docs/middleware/index.js

import fs from 'fs'
import path from 'path'

import express from 'express'
import instrument from '../lib/instrument-middleware.js'
import haltOnDroppedConnection from './halt-on-dropped-connection.js'
import abort from './abort.js'
import timeout from './timeout.js'
import morgan from 'morgan'
import datadog from './connect-datadog.js'
import rateLimit from './rate-limit.js'
import slowDown from './slow-down.js'
import cors from './cors.js'
import helmet from 'helmet'
import csp from './csp.js'
import cookieParser from './cookie-parser.js'
import csrf from './csrf.js'
import handleCsrfErrors from './handle-csrf-errors.js'
import { setDefaultFastlySurrogateKey } from './set-fastly-surrogate-key.js'
import setFastlyCacheHeaders from './set-fastly-cache-headers.js'
import reqUtils from './req-utils.js'
import recordRedirect from './record-redirect.js'
import connectSlashes from 'connect-slashes'
import handleErrors from './handle-errors.js'
import handleInvalidPaths from './handle-invalid-paths.js'
import handleNextDataPath from './handle-next-data-path.js'
import detectLanguage from './detect-language.js'
import context from './context.js'
import shortVersions from './contextualizers/short-versions.js'
import redirectsExternal from './redirects/external.js'
import languageCodeRedirects from './redirects/language-code-redirects.js'
import handleRedirects from './redirects/handle-redirects.js'
import findPage from './find-page.js'
import spotContentFlaws from './spot-content-flaws.js'
import blockRobots from './block-robots.js'
import archivedEnterpriseVersionsAssets from './archived-enterprise-versions-assets.js'
import events from './events.js'
import search from './search.js'
import healthz from './healthz.js'
import remoteIP from './remote-ip.js'
import archivedEnterpriseVersions from './archived-enterprise-versions.js'
import robots from './robots.js'
import earlyAccessLinks from './contextualizers/early-access-links.js'
import categoriesForSupport from './categories-for-support.js'
import loaderio from './loaderio-verification.js'
import triggerError from './trigger-error.js'
import releaseNotes from './contextualizers/release-notes.js'
import whatsNewChangelog from './contextualizers/whats-new-changelog.js'
import graphQL from './contextualizers/graphql.js'
import webhooks from './contextualizers/webhooks.js'
import layout from './contextualizers/layout.js'
import currentProductTree from './contextualizers/current-product-tree.js'
import genericToc from './contextualizers/generic-toc.js'
import breadcrumbs from './contextualizers/breadcrumbs.js'
import earlyAccessBreadcrumbs from './contextualizers/early-access-breadcrumbs.js'
import features from './contextualizers/features.js'
import productExamples from './contextualizers/product-examples.js'
import featuredLinks from './featured-links.js'
import learningTrack from './learning-track.js'
import next from './next.js'
import renderPage from './render-page.js'
import assetPreprocessing from './asset-preprocessing.js'
import archivedAssetRedirects from './archived-asset-redirects.js'
import favicons from './favicons.js'
import setStaticAssetCaching from './static-asset-caching.js'

const { DEPLOYMENT_ENV, NODE_ENV } = process.env
const isDevelopment = NODE_ENV === 'development'
const isAzureDeployment = DEPLOYMENT_ENV === 'azure'
const isTest = NODE_ENV === 'test' || process.env.GITHUB_ACTIONS === 'true'

// Catch unhandled promise rejections and passing them to Express's error handler
// https://medium.com/@Abazhenov/using-async-await-in-express-with-node-8-b8af872c0016
const asyncMiddleware = (fn) => (req, res, next) => {
  Promise.resolve(fn(req, res, next)).catch(next)
}

// By default, `:remote-addr` is described as following in the morgon docs:
//
//    The remote address of the request. This will use req.ip, otherwise
//    the standard req.connection.remoteAddress value (socket address).
//
// But in production, by default, `req.ip` is the IP of the Azure machine
// which is something like "104.156.87.177:28244" which is *not* the
// end user. BUT! Because we configure `app.set('trust proxy', true)`
// *before* morgain is enabled, it will use the first entry from
// the `x-forwarded-for` header which is looking like this:
// "75.40.90.27, 157.52.111.52, 104.156.87.177:5786" which is
// "{USER'S IP}, {FASTLY'S POP IP}, {AZURE'S IP}".
// Incidentally, that first IP in the comma separated list is the
// same as the value of `req.headers['fastly-client-ip']` but
// Fastly will put that into the X-Forwarded-IP.
// By leaning in to X-Forwarded-IP (*and* the use
// `app.set('trust proxy', true)`) we can express ourselves here
// without having to use vendor specific headers.
const productionLogFormat = `:remote-addr - ":method :url" :status - :response-time ms`

export default function (app) {
  // *** Request connection management ***
  if (!isTest) app.use(timeout)
  app.use(abort)

