cache warming

In this article, we’re going to help you tackle the tricky problem of cache warming. We’ll approach it from a general viewpoint first before going into detail about the measures we’ve put in place at Fasterize.

At Fasterize, we work with our clients to put in place a consistent and powerful page caching strategy. We’ve got a whole host of tools available: smart caches, which allow dynamic page caching, cookieless caches, which allow pages to be cached for users without an active session, and caches that manage several page variants for mobile devices, different languages and different stores.

However, the Fasterize cache only performs at its best if it is warmed up and properly fed. Feeding the cache is a passive process based on inbound requests made by visitors. Pre-loading the cache using a crawler brings even more benefits when:

    • site traffic is low compared to the product catalogue size
    • or the probability of several users requesting the same variant of a page is low.

Cache hit vs cache miss

Fasterize’s cache stores one or more variants of the files that form a web page so they can be distributed more quickly. By caching copies of the page’s images, CSS and HTML with Fasterize, the origin server doesn’t need to generate these files each time a new visitor accesses the website. This improves the page loading time and reduces the pressure on the origin server. In turn, this means that a website can serve more visitors at once.

As e-commerce sites constantly update their product inventory, files expire after a set period, which can range from a minute to several hours. Each time a cache file expires, it needs to be retrieved once again from the origin server.

Cache hit vs cache miss

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The first web user to visit a website after a cache is first set up or after it expires goes through an empty or ‘cold’ cache, and is met with a ‘cache miss’. The cache passes on the request to the origin server to retrieve the file and transmits it to the visitor. Of course, it also retains the file in the cache, which is now full or ‘warm’. Each subsequent user who visits the same site before the cache expires will be served the file from the cache. This is referred to as a ‘cache hit’.

In summary, a cache hit is when a request is served by the cache and a cache miss is when it is served by the origin server. A cold cache doesn’t yet contain any files, and a warm cache already contains files and is ready to serve visitors.

Cache warming: pros and cons

In general, websites want visitors to encounter a cache hit so they can be served more quickly. However, unless sites engage in cache warming, some visitors will be met with a cache miss after the content expires or the cache is cleared. This gives these visitors a worse experience.

Warming up the cache involves artificially filling the cache so that real visitors always have cache access. Essentially, it prepares the cache for visitors (hence the term ‘warming’, as in warming up a car engine) rather than allowing the first visitor to be served a cache miss. This ensures that all visitors have the same experience
While cache warming offers benefits at a number of levels, it’s important to watch out for a couple of points. Here are some of the problems that can arise when implementing cache warming:

Too many cache servers to warm

If the pages are cached on a CDN with several hundred edge servers, the system will have to set up all of these caches. The site can use an indexing bot, or crawler. This crawler will have to visit the website multiple times and from multiple locations to make sure that each cache is filled.

In this case, the most reasonable strategy is to target only certain nodes of the CDN – those referred to as the principal nodes (origin shield in origin shieldKeyCDN, regional edge cache in Cloudfront). This reduces the scale of the task.

At Fasterize, pages are generally cached at the platform level. As such, this problem doesn’t arise.

Page lifespans that are too short

If the cache duration is just a few minutes, setting up a crawler won’t be effective. It won’t have enough time to trawl through the entire catalogue before the pages it visits expire.

In this situation, a compromise needs to be found – pre-loading key site pages.

An origin server that can't cope with regular crawling

Being visited by a crawler can result in a non-negligible load on undersized origin servers. Essentially, the crawler requests pages that are loaded from the origin server infrequently, which can result in requests that put heavy strain on the origin server’s database.

In this situation, a compromise needs to be found – pre-loading key site pages.
In this case, the solution is to:

  • reduce the number of pages crawled,
  • or reduce the crawling speed,
  • or carry out crawling at quieter times.

Too many possible variations per page

If there are hundreds of versions of the product page (one version for each physical store, for example), the number of pages to be processed may be too high for the allocated crawling time. In this case, the best thing is to determine which versions to prioritize so the crawler only visits major versions of each page.

The Fasterize crawler

Fasterize’s crawler is a feature available to our clients who take out our Enterprise package. The crawler is implemented by our Customer Success Engineers after considering the impacts of Fasterize connections.
The crawler has two operating methods:

  • crawling through the URLs included in the website’s sitemap
  • crawling through a list of URLs provided by the clientle parcours d’une liste d’URLs fournis par le client

The crawler only takes the URLs of pages included in the sitemap into account. Image URLs are not crawled. We recommend using this mode because of its simplicity. However, if the sitemap can’t be used because it is too big or not up-to-date, a static list of URLs can be provided. The crawler then goes through this list from start to finish.

The crawling speed can be configured. We can configure two aspects:

  • the number of bots crawling at once (set to 4 by default)
  • the wait time between each request (set to 0.2 seconds by default).

The User-Agent header used by the crawler can also be configured. By default, it is: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.111 Safari/537.36k FstrzCrawler.

Finally, the request headers can also be customized. This gives significant flexibility in a number of cases:

  • dealing with several variants depending on the fstrz_vary variant cookie
  • requiring mandatory or optional refreshing of each page.
  • refreshing several Fasterize caches: the main cache or the cookieless cache.

This crawler is currently being used by a great deal of clients, bringing very positive results in terms of performance.

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