Dust.js as a React Delivery Vehicle

Even people in love with Single Page Apps and pooh-poohing server side rendering know that SPAs cannot just materialize in the browser. Something has to deliver them there. Until recently, AngularJS was the most popular client side JS framework, and I have seen all kinds of ADVs (Angular Delivery Vehicles) – from Node.js (N is for Node.js in the MEAN stack), to wonderfully ironic JSP files serving Angular. Particularly exuberant developers go all the way to using static HTML to launch their apps into orbit, extolling the virtues of CDNs.

Of course, I would like to see their faces the first time they realize the need to compute something dynamically in those static files. But that is beside the point. The important takeaway is that AngularJS has a very nice property of being embeddable. You don’t need to surrender your entire page to Angular – you can sequester it to a DIV (in fact, you can have multiple Angular apps running in the same page). This is a very nice feature, but also a necessary one – again, AngularJS is a pure client side framework.

With great power…

React changes this equation because it can be rendered on the server. Similarly to AngularJS, React root component can be mounted to any node inside your page, but unlike Angular, this page can also be rendered on the server by React.

This opens the door for a whole new class of isomorphic or universal apps, and I have written about it already. What you need to decide is whether you want to render the entire page, or a subset of it. If you choose the latter, then the subsequent question becomes – how do you start the rendering process until React takes over?

My first inclination was to go full React. Using the awesome react-engine module, it is easy to configure React as the view engine in the Express framework, and render the entire page. However, we soon hit some snags:

1. As we render our UIs using micro services that use UI composition to glue the pages together, we hit a problem of React being grumpy about having to render HTML snippet arriving from outside of its control – something much easier with a templating library such as Dust.js.
2. JSX turned out to be fairly quirky and finicky when rendering HTML HEAD element, as well as trying to inline JavaScript in tags. In fact, the latter became so error prone that we completely gave up on it, electing to put all our script in files. I didn’t like this particular restriction, and this remains an area where I am sour on JSX and the fact it most decidedly isn’t HTML.

Of course the problems I listed above did not show up in the small example I published in the initial article. They reared their ugly head once we started getting serious. In my original example, the header was a React component. This works great if you control the entire app, but the whole deal of UI composition is to integrate with apps not necessarily written using React, and you can only do that using the least common denominator (HTML, CSS and vanilla JS).

Remind me again about UI composition

In a nutshell, UI composition is the approach where common areas on the page are served as HTML snippets from a REST API. The API service can also serve CSS and JavaScript files that accompany the snippet (if you don’t serve them from a CDN). The idea is that a dedicated microservice can be providing these common areas this way, allowing other microservices to pull the content and inline it in their pages regardless of their stack.

UI composition inverts the flow you would normally use with iframes. Instead of rendering the common areas, then placing an iframe for the content and loading the content from an external URL, we load the common area as HTML and inline it into our page. In the olden days this approach was often accomplished using ‘edge side includes’ or ESIs. This approach has many benefits, including no need for the dreaded iframes, full control over the entire page, and the ability to integrate microservices implemented using different stacks. For example, the teams in my current project use:

1. Node.js microservices using React for isomorphic rendering
2. Node.js microservices using Dust.js and jQuery
3. Java microservices using AngularJS

All of these microservices render the exact same header even though they use different stacks and libraries. Powerful stuff.

OK, so React?

The problem here with React is that when we use it to render the entire page server-side, it starts to squirm in discomfort. Inlining HTML is a problem for React because, while it can be used on the server, it was designed to shine on the client. Inlining raw HTML presumably obtained as a user input into DOM is a major security exposure without sanitization, and React will tell you so in no uncertain terms. Here is the line that inlines the shared header taken straight out of our production code:

<div dangerouslySetInnerHTML={{__html: decodeURIComponent(this.props.header)}} />

This is React telling us very clearly: “I really, really, really don’t like what you are asking me to do”. And this is even before we bring in react-engine. React engine is a PayPal module that enables isomorphism when combined with Node.js/express and react-router. We can render on the server, then pack the properties as part of the HTML response, hydrate the React components on the client and continue where we left off.

Only we have a little problem: notice that in the snippet above we have inlined the header by passing it down to the React component as a prop. What this means is that this prop (‘header’) will be sent to the client alongside other properties. The only problem is that this property contains HTML snippet for the entire header, and because it can really mess up react-engine, it is also URLencoded, making it even bulkier.

