Everything about website caching: how it works, benefits, and risks

Cache, which is a mechanism for storing partially processed data in an easily accessible location, enables faster page loading, reduced server load, and improved overall performance and scalability of websites. Despite its undeniable benefits, caching also brings certain challenges and risks, such as issues with data freshness, security and privacy concerns, and complications related to its configuration. In this article, we will take a closer look at both the benefits and potential problems associated with using cache in the context of websites.

Cache – what is it?

Cache on a website refers to a mechanism for storing copies of website elements, such as HTML files, CSS stylesheets, JavaScript scripts, and multimedia, either on the user’s device (e.g., in the browser) or on the server. The main goal of website caching is to speed up page loading by reducing the amount of data that needs to be downloaded during subsequent visits.

How does website caching work?

• Downloading and storing data
  When a user visits a website for the first time, the browser downloads all necessary resources from the server. After downloading, the browser can store these resources in its local cache. These resources may include:
  • HTML files
  • CSS stylesheets
  • JavaScript scripts
  • Images
  • Multimedia files
  • Fonts
• Checking cache validity
  When a user visits the same website again, the browser first checks if it has saved copies of the website’s resources in its cache. It also checks whether these resources are still current. Browsers use HTTP headers, such as Last-Modified or ETag, to determine if the resource has changed on the server since the last download.
• Loading data from the cache
  If the resources are still current, the browser loads them directly from the cache, which significantly speeds up page loading. If the resources have been changed or are not present in the cache, the browser downloads them again from the server.

Types of cache

Regarding websites, we distinguish three types of cache:

1. Browser cache:
   Web browsers use a local cache to store resources from websites that have been visited before. This way, when a user returns to the site, the browser can load part or all of the content from the local storage instead of downloading the data again from the internet.
2. Server cache:
   In large networks, such as in companies or universities, proxy servers can store copies of popular websites accessible to all network users. This allows for faster page loading and reduces the load on the internet connection.
3. CDN cache (Content Delivery Network):
   A CDN is a network of servers distributed around the world that store static website resources, such as images and scripts. A CDN enables faster content delivery to users, regardless of their geographic location, by serving data from the nearest physical server.

Using cache is one of the few ways to speed up a website. You can read more about improving site performance in our article:

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Benefits of using cache

Faster page loading

Cache allows frequently used resources, such as images, scripts, and styles, to be stored directly on the user’s device or on the server. As a result, when the user revisits the site, most resources can be loaded locally without the need to download them again from the server.

Reduced server load

When resources are stored in the cache on users’ devices or proxy servers, fewer requests reach the main server. This means that the server has to handle less traffic and can operate more efficiently. Reduced server load can also translate into lower resource usage, such as network bandwidth and processing power, which results in lower infrastructure maintenance costs.

Improved user experience

Website caching significantly improves the overall user experience, as it contributes to smoother and faster browsing. Users appreciate faster load times, which can also lower bounce rates since they are less likely to abandon a page while waiting for it to load.

Scalability

Thanks to caching, websites can better handle an increase in the number of users. Scalability is crucial for large websites that may experience sudden traffic spikes. With cache, servers can manage more requests without the need to immediately increase available resources.

Potential issues with caching

1. Outdated content: Resources on the server may be updated, but the user’s browser may still display old versions.
2. Aggressive caching: Overly long caching periods can lead to displaying outdated content.
3. Duplicate resources: Storing different versions of the same resource burdens the cache.
4. Issues with CDN cache: CDNs may not synchronize properly with the server.
5. Conflicts between browser and server cache: Different caching strategies can lead to inconsistencies.
6. Issues with caching in mobile applications: Caching behavior can differ between mobile apps and desktop browsers.
7. Debugging issues: Difficulties in identifying and resolving caching problems.

Solutions to these issues include using cache busting techniques, appropriate HTTP header settings, TTL optimization, clear file versioning, and automated cache clearing in CDNs. It is also advisable to adopt consistent caching policies between the server and the browser, tailor caching strategies for mobile applications, and utilize debugging tools.

Cache management

HTTP headers

HTTP headers play a key role in cache management. They allow you to control how long and where resources are stored. The most important HTTP headers related to caching are:

• Cache-control: A header that specifies the rules for storing resources. It can include values such as max-age, no-cache, no-store, must-revalidate, etc.
  Examples:
  ‘Cache-Control: no-cache’ – forces the resource to be re-downloaded on every visit.
  ‘Cache-Control: no-store’ – prevents the resource from being stored in the cache.
  ‘Cache-Control: public, max-age=3600’ – allows the resource to be cached by any server for one hour.
  ‘Cache-Control: private, max-age=3600’ – restricts caching of the resource to the user’s browser for one hour.
• Expires: Specifies the date and time after which the resource is considered outdated.
  Example:
  ‘Expires: Wed, 21 Oct 2021 07:28:00 GMT’ – the resource will be cached until the specified date and time.
• ETag (Entity Tag): A unique identifier for a resource’s version, used to detect changes.
  Example:
  ‘ETag: “686897696a7c876b7e”‘ – the server can compare this identifier with the version stored in the browser to determine if the resource is current.

Cache busting

Cache busting is a technique that forces cached resources to be refreshed when they are updated. Here are the most common methods:

• File versioning: Adding version numbers to file names, causing the browser to treat them as new resources.
  Example: Instead of style.css, we use style.v1.css or style.12345.css. Any change to the file results in a name change, which forces the download of the new version.
• Query string in URL: Adding query parameters to the resource URL.
  Example: Instead of style.css, we use style.css?v=1.2. Each change in version adds a new parameter, which forces the resource to refresh in the cache.

TTL

TTL (time to live) is a parameter that specifies how long a resource can be stored in the cache before it is considered outdated. Proper TTL settings are crucial for effective cache management.

• Defining TTL: TTL is most commonly defined using the header Cache-Control: max-age=<seconds>.
  Example: Cache-Control: max-age=86400 sets the resource’s lifetime to 24 hours (86,400 seconds).
• Optimal TTL settings: Static resources (e.g., images, CSS, JavaScript files) should have a long lifetime, such as several days to several months. Dynamic resources (e.g., HTML pages, API data) should have a short lifetime, ranging from a few seconds to a few minutes, depending on how frequently they change.
• Balancing freshness and performance: Setting TTL requires balancing data freshness with site performance. Too short a lifetime may lead to frequent downloads, while too long a lifetime may result in outdated content being displayed.

Summary

Website caching, despite its challenges, remains an invaluable tool in the arsenal of web technologies, significantly impacting performance, scalability, and overall user experience. However, it is crucial for website administrators and developers to be aware of both the benefits and the potential issues associated with its use. Careful configuration and regular monitoring of the cache are essential to ensure fast and secure website loading, while also protecting user privacy and maintaining the freshness of the displayed data.