Content Introduction Cache Control Headers Validation ETag Validation Request Validation Request outcomes Vary Request Collapsing Response Staleness Serve Stale, while revalidating Revalidate before reuse Deleting From The Cache Conclusion Introduction HTTP caching is a standard that many software tools follow and are built on. The tools include browsers, content delivery networks ( ), proxy caches, gateways, and many more. CDNs So, that means these tools either support it or are at least compatible with it because it is so widely used. This is why it is important to understand it because so many tools are built on this caching standard. After going through this guide, you should have a better understanding of how HTTP caching works! It’s going to save you hours of reading and researching! There are many details that you can learn about HTTP caching but of all of them, I think these 6 concepts would give you enough to build a solid foundation. It would be like learning the 80/20 of HTTP caching! Here are the 6 concepts: Cache-Control headers Validation Vary Request Collapsing Response Staleness Deleting from the cache Let’s dive right in! 1. Cache Control headers A cache control header provides a way for the server to give instructions to the client or the shared cache about the caching behavior for a particular resource. Here is an example: Cache-Control: max-age=3600 // 60 minutes is one of the most common directives to use. It controls caching which allows us to specify how long to cache a particular response. max-age This is not the only directive, there are several other types that you can use. Here are some of the more common types: For a full list of directives, check out . MDN - Cache Control You can also add several directives by separating them with a comma. 💡 Tip: Here is an example: Cache-Control: public, max-age=3600 2. Validation Of all the concepts, this is probably the most important. It has to do with validation. When a resource in a cache becomes stale, the cache does not immediately remove it. Rather it “validates” this resource with the server by making a request with the header and the value. If-None-Match ETag In a later section, we’ll also discuss the different strategies for handling stale responses. ETag ETags are typically a checksum (it can be a hash or a version number) generated by the server to check whether or not a resource has changed. Let’s see how this fits in with the validation request. Validation request To perform validation for a cached response, the client sends a request to the server with the appropriate details in the request including the header with the as the value. GET If-None-Match ETag The response to this request has two outcomes. The validation request outcomes There are only two outcomes from the validation request. - The server responds with a HTTP , the response remains the same Resource is not modified 304 Modified - The server responds with a HTTP , and returns the latest version of the response New resource is available 200 OK Now, what happens if we have variations of the resource but the same URL? How do we go about caching that? That’s where the header comes in. Vary 3. Vary When a response comes back from the server, it is typically cached based on its URL. However, there are times when the same URL can produce many variations (ie languages, and compression formats). Some examples include: - A different language or localization Accept-Language - a type of compression used for transferring between the client & origin server Content-Encoding So, what we can do in this scenario is to provide a header in the response from the server to alter the cache key. Vary With the header, you are not limited to just one header. Vary You can provide one or many of them in a comma-separated format (ie ), those values will be used to create the cache key for storing the cached response. Vary: Accept-Language,Content-Encoding 4. Request collapsing Another concept to understand in regard to HTTP caching is the idea of request collapsing. When there is a shared cache, and multiple clients are requesting the same resource, it would reduce the number of forwarded requests to the origin server by collapsing it. Let’s illustrate this. When a request comes in for a resource and assuming a response is not in the cache, the shared cache would forward the request to the origin server. During this time, let’s say more requests arrive for the same resource. In this case, the shared cache would not forward any more requests to the server, rather it would wait for the first request. Then when the request returns, it can use the same response to serve multiple clients who requested the same resource. The important thing to note here is that this only applies to responses that can be shared across the clients. ⚠️ Important: 5. Response Staleness When working with cached responses, it is also important to talk about how to handle situations when they become stale. There are two common directives available that can be used to manage responses that have expired. The strategy you choose to use will depend on the type of resource you are serving, and the experience you wish to provide. The Two options are: Serve stale, while revalidate Revalidate before reuse Let’s go through these options. 1. Serve stale, while revalidate When the response expires, this option would serve the stale response. Then it would revalidate with the server in the background. This means that some clients will temporarily get a stale response while the cache revalidates in the background. Just keep this in mind. In some cases, you may want to always serve the latest response. So, this may or may not work depending on your use case. 2. Revalidate before reuse When the response expires, this option will revalidate with the server before reusing a response. If the response has changed, the server will return with the new response, which will be served to the client. This means that the cache will revalidate immediately before serving a response to check if the current response is still up-to-date. After doing so, it will either serve the existing response (if it hasn’t changed) or the new response to the client. There is also which is identical to but it is used by shared caches. Cache-Control: proxy-revalidate must-revalidate What happens if the validation request fails? This is not directly related to stale responses but since we are discussing the validation request, it may be worth mentioning. Upon receiving an error response from the origin server, the default behavior of the shared cache varies. shared caches will try to serve the stale response (if applicable) — Please keep in mind that this is not always the case. Most If you want to support this behavior, you can use the directive. Cache-Control: stale-if-error=[time-in-seconds] If the shared cache receives an error response then serve the stale response for the defined duration. This translates to: Ok, we talked about managing stale responses but what about the cached responses that are no longer valid? 6. Deleting from the cache Sometimes you may want to remove a stored response from the cache. when working with a client or a shared cache, there are differences in how this is handled. Client caches (Browser Cache) When working with a client cache (browser cache), there isn’t a direct way to delete responses from the cache after caching it. That means you would have to wait until the cached response expires before it can be changed. There are proposals for a new HTTP header, , which can be set to clear the cache in the browser. Clear-Site-Data: cache However, this may not be supported on all browsers, so just keep that in mind! Shared caches When working with a shared cache, you typically have more control over the items in the cache (if it is a self-managed service). Most shared caches (ie CDNs, proxy caches, gateways) will provide an API to delete (or invalidate) the items in the cache. Ultimately, this means when storing responses in the client (browser cache), there will be less control. When devising a caching strategy, you should take that into consideration. Conclusion As I mentioned in the introduction, this guide is not meant to be a guide that covers everything. Rather it covers some of the major elements (The 6 core concepts) of HTTP caching that would help you to better understand how it works. Once you understand these elements, it should be just a matter of looking up the other details to fill in the gaps. Let’s do a recap. The Takeaways: - This is the HTTP header that controls the caching behaviour, and you do so via directives Cache-Control headers - When a cached response goes stale, validation is when the cache reaches out to the server to confirm whether or not this response is up-to-date ⭐️ Validation - When working with variations of the response from the same URL, you can adjust the cache key based on other properties by using the header Vary Vary - When multiple clients request the same resource, the shared cache will combine the forwarded request into one then return the same server response to all the clients (assuming this is a shared response) Request Collapsing - There are two common strategies for managing stale responses Response staleness - Serve the stale response while the cache revalidates in the background stale-while-revalidate (or for shared cache) - The cache must revalidate with server before serving the response must-revalidate proxy-revalidate - Keep in mind that if you are caching responses using browser cache, there isn’t an easy way to invalidate or delete the cached responses Deleting from the cache That’s it! I hope this guide was helpful. If you found this helpful or learned something new, please share this article with a friend or co-worker 🙏🧡 (Thanks!)

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