This is an unhappy situation: first we inline the header HTML (which is fine), then we send that same inlined HTML, this time encoded, as a React prop to the client. This unnecessarily bloats the generated HTML.

RDV

This is what we are going to do to solve this problem: we are going to call back our trusty Dust.js sitting sad on the sidelines ever since we got smitten with React. We can press it into service to do what it does best – outline the page, prep it up to the point where React can take over.

It’s a good thing that Express framework will happily handle more than one view engine. Of course, only one of them can be registered as default, but this just means that we will need to spell out the extension of dust files (because React is the default). Big deal.

var path = require('path')
, express = require('express')
, renderer = require('react-engine')
, dust = require('dustjs-linkedin')
, helpers = require('dustjs-helpers')
, cons = require('consolidate');

...

// create the view engine with `react-engine`
var engine = renderer.server.create({
  routes: require(path.normalize(__dirname + '/public/routes.jsx')), 
  docType: ""
});

// set the engines
app.engine('.jsx', engine);
app.engine('.dust', cons.dust);

// set the view directory
app.set('views', __dirname + '/public/views');

// set jsx as the view engine
app.set('view engine', 'jsx');

// finally, set the custom view
app.set('view', renderer.expressView);

Once we have both engines set up, we can arrange our delivery vehicle like this. We will first set up a reusable layout template using Dust.js that will deliver the outline of each page:

<!DOCTYPE html>
<html>
 <head>
   <meta charset='utf-8'>
   <meta http-equiv="X-UA-Compatible" content="IE=edge">
   <meta name="viewport" content="width=device-width, initial-scale=1" >
   {+headContent/}
   <title>
     {title}
   </title>
   <link rel="stylesheet" href="/css/styles.css">
 </head>
 <body>
   {header|s}
   <div class="main-content">
     {+mainContent/}
   </div>
   {+javascript/}
 </body>
</html>

Notice how inlining of the header now happens in Dust.js, relieving React from doing it. We can now use this template to render our page like this:

{>"layout"/}
{<mainContent}
<div id="react-mount-node">{spa|s}</div>
{/mainContent}
{<javascript}
<script src="/bundle.js"></script>
{/javascript}

We have created a DIV in the template above where we are mounting React, and we will inline the React server-side content using the ‘spa’ variable.

Our Express controller that will handle both Dust.js and React can look something like this:

var request = require("request");

module.exports.get = function(req, res) {
  var settings = {
    title: 'SPA - Demo',
    name: 'React SPA',
    selection: 'header-spa'
  };

  var headerUrl = "http://"+req.headers.host+"/header?selection=header-spa";

  request.get(headerUrl, function (err, response, body) {
    if (err)
      console.log(err);
    else
      settings.header = body;
    res.render(req.url, { name: settings.name }, function(err, html) {
      if (err) {
        settings.spa = err.message;
        console.log(JSON.stringify(err.stack));
      }
      else
        settings.spa = html;
      res.render("spa.dust", settings); 
    });
  });
}

The code above is what I call ‘React Delivery Vehicle’. It has three steps:

1. First we fetch the header from the URL that is providing us with the header HTML snippet. We capture it into a variable ‘header’. In production, we will heavily cache this step.
2. Then we render the React root component as usual. This will employ react-engine and react-router to render all the views necessary for the provided request URL. However, we don’t send the rendering to the response. Instead, we capture it in the variable ‘spa’.
3. Finally, we invoke Dust.js view engine to render the full page, passing in the ‘settings’ object. This object will contain both the header and the content rendered by React. Dust will simply inline both while rendering the page outline. The result will be sent directly to the Express response.

Commentary

The solution described above plays to the strengths of both Dust.js as a server side template engine and React as a component library. Dust.js is great at rendering HTML outline the way you would expect, with no need to worry about JSX quirks around HEAD meta tags, or JavaScript inlining. React takes over to render isomorphic components into the mount node. This means we don’t need to send any data that is not needed on the client. This fixes the HTML bloat problem I mentioned before.

As for the negatives, setting up two rendering engines on the server has a slight overhead, and switching mentally from Dust.js to JSX adds context switching tax. Luckily, you can set up reusable Dust.js templates and not really worry about them too much – most of the action will be in JSX anyway.

We like this approach and are currently switching to it across the board. If you have alternative ideas or comments, drop me a line. Meanwhile, the source code for the entire example is available on GitHub as usual, and the sample app employing this mechanism runs on Bluemix.

© Dejan Glozic, 2015