Understanding the Ineffectiveness of Cache Invalidation
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Chapter 1: The Problem with Cache Invalidation
Utilizing cache invalidation as a means to launch a new version of a website is often ineffective. For smaller websites, it can be tempting to bypass proper caching management and serve content directly. However, as a website grows and begins to utilize a Content Distribution Network (CDN), establishing an effective caching strategy becomes crucial. If developers neglect this process, users may encounter outdated content or even malfunctioning sites.
Why CDNs Rely on Caching
Content Distribution Networks (CDNs) are designed to cache responses to web requests to enhance performance. By caching a website, a CDN can deliver content without having to return to the origin server, which significantly reduces load times and improves the user experience. Nevertheless, if the CDN serves cached content, it may present users with outdated information. Developers often attempt to refresh this content by invalidating the cache using tools provided by the CDN.
The Limitations of Cache Invalidation
Invalidating the cache at the CDN level does not affect the caching mechanisms in users' browsers or those employed by Internet Service Providers (ISPs). As a result, even if a cache is invalidated, it’s impossible to gauge the situation from the customer's viewpoint.
Browser and ISP Caching Explained
Browsers implement caching headers similar to those used by CDNs. Consequently, if a user visits a site, the cache is invalidated, and they return to the site later, they may still access the old version. Cache invalidation should be reserved for exceptional circumstances, not treated as a routine part of the development workflow. As noted by Google, "Invalidations do not impact cached copies in web browser caches or those managed by third-party ISPs." It is also common for ISPs to cache frequently accessed websites to reduce network costs and server load. Once content is cached in a user's browser or by an ISP, it cannot be removed, highlighting the challenges of cache invalidation and the need for alternative strategies.
Chapter 2: Alternative Strategies for Cache Management
Instead of relying on cache invalidation when updating websites, we should utilize appropriate cache headers or implement cache busting techniques. While it's typical for websites to set caching headers by default, proper usage is key to minimizing the need for invalidation.
Cache Control Headers
Using the expires header allows us to specify when a particular object should be removed from the cache. However, managing this header can be challenging due to parsing difficulties, potential implementation bugs, and issues arising from system clock adjustments.
Max-Age Directive
Typically, the max-age header is recommended to indicate how long an object should be cached. A sensible starting point is to set this to 86400 seconds (equivalent to one day). This way, when a new version of a website is deployed, it will be visible to all users after 24 hours.
Cache Busting Techniques
For immediate updates while utilizing caching, consider employing cache busting. By appending a unique URL or query string to requests, caches will recognize each version as a distinct object. For instance, deploying updated links with each change—while never caching the initial index.html—allows for loading new files without waiting for previous versions to clear.
Conclusion
Next time you contemplate invalidating a cache for a website update, consider using the max-age or cache busting techniques instead. These methods will streamline your deployments and ensure that users receive the correct files promptly.
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