  // Don't use the proxy's IP, use the requester's for rate limiting or
  // logging.
  // See https://expressjs.com/en/guide/behind-proxies.html
  // Essentially, setting this means it believe that the IP is the
  // first of the `X-Forwarded-For` header values.
  // If it was 0 (or false), the value would be that
  // of `req.socket.remoteAddress`.
  // Now, the `req.ip` becomes the first entry from x-forwarded-for
  // and falls back on `req.socket.remoteAddress` in all other cases.
  // Their documentation says:
  //
  //   	If true, the client's IP address is understood as the
  //    left-most entry in the X-Forwarded-For header.
  //
  app.set('trust proxy', true)

  // *** Request logging ***
  // Enabled in development and azure deployed environments
  // Not enabled in Heroku because the Heroku router + papertrail already logs the request information
  app.use(
    morgan(isAzureDeployment ? productionLogFormat : 'dev', {
      skip: (req, res) => !(isDevelopment || isAzureDeployment),
    })
  )

  // *** Observability ***
  if (process.env.DD_API_KEY) {
    app.use(datadog)
  }

  // Must appear before static assets and all other requests
  // otherwise we won't be able to benefit from that functionality
  // for static assets as well.
  app.use(setDefaultFastlySurrogateKey)

  // Must come before `csrf` otherwise you get a Set-Cookie on successful
  // asset requests. And it can come before `rateLimit` because if it's a
  // 200 OK, the rate limiting won't matter anyway.
  // archivedEnterpriseVersionsAssets must come before static/assets
  app.use(
    asyncMiddleware(
      instrument(archivedEnterpriseVersionsAssets, './archived-enterprise-versions-assets')
    )
  )

  app.use(favicons)

  // Any static URL that contains some sort of checksum that makes it
  // unique gets the "manual" surrogate key. If it's checksummed,
  // it's bound to change when it needs to change. Otherwise,
  // we want to make sure it doesn't need to be purged just because
  // there's a production deploy.
  // Note, for `/assets/cb-*...` requests,
  // this needs to come before `assetPreprocessing` because
  // the `assetPreprocessing` middleware will rewrite `req.url` if
  // it applies.
  app.use(setStaticAssetCaching)

  // Must come before any other middleware for assets
  app.use(archivedAssetRedirects)

  // This must come before the express.static('assets') middleware.
  app.use(assetPreprocessing)

  app.use(
    '/assets/',
    express.static('assets', {
      index: false,
      etag: false,
      // Can be aggressive because images inside the content get unique
      // URLs with a cache busting prefix.
      maxAge: '7 days',
      immutable: process.env.NODE_ENV !== 'development',
      // This means, that if you request a file that starts with /assets/
      // any file doesn't exist, don't bother (NextJS) rendering a
      // pretty HTML error page.
      fallthrough: false,
    })
  )
  app.use(
    '/public/',
    express.static('data/graphql', {
      index: false,
      etag: false,
      maxAge: '7 days', // A bit longer since releases are more sparse
      // See note about about use of 'fallthrough'
      fallthrough: false,
    })
  )

  // In development, let NextJS on-the-fly serve the static assets.
  // But in production, don't let NextJS handle any static assets
  // because they are costly to generate (the 404 HTML page)
  // and it also means that a CSRF cookie has to be generated.
  if (process.env.NODE_ENV !== 'development') {
    const assetDir = path.join('.next', 'static')
    if (!fs.existsSync(assetDir))
      throw new Error(`${assetDir} directory has not been generated. Run 'npm run build' first.`)

    app.use(
      '/_next/static/',
      express.static(assetDir, {
        index: false,
        etag: false,
        maxAge: '365 days',
        immutable: true,
        // See note about about use of 'fallthrough'
        fallthrough: false,
      })
    )
  }

  // *** Early exits ***
  app.use(slowDown)
  app.use(rateLimit)
  app.use(instrument(handleInvalidPaths, './handle-invalid-paths'))
  app.use(asyncMiddleware(instrument(handleNextDataPath, './handle-next-data-path')))

  // *** Security ***
  app.use(cors)
  app.use(
    helmet({
      // Override referrerPolicy to match the browser's default: "strict-origin-when-cross-origin".
      // Helmet now defaults to "no-referrer", which is a problem for our archived assets proxying.
      referrerPolicy: {
        policy: 'strict-origin-when-cross-origin',
      },
    })
  )
  app.use(csp) // Must come after helmet
  app.use(cookieParser) // Must come before csrf
  app.use(express.json()) // Must come before csrf
  app.use(csrf)
  app.use(handleCsrfErrors) // Must come before regular handle-errors

