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

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 https://render-tron.appspot.com/. It is not designed to be used as a production endpoint. You can use it, but there are no uptime guarantees.

• Middleware
• API
  • Render
  • Screenshot
  • Invalidate cache
• FAQ
  • Query parameters
  • Page render timing
  • Rendering budget timeout
  • Web components
  • Status codes
• Installing & deploying
  • Building
  • Running locally
  • Deploying to Google Cloud Platform
  • Deploying using Docker
  • Config
  • Troubleshooting

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:

• Express.js middleware
• Firebase functions (Community maintained)
• ASP.net core middleware (Community maintained)
• Python (Django) middleware and decorator (Community maintained)

Rendertron is also compatible with prerender.io 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:

• mobile defaults to false. Enable by passing ?mobile to request the mobile version of your site.
• refreshCache: Pass refreshCache=true to ignore potentially cached render results and treat the request as if it is not cached yet. The new render result is used to replace the previous result.

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.
• timezoneId - specifies rendering for timezone.

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.

GET /invalidate/<url>

The invalidate endpoint will remove cache entried for <url> from the configured cache (in-memory, filesystem or cloud datastore).

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:

https://render-tron.appspot.com/render/http://my.domain/?page=home

The service attempts to detect 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.

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.

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 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" />

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

npm install -g rendertron

With Chrome installed on your machine run the Rendertron CLI:

rendertron

Clone and install dependencies:

git clone https://github.com/GoogleChrome/rendertron.git
cd rendertron
npm install
npm run build

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

npm run start

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

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. Available configuration options:

• timeout default 10000 - set the timeout used to render the target page.
• port default 3000 - set the port to use for running and listening the rendertron service. Note if process.env.PORT is set, it will be used instead.
• host default 0.0.0.0 - set the hostname to use for running and listening the rendertron service. Note if process.env.HOST is set, it will be used instead.
• width default 1000 - set the width (resolution) to be used for rendering the page.
• height default 1000 - set the height (resolution) to be used for rendering the page.
• reqHeaders default {} - set the additional HTTP headers to be sent to the target page with every request.
• cache default null - set to datastore to enable caching on Google Cloud using datastore only use if deploying to google cloud, memory to enable in-memory caching or filesystem to enable disk based caching
• cacheConfig - an object array to specify caching options
• renderOnly - restrict the endpoint to only service requests for certain domains. Specified as an array of strings. eg. ['http://render.only.this.domain']. This is a strict prefix match, so ensure you specify the exact protocols that will be used (eg. http, https).
• closeBrowserdefault false - true forces the browser to close and reopen between each page render, some sites might need this to prevent URLs past the first one rendered returning null responses.
• restrictedUrlPatterndefault null - set the restrictedUrlPattern to restrict the requests matching given regex pattern.

• cacheDurationMinutes default 1440 - set an expiry time in minues, defaults to 24 hours. Set to -1 to disable cache Expiration
• cacheMaxEntries default 100 - set the maximum number of entries stored in the selected cache method. Set to -1 to allow unlimited caching. If using the datastore caching method, setting this value over 1000 may lead to degraded performance as the query to determine the size of the cache may be too slow. If you want to allow a larger cache in datastore consider setting this to -1 and managing the the size of your datastore using a method like this Deleting Entries in Bulk
• snapshotDir default <your os's default tmp dir>/renderton - filesystem only the directory the rendertron cache files will be stored in

An example config file specifying a memory cache, with a 2 hour expiration, and a maximum of 50 entries

{
    "cache": "memory",
    "cacheConfig": {
        "cacheDurationMinutes": 120,
        "cacheMaxEntries": 50
    }
}

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