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Keeping It Light With Effective Caching

Published Sep 28, 2017 by Sarah Chambers

 

Technology and business goals collide at the intersection of page load time and conversion rate.

Marketing wants a fully featured page, with lots of images and services to track user behavior.

Page load time has a huge effect on conversion rate and customer happiness:

  • Half of all customers expect a webpage to load in under 2 seconds. If it doesn’t, they lose trust in and patience with the site, and click the back button to navigate to the next search result on Google.
  • User expectations have increased big time. SOASTA conducted an experiment between 2014 and 2015, looking at conversion rates based on page load time. In 2015 they saw a peak conversion rate at for sites loading in 2.4 seconds. This was 30% faster than the peak conversion load time in 2014.  
  • Cedexis found that decreasing load time by just 1 second improves conversion rate by an average of 27.3%, where a conversion is defined as a purchase, download or sign-up.

The necessity of keeping page load time low for a good customer experience means that tech teams need to exercise every option available to them for performance. Effective caching techniques can bring improvements to even the leanest of websites.

Why optimize caching?

Caching is the process of storing elements so that clients can retrieve resources from memory without needing to put strain on the main server. Utilizing caches has three main benefits.

First of all, caching can make web pages load faster, especially if the user has visited before. If you utilize caching to distribute content globally, visitors will see a reduction in latency, which is the time it takes for a request to physically travel from their browser through the network to the server and back again. If your page is cached locally on the user’s browser, they don’t need to download every resource from your server, every time.

Secondly, caching reduces the amount of bandwidth needed. Instead of the server being responsible for delivering resources for every request, it only needs to deliver new content. Everything else can be returned from a cache along the network.

Finally, caching increases how robust your site is. A client can retrieve resources from a cache, even if your server is down or experiencing high traffic. A plan for preparing for volume spikes should include a caching strategy.

Levels of caching

Caching can happen at lots of different checkpoints along the network, right from the browser to the server itself. Every checkpoint has different benefits and challenges associated with them.

Let’s start with the caching options closest to the end user, then move up the chain to the server where the resource being retrieved originates from.

Browser caching - imagine scrolling through the search results on an online shop. You click on an image link to load the product page, decide it’s not quite right, and hit the back button. If your browser had to request the entire search page again, you’d have to wait for all the images to be downloaded to your browser for a second time. Fortunately, browsers use memory to store a version of sites they’ve already visited. Instead of going all the way to the server and back again, your browser just pulls up the version it’s already stored for you. It will also do this for constant pieces of your site, like your logo, for example.

Proxy cache (Web Server Accelerator) - caches can also be shared between many users. ISP use caches to reduce bandwidth requirements by sharing resources. That way, if one user has already requested a static resource (like an image or file) the ISP doesn’t need to request it again from the server - it can provide it instantly.

Content Delivery Network (CDN) - remember how distance between user and server affects load time? CDNs are caches designed to reduce latency by distributing copies of cached files to local servers all over the world. When a user requests a resource, they are connected to their local CDN. Companies with international users should consider using a CDN to reduce latency.

Server side caching/ reverse proxy - if most of your content is static, you can cache it for yourself, so customers won’t need to hit your server to load static content. There  are several tools that do this for you - Redis, Varnish, and phpfm are all popular options.

Database caching - database servers are often separated from the rest of the server. This means that when your server receives a request from a user, they need to request something extra from the database. If a frequent request always returns the same result, you can cache this in a database cache. This prevents the database from crunching the same request over and over again, resulting in better performance, even during busy periods. Search servers  for ecommerce sites also return cacheable queries.

When should you optimize caching?

“I’m not lazy, I’m just efficient” - ever heard that before? Well, think of your servers as the absolute laziest pieces of hardware you own. Never ask them to do something time consuming twice if there’s a way for them to hold onto results in a cache down the line

For example, you sell jewelry online and one of your top link destinations is a list featuring the 20 most popular items. If you didn’t utilize caching, every time a visitor clicked on that link, they’d need to send a new request through their ISP to your server, which would ask the database to calculate the top 20 items and then send back each of the corresponding images and prices. But realistically, you don’t need to compute this full page every time it’s requested. The top 20 items don’t change often enough to require real-time results. Instead, cache the page in a reverse proxy - located in the same country as the customer - and deliver it much faster.

When you start optimizing your caching strategy a good place to begin is by identifying the most popular and largest representations first. You’ll get the biggest benefit from focusing on caching improvements for pages that are resource heavy and requested often. Looking at the waterfall diagrams on the Network tab of your browser can help identify resource intensive modules on the page.

Time To First Byte (TTFB) is a good way to measure the responsiveness of your web server. Improving your caching strategy through reverse proxies, CDNs and compression will help customers experience shorter TTFB, and help your website feel snappier.

However, don’t forget that most customers will have a poorer experience than that seen in testing. They might, for example, be located on the opposite side of the world using a mobile device or an older computer. By utilizing caching best practices, you’ll ensure customers have a great experience, no matter where they are.

When you need to refresh your data

Because we work in a world where everything is frequently updated, it’s important to understand the different methods we have of forcing a cache reset. There are a few ways we can force browsers and other caches to retrieve a fresh copy of data straight from the server.

  1. Set expiration date  - when the site doesn’t need to stay perfectly up to date in real time, but does need to stay reasonably fresh. If you set an expiration date in your header, the browser will dump the cache after that time. If the resource is requested again, a fresh copy will be retrieved.
  2. Set modified-since - the client will download the updated resource only if the server confirms it’s been updated after the modified-since date. Instead of sending everything again, the server can send back a short 304 response without a body, thus saving bandwidth and time.
  3. Clear specific module - you don’t need to refresh your entire blog cache just to display a new comment. Segmenting a page into different modules can help with cache refreshes.
  4. Fingerprinting - caches work by storing and retrieving a specific file when requested. If you change the name of the file, the cache won’t be able to find the file, and the new copy will be downloaded. This is how fingerprinting works to keep assets up to date. By attaching a unique series of characters to the filename, each asset is considered a new file and requested from the server. Because the content is updated every time, you can set an expiration date years in the future and never worry about a stale cache. Many compilers will automatically fingerprint assets for you, so you can keep the same base filename.

Don’t forget that a cache is not long term storage! If you decide to cache something for later, you might find that it’s been invalidated and you need to retrieve the resource again.

Making caching work for you

Determining the perfect solution for your site can be difficult. Rely too much on caching and you might find users have outdated sites, or memory troubles in their browser. Ignore caching entirely and you’ll see page loading times increase and user experience suffer.

By understanding your user’s needs, you can create a great experience from the beginning. If caching is important, it’s worth using a framework that provides out-of-the-box caching optimization. If caching is less relevant because accuracy is more important than speed, then you can make allowances either way.

Caching strategy is a problem to be solved uniquely for each app. Determining where you can utilize caching to save bandwidth is an ongoing learning experience. Keep making incremental improvements and keep it light for your customers.

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