Renders page using headless Chrome for usage by bots.
Last updated a year ago by himanshupandey .
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Rendertron Build status NPM rendertron package

Rendertron is a headless Chrome rendering solution designed to render & serialise web pages on the fly.

:hammer: Built with Puppeteer

:cloud: Easy deployment to Google Cloud

:mag: Improves SEO

Rendertron is designed to enable your Progressive Web App (PWA) to serve the correct content to any bot that doesn't render or execute JavaScript. Rendertron runs as a standalone HTTP server. Rendertron renders requested pages using Headless Chrome, auto-detecting when your PWA has completed loading and serializes the response back to the original request. To use Rendertron, your application configures middleware to determine whether to proxy a request to Rendertron. Rendertron is compatible with all client side technologies, including web components.

Demo endpoint

A demo Rendertron service is available at It is not designed to be used as a production endpoint. You can use it, but there are no uptime guarantees.



Once you have the service up and running, you'll need to implement the differential serving layer. This checks the user agent to determine whether prerendering is required.

This is a list of middleware available to use with the Rendertron service:

Rendertron is also compatible with middleware. Note: the user agent lists differ there.



GET /render/<url>

The render endpoint will render your page and serialize your page. Options are specified as query parameters:

  • width defaults to 1000 - specifies viewport width.
  • height defaults to 1000 - specifies viewport height.
  • mobile defaults to false. Enable by passing ?mobile to request the mobile version of your site.


GET /screenshot/<url>
POST /screenshot/<url>

The screenshot endpoint can be used to verify that your page is rendering correctly.

Both endpoints support the following query parameters:

  • width defaults to 1000 - specifies viewport width.
  • height defaults to 1000 - specifies viewport height.
  • mobile defaults to false. Enable by passing ?mobile to request the mobile version of your site.

Additional options are available as a JSON string in the POST body. See Puppeteer documentation for available options. You cannot specify the type (defaults to jpeg) and encoding (defaults to binary) parameters.


Query parameters

When setting query parameters as part of your URL, ensure they are encoded correctly. In JS, this would be encodeURIComponent(myURLWithParams). For example to specify page=home:

Auto detecting loading function

The service detects when a page has loaded by looking at the page load event, ensuring there are no outstanding network requests and that the page has had ample time to render.

Rendering budget timeout

There is a hard limit of 10 seconds for rendering. Ensure you don't hit this budget by ensuring your application is rendered well before the budget expires.

Web components

Headless Chrome supports web components but shadow DOM is difficult to serialize effectively. As such, shady DOM (a lightweight shim for Shadow DOM) is required for web components.

If you are using web components v0 (deprecated), you will need to enable Shady DOM to render correctly. In Polymer 1.x, which uses web components v0, Shady DOM is enabled by default. If you are using Shadow DOM, override this by setting the query parameter dom=shady when directing requests to the Rendertron service.

If you are using web components v1 and either webcomponents-lite.js or webcomponents-loader.js, set the query parameter wc-inject-shadydom=true when directing requests to the Rendertron service. This renderer service will force the necessary polyfills to be loaded and enabled.

Status codes

Status codes from the initial requested URL are preserved. If this is a 200, or 304, you can set the HTTP status returned by the rendering service by adding a meta tag.

<meta name="render:status_code" content="404" />

Running locally

To install Rendertron and run it locally, first install Rendertron:

npm install -g rendertron

With Chrome installed on your machine run the Rendertron CLI:


Installing & deploying


Clone and install dependencies:

git clone
cd rendertron
npm install
npm run build

Running locally

With a local instance of Chrome installed, you can start the server locally:

npm run start

Deploying to Google Cloud Platform

gcloud app deploy app.yaml --project <your-project-id>

Deploying using Docker

Rendertron no longer includes a Docker file. Instead, refer to Puppeteer documentation on how to deploy run headless Chrome in Docker.


When deploying the service, set configuration variables by including a config.json in the root. Default configuration is:

     datastoreCache: false;
     rendererConfig: {
        useIncognito: true,
         browserConfig: {
           browserMaxUse: 50,
           poolSettings: {
             idleTimeoutMillis: 300000,
             max: 10,
             min: 2,
             testOnBorrow: true,
           browserArgs: {args: ['--no-sandbox']}

Available configuration options:

  • datastoreCache default false - set to true to enable caching on Google Cloud using datastore
  • rendererConfig
    • useIncognito use incognito context instead of default one
    • browserConfig
      • browserMaxUse number of times a browser object can be used
      • browserArgs arguments object to pass to puppeteer while creating browser instance
      • poolSettings browser pool settings generic pool options
        • max: maximum number of resources to create at any given time. (default=1)
        • min: minimum number of resources to keep in pool at any given time. If this is set >= max, the pool will silently set the min to equal max. (default=0)
        • maxWaitingClients: maximum number of queued requests allowed, additional acquire calls will be callback with an err in a future cycle of the event loop.
        • testOnBorrow: boolean: should the pool validate resources before giving them to clients. Requires that factory.validate is specified.
        • acquireTimeoutMillis: max milliseconds an acquire call will wait for a resource before timing out. (default no limit), if supplied should non-zero positive integer.
        • fifo : if true the oldest resources will be first to be allocated. If false the most recently released resources will be the first to be allocated. This in effect turns the pool's behaviour from a queue into a stack. boolean, (default true)
        • priorityRange: int between 1 and x - if set, borrowers can specify their relative priority in the queue if no resources are available. see example. (default 1)
        • autostart: boolean, should the pool start creating resources, initialize the evictor, etc once the constructor is called. If false, the pool can be started by calling pool.start, otherwise the first call to acquire will start the pool. (default true)
        • evictionRunIntervalMillis: How often to run eviction checks. Default: 0 (does not run).
        • numTestsPerEvictionRun: Number of resources to check each eviction run. Default: 3.
        • softIdleTimeoutMillis: amount of time an object may sit idle in the pool before it is eligible for eviction by the idle object evictor (if any), with the extra condition that at least "min idle" object instances remain in the pool. Default -1 (nothing can get evicted)
        • idleTimeoutMillis: the minimum amount of time that an object may sit idle in the pool before it is eligible for eviction due to idle time. Supercedes softIdleTimeoutMillis Default: 30000
    • allowedRequestUrlsRegex: this attributes allow you to ignore all request not matching regex, it would be particularly beneficial in ignoring third party request
    • internalRequestCacheConfig: this is config for caching of internal requests made by page. Cache used here is in memory LRU cache, where on breach of maxEntries least recently used values would be removed to accommodate new entries Structure of internalRequestCacheConfig object:
            "cacheUrlRegex": "<regex for url patters whose response needs to be cached",
            "imageCacheOptions": "BLANK_PIXEL",
            "cacheExpiry": 10000,
            "maxEntries": 2000
      • cacheUrlRegex: This defined regex for url pattern which should be cached
      • cacheExpiry: This defines duration in milliseconds cache entry would be valid. This does not automatically removes entries at expiry time. But ensures cache is not used beyond expiry and entry is also deleted on fetching the entry beyond expiry.
      • maxEntries: This defines max entries cache would maintain
      • imageCacheOptions: This defines the way image request needed to be made. Possible value is:
        • i) 'BLANK_PIXEL', this make us respond with dummy data for image
        • ii) 'IGNORE', this stops any request made for any image type
        • iii) 'ALLOW', this would allow image requests to pass through, it could be cached or allowed to go through depending upon cacheUrlRegex regex


If you're having troubles with getting Headless Chrome to run in your environment, refer to the troubleshooting guide for Puppeteer.

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