HAProxy's keep-alive functionality (and how it can speed up your site)

A while back I wrote a post about using NginX as a reverse-proxy cache for PHP (or whatever your backend is) and mentioned how I was using HAProxy to load balance. The main author of HAProxy wrote a comment about keep-alive support and how it would make things faster.

At the time, I thought "What's the point of keep-alive for front-end? By the time the user navigates to the next page of your site, the timeout has expired, meaning a connection was left open for nothing." This assumed that a user downloads the HTML for a site, and doesn't download anything else until their next page request. I forgot about how some websites actually have things other than HTML, namely images, CSS, javascript, etc.

Well in a recent "omg I want everything 2x faster" frenzy, I decided for once to focus on the front-end. On beeets, we're already using S3 with CloudFront (a CDN), aggressive HTTP caching, etc. I decided to try the latest HAProxy (1.4.4) with keep-alive.

I got it, compiled it, reconfigured:

	option httpclose


	timeout client  5000
	option http-server-close
Easy enough...that tells HAProxy to close the server-side connection, but leave the client connection open for 5 seconds.

Well, a quick test and site load times were down by a little less than half...from about 1.1s client load time (empty cache) to 0.6s. An almost instant benefit. How does this work?

Normally, your browser hits the site. It requests /page.html, and the server says "here u go, lol" and closes the connection. Your browser reads page.html and says "hay wait, I need site.css too." It opens a new connection and the web server hands the browser site.css and closes the connection. The browser then says "darn, I need omfg.js." It opens another connection, and the server rolls its eyes, sighs, and hands it omfg.js.

That's three connections, with high latency each, your browser made to the server. Connection latency is something that, no matter how hard you try, you cannot control...and there is a certain amount of latency for each of the connections your browser opens. Let's say you have a connection latency of 200ms (not uncommon)...that's 600ms you just waited to load a very minimal HTML page.

There is hope though...instead of trying to lower latency, you can open fewer connections. This is where keep-alive comes in.

With the new version of HAProxy, your browser says "hai, give me /page.html, but keep the connection open plz!" The web server hands over page.html and holds the connection open. The browser reads all the files it needs from page.html (site.css and omfg.js) and requests them over the connection that's already open. The server keeps this connection open until the client closes it or until the timeout is reached (5 seconds, using the above config). In this case, the latency is a little over 200ms, the total time to load the page 200ms + the download time of the files (usually less than the latency).

So with keep-alive, you just turned a 650ms page-load time into a 250ms page-load time... a much larger margin than any sort of back-end tweaking you can do. Keep in mind most servers already support keep-alive...but I'm compelled to write about it because I use HAProxy and it's now fully implemented.

Also keep in mind that the above scenario isn't necessarily correct. Most browsers will open up to 6 concurrent connections to a single domain when loading a page, but you also have to factor in the fact that the browser blocks downloads when it encounters a javascript include, and then attempts to download and run the javascript before continuing the page load.

So although your connection latency with multiple requests goes down with keep-alive, you won't get a 300% speed boost, more likely a 100% speed boost depending on how many scripts are loading in your page along with any other elements...100% is a LOT though.

So for most of us webmasters, keep-alive is a wonderful thing (assuming it has sane limits and timeouts). It can really save a lot of page load time on the front-end, which is where users spend the most of their time waiting. But if you happen to have a website that's only HTML, keep-alive won't do you much good =).