5.A Smoke Alarm for a One-Person App

The Problem in Real Life

Finding Out From Users vs. Finding Out First

Logging, Error Tracking and Basic Monitoring for Your Own Project

• Structured logs — the logbook (Acts 17, 19): replace console.log with a small logging library (like pino) that writes JSON lines with a level, a time, a message and named fields. Remove the debug leftovers. Log the important events: waitlist_joined, organiser_login_failed, entry_removed — without passwords or full email addresses (Act 22: mask them, m***@example).
• Request IDs — following one visitor (Act 23): give every request an ID, include it in every log line for that request, and return it in an X-Request-Id header and in error responses. Now "Something went wrong (ref: req_7f2a)" leads straight to the exact log lines.
• Error tracking — the smoke alarm: an error-tracking service (Sentry and similar, many with free tiers) captures every unhandled error with its stack trace (Act 17), the request and how many users it affected, and emails or messages her. She sees errors before users report them.
• Uptime monitoring — the doorbell camera: a free external service calls https://waitlist.anna-builds.example/health every minute from outside (Act 26's lesson: test the path users take). If it fails twice in a row, her phone buzzes. It also watches the certificate's expiry (chapter four).
• A few metrics — glancing at the till (Act 19): the platform already shows requests, error rate, response time, CPU and memory. She adds two of her own: waitlist joins per hour and failed logins per hour (a jump could mean bots — Act 22).
• Alerts that matter, and only those (Act 19): app down for 2 minutes; error rate above 5% for 5 minutes; certificate within 14 days of expiry. Three alerts. Enough to sleep well, few enough to never ignore.

const pino = require("pino");
const { randomUUID } = require("crypto");
const logger = pino({ level: process.env.LOG_LEVEL || "info" });

app.use((req, res, next) => {
  req.id = req.headers["x-request-id"] || randomUUID();
  req.log = logger.child({ requestId: req.id });
  res.setHeader("X-Request-Id", req.id);
  next();
});

// inside the join handler
req.log.info({ event: "waitlist_joined", eventId: req.params.id, email: mask(email) });

// error handler: log it, never leak details to the user
app.use((err, req, res, next) => {
  req.log.error({ err }, "unhandled error");
  res.status(500).json({ error: { code: "internal_error", requestId: req.id } });
});