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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Leaving express-session on its default MemoryStore in production

// Wrong: default store, fine for a local demo, breaks in production
app.use(session({ secret: 'my-secret', resave: false }));

// Correct: externalize session state to Redis
const RedisStore = require('connect-redis').default;
app.use(session({ store: new RedisStore({ client: redisClient }), secret: process.env.SESSION_SECRET, resave: false }));

The Solution //

express-session prints an explicit terminal warning ('MemoryStore is not designed for a production environment') that teams routinely ignore. MemoryStore leaks memory over time (sessions are never cleaned up efficiently) and breaks entirely once you run more than one server instance, since each instance has its own isolated RAM. Always configure an external store like connect-redis before deploying.

The Error //

Relying on Sticky Sessions instead of a shared session store to 'fix' multi-server logouts

// Anti-pattern: load balancer config pins users to one instance
// upstream backend { ip_hash; server app1; server app2; }

// Correct: any server can serve any request because state lives in Redis
// no ip_hash needed once sessions are externalized

The Solution //

Configuring the load balancer to always route a user to the same server papers over the real problem (session state living in one server's RAM) rather than fixing it — if that specific server restarts or crashes, every user pinned to it is instantly logged out, and sticky sessions also defeat even load distribution across your fleet. Make servers genuinely stateless with a shared store like Redis instead.