.

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Banking Example

Customer Dashboard.

Required Data:

Account Balance

Recent Transactions

Loan Details

Credit Card Summary

---

Without Gateway

4 Separate API Calls

---

With Gateway

flowchart LR

A[Mobile App]

B[API Gateway]

C[Account Service]

D[Loan Service]

E[Card Service]

A --> B

B --> C
B --> D
B --> E

Single API call.

---

Insurance Example

Claim Dashboard.

Services:

Policy Service

Claim Service

Payment Service

Gateway aggregates data.

---

Insurance Architecture

flowchart LR

A[Portal]

B[Gateway]

C[Policy Service]

D[Claim Service]

E[Payment Service]

A --> B

B --> C
B --> D
B --> E

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E-Commerce Example

Order Page requires:

Customer Data

Order Details

Inventory

Payment Status

Gateway fetches all.

---

Retail Architecture

flowchart LR

A[Frontend]

B[API Gateway]

C[Customer Service]

D[Order Service]

E[Inventory Service]

F[Payment Service]

A --> B

B --> C
B --> D
B --> E
B --> F

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Spring Cloud Gateway

Most popular Java implementation.

Dependency:

<dependency>
   <groupId>
      org.springframework.cloud
   </groupId>

   <artifactId>
      spring-cloud-starter-gateway
   </artifactId>
</dependency>

---

Route Configuration

spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          uri: lb://USER-SERVICE
          predicates:
            - Path=/users/**

---

Routing Flow

flowchart LR
    A["Users API Route"]
    B["API Gateway"]
    C["User Service"]

    A --> B
    B --> C

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Gateway Filters

Filters perform:

Authentication

Logging

Validation

Header Modification

---

Filter Flow

flowchart LR

A[Request]

B[Filter]

C[Service]

A --> B
B --> C

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API Gateway vs Load Balancer

Feature	API Gateway	Load Balancer
Routing	Yes	Limited
Authentication	Yes	No
Rate Limiting	Yes	No
Aggregation	Yes	No
Load Balancing	Yes	Yes

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API Gateway vs Reverse Proxy

Feature	Gateway	Reverse Proxy
Security	Advanced	Basic
Routing	Advanced	Basic
Aggregation	Yes	No
Business Rules	Yes	No

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Popular API Gateways

Spring Cloud Gateway

Kong

NGINX

Apigee

AWS API Gateway

Azure API Management

Gloo Gateway

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Enterprise Examples

Banking

Account Services

Payments

Loans

Credit Cards

---

Insurance

Claims

Policies

Payments

---

Retail

Orders

Inventory

Catalog

---

Healthcare

Patient Records

Appointments

Insurance Validation

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Travel

Flights

Hotels

Payments

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Real Enterprise Architecture

flowchart LR

A[Mobile App]

B[Web App]

C[Partner APIs]

D[API Gateway]

E[User Service]

F[Order Service]

G[Payment Service]

H[Inventory Service]

A --> D
B --> D
C --> D

D --> E
D --> F
D --> G
D --> H

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Benefits

✅ Single Entry Point

✅ Centralized Security

✅ Request Routing

✅ Rate Limiting

✅ Load Balancing

✅ Service Aggregation

✅ Better Monitoring

✅ Simplified Clients

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Limitations

❌ Additional Infrastructure

❌ Potential Bottleneck

❌ More Operational Complexity

❌ Gateway Failure Impacts All Services

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When To Use API Gateway

Use when:

Microservices Architecture

Multiple Clients

Security Requirements

Centralized Routing

Cloud Native Systems

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When NOT To Use

Avoid when:

Small Monolithic Applications

Simple Internal Tools

Few APIs

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Interview Questions

What is API Gateway?

A single entry point for all client requests.

---

Why Use API Gateway?

To centralize routing, security, and monitoring.

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Responsibilities?

Routing

Authentication

Authorization

Rate Limiting

Load Balancing

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Popular Java Gateway?

Spring Cloud Gateway

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Difference Between Gateway And Load Balancer?

Gateway provides routing and security.

Load balancer distributes traffic.

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Can API Gateway Aggregate Responses?

Yes.

It can combine responses from multiple services.

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Biggest Benefit?

Centralized management of microservices traffic.

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Key Takeaways

• API Gateway is the front door of Microservices Architecture.
• Provides routing, authentication, authorization, monitoring, and rate limiting.
• Simplifies client interactions.
• Commonly implemented using Spring Cloud Gateway.
• Integrates with Service Discovery and Load Balancing.
• Widely used in Banking, Insurance, Retail, Healthcare, and Cloud Native Systems.
• One of the most important patterns in modern enterprise architecture.

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