Controlling The Cache: Using Edge Side Includes In Varnish

I’m a firm believer that the best way to optimize for fast-loading mobile sites is to optimize for everyone. We don’t know when someone is on a non-mobile device but tethered to their phone, or just on awful Wi-Fi.

In a previous article for Smashing Magazine I explained how you can speed up your websites by serving dynamic pages from a reverse proxy like Varnish. If you are new to Varnish then that article is the place to start as I’ll be diving straight into configuration details here.

In this article I’ll explain how you can benefit from using Varnish even when there are parts of your pages that can’t be cached for long periods, using Edge Side Includes.

The Problem With Caching Link

The bulk of most web pages is content that doesn’t change very often. Generally, when you publish some content it remains fairly static, and even if it is updated, taking a few minutes before the new version appears may not be a problem. If it is vital that the cache is invalidated on publish, you can use cache invalidation to clear that page from the cache. However, there might be some content that you do not want to cache at all, such as personalized content. There may also be content that you would like to cache but for shorter periods of time, perhaps a news widget that updates very frequently. When pages contain this type of content you might think they are uncachable. It is here, though, that Edge Side Includes (ESI) can save the day – and your site performance.

What Are Edge Side Includes? Link

ESI is a language specification for assembling fragments of web pages inside other web pages. The specification was submitted to the W3C in 2001, but remains a “W3C Note” made available for discussion and not endorsed by the W3C or updated by a Working Group.

ESI works in a similar way to other methods of including fragments in your pages, such as Server Side Includes (SSI) or PHP include statements, but it has been designed for reverse proxies like Varnish that sit in front of a web server and cache content.

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