API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) · Lesson

Defining Routes & Predicates

Learn to define dynamic routes using predicates (e.g., Path, Host, Method) to direct requests to specific services.

Defining Routes & Predicates is a free API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Routing with API Gateways

Welcome! In a microservices architecture, clients don't talk directly to every service. Instead, they interact with an API Gateway.

This gateway acts as a single entry point, directing incoming requests to the correct backend service. This process is called routing.

What Are Predicates?

In Spring Cloud Gateway, predicates are key to routing. Think of them as 'conditions' or 'rules'.

A predicate evaluates to true or false.
If a predicate (or set of predicates) for a route evaluates to true, the gateway forwards the request using that route.
If false, the gateway tries the next route.

Gateway Project Foundation

Routes are typically configured in your Spring Boot application's application.yml or application.properties file. Here's a minimal Spring Boot application that *could* host our gateway configuration. The routing logic itself will go into the YAML.

You can run this basic app to see it working:

package com.coddykit.gateway;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
@RestController
public class GatewayApplication {

  public static void main(String[] args) {
    SpringApplication.run(GatewayApplication.class, args);
  }

  @GetMapping("/test-gateway")
  public String testGateway() {
    return "Hello from Gateway!";
  }
}

Basic Route Structure

A route in Spring Cloud Gateway needs three main things:

id: A unique identifier for the route.
uri: The URI of the backend service to which requests will be forwarded.
predicates: A list of conditions that must be met for this route to activate.

We'll primarily define these in application.yml.

The Path Predicate

The Path predicate is one of the most common. It matches the request URI path against a specified pattern.

You can use wildcards like * (matches one path segment) and ** (matches multiple path segments).
For example, /users/* matches /users/1 but not /users/admin/1.
/users/** matches both /users/1 and /users/admin/1.

Path Predicate in Action

Here's how you'd configure a route using the Path predicate to send all requests starting with /api/users to a backend user-service:

spring:
  cloud:
    gateway:
      routes:
        - id: user_service_route
          uri: http://localhost:8081 # Your backend user service
          predicates:
            - Path=/api/users/**

The Host Predicate

The Host predicate matches requests based on the hostname in the request header.

This is useful for routing requests from different domains or subdomains to specific services.
You can also use wildcards here, like *.example.com to match any subdomain of example.com.

Host Predicate Example

Let's say you want to route all requests coming from api.mycompany.com to a specific backend service. Here's how:

spring:
  cloud:
    gateway:
      routes:
        - id: api_domain_route
          uri: http://localhost:8082 # Your backend API service
          predicates:
            - Host=api.mycompany.com

The Method Predicate

The Method predicate matches requests based on their HTTP method (also known as HTTP verb).

Common methods include GET, POST, PUT, DELETE, and PATCH.
This allows you to direct different types of operations (e.g., reading vs. creating data) to distinct backend endpoints or services.

Method Predicate Example

To ensure only POST requests to /products are routed to a specific service, and other methods are handled elsewhere:

spring:
  cloud:
    gateway:
      routes:
        - id: create_product_route
          uri: http://localhost:8083 # Service for creating products
          predicates:
            - Path=/products
            - Method=POST

Predicate Power Check

You've learned about Path, Host, and Method predicates. Now, let's test your understanding!

Recap: Routes & Predicates

Great job! You've learned the fundamentals of defining routes and using predicates in Spring Cloud Gateway.

Routes direct traffic to backend services.
Predicates are the conditions (like Path, Host, Method) that determine if a route matches.
Multiple predicates on a single route act as an AND condition.

This powerful combination allows for flexible and intelligent traffic management in your microservices.

Frequently asked questions

Is the “Defining Routes & Predicates” lesson free?

Yes — the full text of “Defining Routes & Predicates” is free to read here on the web, and the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) course, upgrade to CoddyKit PRO.

What will I learn in “Defining Routes & Predicates”?

Learn to define dynamic routes using predicates (e.g., Path, Host, Method) to direct requests to specific services. You practise API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway)?

No prior experience is required. API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Defining Routes & Predicates” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson?

Yes. Every API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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