feat(prerenderer): improving under high load

prerender-server/package.json

@@ -6,8 +6,9 @@
   "dependencies": {
     "body-parser": "^1.19.0",
     "cookie-parser": "^1.4.5",
-    "superagent": "^6.1.0",
-    "prom-client": "^14.0.0"
+    "express-queue": "^0.0.13",
+    "prom-client": "^14.0.0",
+    "superagent": "^6.1.0"
   },
   "scripts": {
     "start": "./start.sh",

prerender-server/src/cluster.js

@@ -5,7 +5,7 @@ if (cluster.isMaster) {
   console.log(`Master ${process.pid} is running`);
 
   let activeRenders = 0
-  const MAX_TOTAL_RENDERS = parseInt(process.env.MAX_TOTAL_RENDERS || '1')
+  const MAX_TOTAL_RENDERS = parseInt(process.env.MAX_TOTAL_RENDERS || numCPUs.toString())
 
   for (let i = 0; i < numCPUs; i++) {
     cluster.fork()

prerender-server/src/server.js

@@ -8,6 +8,10 @@ import promClient from 'prom-client'
 import l10n from '../client/l10n'
 import render from '../client/run-server'
 
+if (!process.env.API_URL) {
+  throw new Error('API_URL environment variable is required but not defined.');
+}
+
 const themes = ['light', 'dark']
   , langs = Object.keys(l10n)
   , baseHref = process.env.BASE_HREF || '/'
@@ -56,17 +60,30 @@ if (app.settings.env == 'development')
 app.use(require('cookie-parser')())
 app.use(require('body-parser').urlencoded({ extended: false }))
 
-app.use((req, res, next) => {
-  // TODO: optimize /block-height/nnn (no need to render the whole app just to get the redirect)
+const queue = process.env.MAX_PENDING_RENDERS && require('express-queue')({
+  activeLimit: 1, // handled by the master process, see below
+  queuedLimit: parseInt(process.env.MAX_PENDING_RENDERS, 10)
+});
 
+app.use((req, res, next) => {
+  // Middleware to check theme and lang cookies
   let theme = req.query.theme || req.cookies.theme || 'dark'
   if (!themes.includes(theme)) theme = 'light'
   if (req.query.theme && req.cookies.theme !== theme) res.cookie('theme', theme)
 
   let lang = req.query.lang || req.cookies.lang || 'en'
   if (!langs.includes(lang)) lang = 'en'
   if (req.query.lang && req.cookies.lang !== lang) res.cookie('lang', lang)
+  req.renderOpts = { theme, lang }
+  next()
+})
+
+if (queue) app.use(queue)
+
+app.use((req, res, next) => {
+  // TODO: optimize /block-height/nnn (no need to render the whole app just to get the redirect)
 
+  // IPC-based queuing for cluster mode
   if (typeof process.send === 'function') {
     const requestId = ++requestCounter
     process.send({ type: 'startRender', requestId })
@@ -77,8 +94,9 @@ app.use((req, res, next) => {
         clearTimeout(timeout)
         process.removeListener('message', handler)
         if (msg.type === 'renderAllowed') {
-          doRender()
+          doRender(req, res, next)
         } else if (msg.type === 'renderDenied') {
+          // received when the master's render queue is full
           res.status(503).send('Server overloaded')
         }
       }
@@ -93,40 +111,45 @@ app.use((req, res, next) => {
       }
     }, 5000) // 5 second timeout
   } else {
-    doRender()
+    // standalone mode
+    doRender(req, res, next)
   }
+})
 
-  function doRender() {
-    activeRenders.inc()
-    const end = renderDuration.startTimer()
-    let metricsUpdated = false
-    render(req._parsedUrl.pathname, req._parsedUrl.query || '', req.body, { theme, lang, isHead: req.method === 'HEAD' }, (err, resp) => {
-      if (!metricsUpdated) {
-        metricsUpdated = true
-        if (typeof process.send === 'function') process.send({ type: 'endRender' })
-        activeRenders.dec()
-        end()
-        totalRenders.inc()
-      }
-      if (err) return next(err)
-      if (resp.redirect) return res.redirect(301, baseHref + resp.redirect)
-      if (resp.errorCode) {
-        console.error(`Failed with code ${resp.errorCode}:`, resp)
-        return res.sendStatus(resp.errorCode)
-      }
+function doRender(req, res, next) {
+  activeRenders.inc()
+  const end = renderDuration.startTimer()
+  let metricsUpdated = false
+  render(req._parsedUrl.pathname, req._parsedUrl.query || '', req.body, { ...req.renderOpts, isHead: req.method === 'HEAD' }, (err, resp) => {
+    if (!metricsUpdated) {
+      metricsUpdated = true
+      // inform the master process that we're done rendering and can accept new requests
+      if (typeof process.send === 'function') process.send({ type: 'endRender' })
+      // and tell express-queue that we're ready for the next one
+      if (queue) queue.next()
+      activeRenders.dec()
+      end()
+      totalRenders.inc()
+    }
 
-      res.status(resp.status || 200)
-      res.render(indexView, {
-        prerender_title: resp.title
-        , prerender_html: resp.html
-        , canon_url: canonBase ? canonBase + req.url : null
-        , noscript: true
-        , theme
-        , t: l10n[lang]
-      })
+    if (err) return next(err)
+    if (resp.redirect) return res.redirect(301, baseHref + resp.redirect)
+    if (resp.errorCode) {
+      console.error(`Failed with code ${resp.errorCode}:`, resp)
+      return res.sendStatus(resp.errorCode)
+    }
+
+    res.status(resp.status || 200)
+    res.render(indexView, {
+      prerender_title: resp.title
+      , prerender_html: resp.html
+      , canon_url: canonBase ? canonBase + req.url : null
+      , noscript: true
+      , ...req.renderOpts
+      , t: l10n[req.renderOpts.lang]
     })
-  }
-})
+  })
+}
 
 // Cleanup socket file from previous executions
 if (process.env.SOCKET_PATH) {