Sharing HTTP Session Across a Cluster with Redis in Spring Boot

Background: In typical web development, the server stores user state in a Session object, which works fine for a single‑node application but fails in a clustered environment because the request may be routed to a different server that cannot locate the original Session.

Problem Reproduction: A simple Spring Boot 2 web app provides two endpoints ( /set-session and /get-session) to write and read a session attribute. The app runs on ports 9001 and 9002, and Nginx is configured for load balancing. When accessed through Nginx, the session data is lost.

Cause Analysis: The original server stores a cookie (e.g., Cookie_for_Session_A) that identifies the Session. A subsequent request routed to another server cannot retrieve the Session using that cookie, leading to failure.

Solution Overview: Introduce a third‑party store (Redis) to hold session data so that all application instances share the same session information.

Implementation Steps:

Add Redis dependencies to build.gradle:

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'org.springframework.boot:spring-boot-starter-data-redis'
    compile group: 'org.springframework.session', name: 'spring-session-data-redis', version: '2.1.2.RELEASE'
}

Configure Redis connection in application.yml (host 127.0.0.1, port 6379).

Create a configuration class annotated with @Configuration and @EnableRedisHttpSession (optionally set maxInactiveIntervalInSeconds).

Build the project with Gradle’s bootJar task and run the generated JAR on both ports 9001 and 9002.

Test the endpoints again; the session is now shared across the cluster and the problem is resolved.

Nginx load‑balancing configuration used in the test:

http {
    upstream app_server {
        server 127.0.0.1:9001;
        server 127.0.0.1:9002;
    }
    server {
        listen 9000;
        location / {
            proxy_pass http://app_server;
        }
    }
}

After applying the Redis‑backed session configuration, the test passes, confirming that session data is correctly shared among the clustered instances.