Question: What is a distributed session cache and how does it work?

Answer

A distributed session cache refers to the method of storing user session data across multiple servers in a distributed system. This is a significant concept in web applications, where scalability and reliability are crucial.

How Does It Work?

In a traditional non-distributed setup, the user's session data is typically stored on the server they connect to. However, this can present issues when there's need to scale horizontally (add more servers) or if a server goes down causing loss of session data.

Distributed session caching mitigates these issues by storing the session data across several servers. This increases redundancy (if one server fails, the session isn't lost) and allows any server to handle any user request because the session data is accessible across all servers.

This can be achieved via different strategies:

1. Session Replication: Each session is copied onto every node in the cluster. This ensures high availability and fault tolerance but may consume significant network bandwidth and memory.
2. Session Affinity or Sticky Sessions: A user's session is associated with a specific node in the cluster. For subsequent requests from the same user, the load balancer directs to the same node ensuring the user's session data is retained.
3. Distributed Cache Systems: Systems like Redis, Memcached are used as a central repository for storing session data. These systems are highly efficient and reduce the need to frequently access databases.

Now, let's see an example of using Redis for distributed session caching in Node.js.

const session = require('express-session');
const RedisStore = require('connect-redis')(session);

app.use(
  session({
    store: new RedisStore({ host: 'localhost', port: 6379 }),
    secret: 'your-secret-key',
    resave: false,
    saveUninitialized: false
  })
);

In this example, we're using express-session for session handling and connect-redis to store sessions in Redis. The RedisStore is initialized with the address of the Redis server. With this setup, session data would be saved in Redis and could be accessed across multiple servers.

Remember, choosing the right strategy depends on the nature of your application and the trade-offs you're willing to make between performance, consistency, and resource usage.