A little bit ago I talked about Varnish, how to install and configure it, and why I’m not using it at the moment. The actual goal of all this stuff is to speed up a website. Site speed is an insanely fickle beast, and measuring it without going insane is nigh impossible.
When we talk about site speed we don’t just mean how fast the site loads. We mean how well it performs on the front and back. Does it load everything it needs to be a page in a non-jumpy way? Does it load and then magically change to another format because you’re on an iPad? Does it hang and then load? We mean all those aspects that go into a site and make it zippy.
Which brings us to caching. The goal of caching is blindingly simple: Don’t put extra load on the server while serving up webpages and make it faster. The how of caching is crazy.
When I talked about Multisite Caching, I brought up the different types and why and where I saw each one being used. But I didn’t really explain why very well. In order to understand it, you need to understand why we need to cache.
If your website was all plain, static, HTML files, it would be really fast. The web was (initially) built for that stuff and it was all basic. “I want this page.” And the server would reply “Okay, here it is and some images.” When we start adding in stuff like dynamic PHP (WordPress), we put more load on the server because for every time someone visits your site, it has to ask WordPress “Do you have this page?” and WordPress has to check, see if it does, generate the output, and then it loads. The more complex the site is, the more big images and javascript fancy stuff, the slower the site gets.
Logical stuff, right? You’re also going to be limited by how fast your server is and how much of it you can use. If you’re on a dedicated server, the limit is your hardware and bandwidth pipe. If you’re on shared, though, the limit is lower, and really varied and complicated. While I mention a ‘bandwidth pipe’ and we techs always joke about the sturdy internet tubes, it’s not a fully accurate analogy, and even with all the bandwidth available in the world, the speed of your server is going to limit you far more.
There’s a phenomena called the “noisy neighbor” which impacts people on shared hosts a lot and is a lot of why people get confused about the bandwidth thing. You see, if you’re on shared servers, you share services. If one of your neighbors uses a lot of memory, there’s less available for you. This makes perfect sense, and hosts combat this by limiting how much you can do. I know a lot of companies say that you have ‘unlimited’ space and bandwidth, and while that’s true, it doesn’t mean you get to use all the power available to the server. Basically on shared servers, when you see ‘unlimited’ you should read it as ‘unlimited until you start making other people’s sites run worse.’
What does this have to do with caching? It’s the reason why we cache! WordPress does not make static HTML pages at all. If you look on your server for a file named ‘about’ you won’t find one. Instead, WordPress uses the .htaccess file to magically run your request for example.com/about/ through the index.php file, which then checks the database and pulls the content for that page. It’s entirely dynamic, and every single page request is run through the database. And yeah, that gets slow over time. The dynamism is fantastic though, and that’s why things like comments magically update the rendered page right away.
Thus, in order to make our super dynamic websites run by WordPress run faster, we turn to methods to generate static file caches. Converting a WordPress page from the PHP queries to a static file is complicated, and in essence every single tool has to generate that dynamic page, copy the output, and save it to a location where it can be pulled from directly. At the same time, it has to alter the server in some way to say “If I have a static file, use that instead.” When you use a plugin, generally it does this via your .htaccess file.
The obvious problem with this is that while the page may be faster for visitors, you’re still putting load on your server by having it generate these html files and serve them. And you, the logged in user, won’t get the cached page, generally, not even with something as cool as Varnish, so we have to still consider the rest of the server.
Speaking of Varnish … the simplest explanation I can give you about it is this: Instead of having WordPress use a plugin to generate the page, Varnish lets WordPress load, takes a snapshot of the resulting page, and saves it somewhere else. That means that in-between your visitor and the WordPress install is the Varnish cached page, which means the load is off your server more! No more loading the html page, Varnish is going to do it and make it a little faster. You’ll still want a plugin to allow WordPress to tell Varnish to delete pages, but it can significantly run faster.
But … what about the server speed itself? Is there a way to cache that and speed it up to? There is! But that’s a longer post, all it’s own.
