Two Express monitoring gotchas: load order and where the error handler goes

[virajrakholiya]

Two mistakes that silently break error and request monitoring in Express apps.

1. Instrumentation loaded too late. Most Node tracing (OpenTelemetry and tools built on it) works by patching modules when they are first required. If express, pg or http is required before the tracing setup runs, those calls are never recorded, with no error. Load it first:

// first line of your entry file
require("./tracing");
const express = require("express");

or start with node --require ./tracing.js server.js.

2. Error handler in the wrong place. Express error middleware (four arguments) only sees errors from routes registered before it. Put the error handler that records errors after all your routes:

app.use(requestMiddleware);
// ... routes ...
app.use(errorHandler); // last

Also worth doing: record the route pattern (req.baseUrl + req.route.path, e.g. /orders/:id) rather than the raw URL, so requests group properly.

Question: any other ordering traps you have hit with Express 5?

(Disclosure: I'm building Relia, a monitoring tool (https://tryrelia.com/?utm_source=express-show-and-tell&utm_medium=community&utm_campaign=places); its Express setup has exactly these two steps.)