How to match Remix routes with route patterns so we can track using tracing libraries like Datadog

routeMatcher.ts

import { matchPath } from '@remix-run/react'
import fs from 'node:fs'
import * as path from 'node:path'

/**
 * Given a request path, tries to find a Remix route path pattern
 * that matches with an existing route in your Remix app.
 * 
 * Make sure to generate the Remix routes file during your build with:
 * 
 * ```shell
 * npx @remix-run/dev routes --json >> build/remix_routes.json
 * ```
 */
export function matchRemixRoutePattern(requestPath: string) {
  const routes = loadRoutesFile()
  for (const routePattern of routes) {
    if (matchPath(routePattern, requestPath) !== null) {
      return routePattern
    }
  }
  return null
}

let routesCache: string[] | null = null

function loadRoutesFile() {
  if (routesCache) {
    return routesCache
  }

  const buildFolder = path.resolve(process.cwd(), 'build')
  const routesData = fs.readFileSync(
    path.resolve(buildFolder, 'remix_routes.json'),
    { encoding: 'utf8', flag: 'r' }
  )
  const routesJson = JSON.parse(routesData)
  routesCache = flattenRoutes(routesJson)
  return routesCache!
}

function flattenRoutes(json: any[]) {
  let paths: string[] = []
  for (const entry of json) {
    if (entry.children) {
      const mappedPaths = flattenRoutes(entry.children)
        .filter(path => typeof path !== 'undefined')
        .map(path => `${entry.path}/${path}`)
      paths = [...paths, entry.path, ...mappedPaths]
    } else {
      paths.push(entry.path)
    }
  }

  return paths
}

tracer.ts

import tracer from 'dd-trace'
import { matchRemixRoutePattern } from './routeMatcher'

tracer.init()

tracer.use('express', {
  hooks: {
    request: (span, req, res) => {
      if (req?.url) {
        try {
          const url = new URL(req.url)
          const matchPattern = matchRemixRoutePattern(url.pathname)
          if (matchPattern) {
            span?.setTag('http.route', matchPattern)
          }
        } catch (error: unknown) {
          console.error('Invalid referer', error)
        }
      }
    },
  },
})

export default tracer