Distributed Session Management

Imagine you're building a web application that needs to handle millions of users. When a user logs in, you need to keep track of their session across multiple servers. This is where distributed session management becomes crucial.

In a single-server setup, managing sessions is straightforward. You just store session data in the server's memory. But when your application runs on multiple servers behind a load balancer, this approach breaks down. If a user's next request goes to a different server, their session data won't be available.

There are several ways to handle sessions in a distributed environment:

Sticky Sessions is the simplest approach. The load balancer ensures that all requests from a user go to the same server where their session was created. While easy to implement, this method has drawbacks. If a server fails, all its sessions are lost. It also makes load balancing less effective since requests can't be freely distributed.

Client-side Sessions store all session data in cookies on the user's browser. Each request carries the complete session data, eliminating the need for server-side storage. However, cookies have size limitations and sending session data with every request increases network traffic. Also, storing sensitive data in cookies can be a security risk.

Centralized Session Storage uses a shared database or cache (like Redis) to store session data. When a request arrives at any server, it can fetch the session data from this central storage. This approach is reliable and scalable, but adds some latency since each request needs to fetch session data from the central store.

Here's how these approaches compare: