server.js

const express = require('express')
const next = require('next')
const LRUCache = require('lru-cache')

const port = parseInt(process.env.PORT, 10) || 3000
const dev = process.env.NODE_ENV !== 'production'
const app = next({ dev })
const handle = app.getRequestHandler()

// This is where we cache our rendered HTML pages
const ssrCache = new LRUCache({
  max: 100 * 1024 * 1024, /* cache size will be 100 MB using `return n.length` as length() function */
  length: function (n, key) {
    return n.length
  },
  maxAge: 1000 * 60 * 60 * 24 * 30,
})

app.prepare()
  .then(() => {
    const server = express()

    server.get('/_next/*', (req, res) => {
      /* serving _next static content using next.js handler */
      handle(req, res)
    })

    server.get('*', (req, res) => {
      /* serving page */
      return renderAndCache(req, res)
    })

    /* starting server */
    server.listen(port, (err) => {
      if (err) throw err
      console.log(`> Ready on http://localhost:${port}`)
    })
  })

/*
 * NB: make sure to modify this to take into account anything that should trigger
 * an immediate page change (e.g a locale stored in req.session)
 */
function getCacheKey(req) {
  return `${req.path}`
}

async function renderAndCache(req, res) {
  const key = getCacheKey(req)

  // If we have a page in the cache, let's serve it
  if (ssrCache.has(key)) {
    // console.log(`serving from cache ${key}`)
    res.setHeader('x-cache', 'HIT')
    res.send(ssrCache.get(key))
    return
  }

  try {
    // console.log(`key ${key} not found, rendering`)
    // If not let's render the page into HTML
    const html = await app.renderToHTML(req, res, req.path, req.query)

    // Something is wrong with the request, let's skip the cache
    if (res.statusCode !== 200) {
      res.send(html)
      return
    }

    // Let's cache this page
    ssrCache.set(key, html)

    res.setHeader('x-cache', 'MISS')
    res.send(html)
  } catch (err) {
    app.renderError(err, req, res, req.path, req.query)
  }
}