Monitoring

Fulmine publishes what Express and Node.js publish for tracers, on node:diagnostics_channel, whether app.listen() serves the app on µWS or http.createServer(app) serves it through node:

• http.server.request.start, http.server.response.created and http.server.response.finish, node:http's own. On µWS the server in them is the app listen() returns, and every request goes through its emit("request"), as node sends it.
• express.router.request, the TracingChannel for every middleware, route handler and error handler that pillarjs/router#196 adds to Express's router, merged and out with the next router release.

Without a subscriber they cost nothing. A tracer has to subscribe before listen(): routes µWS can answer on its own are compiled then, and none is while something listens, so every handler runs.

Sentry

As for Express: initialise Sentry first, then add its error handler after the routes.

// instrument.js, loaded before anything else
const Sentry = require("@sentry/node");

Sentry.init({ dsn: process.env.SENTRY_DSN, tracesSampleRate: 1.0 });

// app.js
require("./instrument.js");
const Sentry = require("@sentry/node");
const express = require("fulmine.js");

const app = express();

app.get("/", (req, res) => {
    res.send("Hello World");
});

// after the routes, before your own error handlers
Sentry.setupExpressErrorHandler(app);

app.listen(3000);

Tested with @sentry/node 11.8 in CI (the case), on app.listen() and on http.createServer(app), against the same app on Express:

• an error thrown in a route, synchronous or after an await, is reported with the request: method and URL;
• each request has its own scope, so Sentry.setTag() in a middleware stays with its request, with requests running at the same time.

What is not there yet: a span per middleware and route handler, and the route as the transaction name. The transaction is named after the path, GET /users/7, where Express gets GET /users/:id. Sentry takes both from rewriting the router package as it loads, which Fulmine does not use. Once Sentry reads express.router.request, the channel Express's router will publish, they come from Fulmine too, with nothing to change here.

Other tracers

A tracer that reads the channels above sees Fulmine as it sees Express. One that patches node's http module, as OpenTelemetry's HTTP instrumentation does, sees the requests only through http.createServer(app), where node itself serves them.