Logging and Debugging

• One JSON object per line: level, time, msg plus fields
• Machine-searchable, not free-text strings

• Log to stdout only
• Docker, systemd, k8s collect and ship the logs
• App never manages files or rotation

const logger = require('pino')({
  level: process.env.LOG_LEVEL || 'info'
})
logger.info({ userId }, 'login ok')

object first, message second

• Request ID = unique ID per request, on every log line
• Filter one request's logs across the whole flow
• Reuse incoming x-request-id, else crypto.randomUUID()
• Echo it back in the response header

const id = req.headers['x-request-id']
  || crypto.randomUUID();
res.setHeader('x-request-id', id);
req.log = logger.child({ reqId: id });

middleware: one child logger per request

• node --inspect app.js: V8 inspector on 127.0.0.1:9229
• --inspect-brk: pauses on the first line
• Set breakpoints before startup code runs

• heapUsed from process.memoryUsage() keeps climbing after GC
• Healthy: a sawtooth that stays level
• Leak: the sawtooth floor never returns to baseline