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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Storing session data or an in-memory cache as a plain JS object, then clustering the app

// Wrong: works with 1 process, breaks silently once clustered
const sessions = {};
sessions[userId] = true;

// Correct: shared across every worker
await redisClient.set(userId, 'true');

The Solution //

Each cluster worker is a fully separate OS process with its own isolated memory — a variable set in Worker A's RAM is invisible to Worker B. If a login request lands on Worker A but a later request from the same user lands on Worker B (which round-robin load balancing guarantees will happen), the user appears logged out. Move session state to a shared external store like Redis before clustering, never a local object.

The Error //

Assuming a crashed worker means the whole application is down

// Correct: self-healing, logged but not necessarily paged
cluster.on('exit', (worker, code, signal) => {
  console.error(`Worker ${worker.process.pid} died (code ${code}), replacing it`);
  cluster.fork();
});

The Solution //

Teams sometimes page on-call for every worker crash assuming total outage, or worse, don't restart crashed workers at all, silently reducing capacity over time. A cluster's Primary process should listen for the 'exit' event and immediately cluster.fork() a replacement — a single worker crashing under a real cluster setup should be self-healing and largely invisible to users, not a page-worthy incident by itself.