  // *** Headers ***
  app.set('etag', false) // We will manage our own ETags if desired

  // *** Config and context for redirects ***
  app.use(reqUtils) // Must come before record-redirect and events
  app.use(recordRedirect)
  app.use(instrument(detectLanguage, './detect-language')) // Must come before context, breadcrumbs, find-page, handle-errors, homepages
  app.use(asyncMiddleware(instrument(context, './context'))) // Must come before early-access-*, handle-redirects
  app.use(asyncMiddleware(instrument(shortVersions, './contextualizers/short-versions'))) // Support version shorthands

  // Must come before handleRedirects.
  // This middleware might either redirect to serve something.
  app.use(asyncMiddleware(instrument(archivedEnterpriseVersions, './archived-enterprise-versions')))

  // *** Redirects, 3xx responses ***
  // I ordered these by use frequency
  app.use(connectSlashes(false))
  app.use(instrument(redirectsExternal, './redirects/external'))
  app.use(instrument(languageCodeRedirects, './redirects/language-code-redirects')) // Must come before contextualizers
  app.use(instrument(handleRedirects, './redirects/handle-redirects')) // Must come before contextualizers

  // *** Config and context for rendering ***
  app.use(asyncMiddleware(instrument(findPage, './find-page'))) // Must come before archived-enterprise-versions, breadcrumbs, featured-links, products, render-page
  app.use(asyncMiddleware(instrument(spotContentFlaws, './spot-content-flaws'))) // Must come after findPage
  app.use(instrument(blockRobots, './block-robots'))

  // Check for a dropped connection before proceeding
  app.use(haltOnDroppedConnection)

  // *** Rendering, 2xx responses ***
  app.use('/events', asyncMiddleware(instrument(events, './events')))
  app.use('/search', asyncMiddleware(instrument(search, './search')))
  app.use('/healthz', asyncMiddleware(instrument(healthz, './healthz')))
  app.get('/_ip', asyncMiddleware(instrument(remoteIP, './remoteIP')))

  // Check for a dropped connection before proceeding (again)
  app.use(haltOnDroppedConnection)

  app.use(instrument(robots, './robots'))
  app.use(
    /(\/.*)?\/early-access$/,
    instrument(earlyAccessLinks, './contextualizers/early-access-links')
  )
  app.use(
    '/categories.json',
    asyncMiddleware(instrument(categoriesForSupport, './categories-for-support'))
  )
  app.use(instrument(loaderio, './loaderio-verification'))
  app.get('/_500', asyncMiddleware(instrument(triggerError, './trigger-error')))

  // Check for a dropped connection before proceeding (again)
  app.use(haltOnDroppedConnection)

  // *** Preparation for render-page: contextualizers ***
  app.use(asyncMiddleware(instrument(releaseNotes, './contextualizers/release-notes')))
  app.use(instrument(graphQL, './contextualizers/graphql'))
  app.use(instrument(webhooks, './contextualizers/webhooks'))
  app.use(asyncMiddleware(instrument(whatsNewChangelog, './contextualizers/whats-new-changelog')))
  app.use(instrument(layout, './contextualizers/layout'))
  app.use(instrument(currentProductTree, './contextualizers/current-product-tree'))
  app.use(asyncMiddleware(instrument(genericToc, './contextualizers/generic-toc')))
  app.use(asyncMiddleware(instrument(breadcrumbs, './contextualizers/breadcrumbs')))
  app.use(
    asyncMiddleware(
      instrument(earlyAccessBreadcrumbs, './contextualizers/early-access-breadcrumbs')
    )
  )
  app.use(asyncMiddleware(instrument(features, './contextualizers/features')))
  app.use(asyncMiddleware(instrument(productExamples, './contextualizers/product-examples')))

  app.use(asyncMiddleware(instrument(featuredLinks, './featured-links')))
  app.use(asyncMiddleware(instrument(learningTrack, './learning-track')))

  // *** Headers for pages only ***
  app.use(setFastlyCacheHeaders)

  // handle serving NextJS bundled code (/_next/*)
  app.use(instrument(next, './next'))

  // Check for a dropped connection before proceeding (again)
  app.use(haltOnDroppedConnection)

  // *** Rendering, must go almost last ***
  app.get('/*', asyncMiddleware(instrument(renderPage, './render-page')))

  // *** Error handling, must go last ***
  app.use(handleErrors)
}