HarpJS and Macros, Static Delivery for Static Content


HarpJS Macros and Multiple Static Domains

I very recently bought my new domain name and redesigned my website. I'm still using Harp to generate my site, and apache to serve it.

Any good web developer worth their salt will tell you that if you've got static content, you'll do the internet and your site visitors a favor if you use the proper cache headers. This isn't a cache header tutorial though, instead, I'd like to show you how you can create a powerful Macro on your Harp Site to enable proper CDN/static domain usage for any static content you might have.

If you don't know what I'm talking about, you've probably seen things like https://fbstatic-a.akamaihd.net/rsrc.php/v2/yL/r/f2IiBfPD5Mj.png or things like somesites.static.1.domain.com. The use of static, cookieless, subdomains as hosts for resources like images, javascript, or css is something that enables browsers to download and display a webpage faster. Your browser is able to download a site faster (in general) if it can open up connections to multiple hosts. I won't go into all the details, you can always read wikipedia for that.

As a case study, I'm going to tell you how it's done on this site. I assume you're somewhat familiar with Harp, and if not I recommend checking out their excellent documentation.

First, HarpJS runs as a Node module and is part of the new-fangled server-side javascript paradigm. As such, we get to use things like EJS to use Javascript to generate our pages in a similar way to PHP. Something that isn't immediately obvious from following the many good recipes on Harp's site is that full blown javascript means full blown javascript. This means functions my friends!

The best way to enable any image to be served from our static subdomain is to write what I would call a Macro (I've been in the C world recently, don't mind my jargon). This macro is pretty simple and uses a couple globals that we'll declare in our harp.json file like so:

    "globals" : {
        "macros" : {},
        "nextDomain" : 0,
        "maxDomains" : 8,
        "staticimagesurl" : [


Each of the staticimagesurl will function as one of our 'cdn' servers. If you're using S3, CloudFlare, or MaxCDN then you'd be replacing each of those items with your corresponding urls. The maxDomains field could be replaced by an array length check during our macro, but for this case I figured it's simple enough to just count them myself.

With our globals in place, we can write our function, to make it accessible to all pages, I created a partial for it that can be easily injected wherever I need it:


macros.rrImageDomain = function rrImageDomain(){
    //Round robin style
    //You could also do staticimagesurl[Math.floor(Math.random() * staticimagesurl.length)]; for random urls
    return staticimagesurl[nextDomain++ % maxDomains];

Simply put, we are adding a function to the global macros object. Within this macro we are using our list of static content domains and whenever we call the macro we'll output the next item in the list (wrapping around when nextDomain is greater than 7). This will set us up to distribute the requests evenly over the static content servers.

To use the macro we can do this (From one of my _layout files):

 <%- partial('_shared/macros') %>
        <title>Ethan J. Eldridge | <%= title %></title>
        <!-- snip -->

        <link rel="shortcut icon" href="//<%= macros.rrImageDomain() %>//favicon.ico" type="image/x-icon" />
        <link rel="icon" href="//<%= macros.rrImageDomain() %>//favicon.ico" type="image/ico">

This will result in the following markup:

        <title>Ethan J. Eldridge | Ethan Joachim Eldridge&#39;s Webspace</title>
        <!-- snip -->

        <link rel="shortcut icon" href="//static6.ethanjoachimeldridge.info/favicon.ico" type="image/x-icon" />
        <link rel="icon" href="//static7.ethanjoachimeldridge.info/favicon.ico" type="image/ico">

You can see that we simply have incrementing subdomains for our static content (in this case, domains 6 and 7)

By using macros like this it is easy to generate a site that is not only fast to serve (due to the static content itself), but also fast to load (due to leveraging the browsers ability to download images in parallel from multiple hosts). Feel free to leave questions or comments in the disqus thread below!

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