Writing Express middleware – Effective Volto – Backend

Writing Express middleware

Writing Express middleware#

When working with Volto projects, we consider the backend to be Plone. But we actually have another backend, the Volto Nodejs HTTP server, which is based on the popular Express web framework. Volto, the SSR server is an Express app and we can extend that app with new capabilities.

Express extensions are usually Express middlewares for routes (URL matchers) and Volto already comes with several of them, useful in proxy-ing some of Plone's browser views (for example the @@images or robots.txt, sitemap.xml.gz, etc).

Here's, as an example, the Sitemap middleware:

import express from 'express';
import { generateSitemap } from '@plone/volto/helpers';

export const sitemap = function (req, res, next) {
  generateSitemap(req).then((sitemap) => {
    if (Buffer.isBuffer(sitemap)) {
      res.set('Content-Type', 'application/x-gzip');
      res.set('Content-Encoding', 'gzip');
      res.set('Content-Disposition', 'attachment; filename="sitemap.xml.gz"');
    } else {
      // {"errno":-111, "code":"ECONNREFUSED", "host": ...}
      // Some data, such as the internal API address, may be sensitive to be published
      res.send(`Sitemap generation error: ${sitemap.code ?? '-'}`);

export default function () {
  const middleware = express.Router();

  middleware.all('**/sitemap.xml.gz', sitemap);
  middleware.id = 'sitemap.xml.gz';
  return middleware;

Notice that this code should not get into the client bundle, so you should load it conditionally by placing it in a separate module.

export default applyConfig(config) {
  if (__SERVER__) {
    const makeMiddleware = require('./mymiddleware');
    const middleware = makeMiddleware();

  return config;

An example of a Volto addon that implements an Express middleware is the volto-corsproxy.