Storing Without Having to Make Http Request Again

Prevent unnecessary network requests with the HTTP Cache

Nov v, 2018 — Updated Apr 17, 2020

Fetching resources over the network is both wearisome and expensive:

• Large responses require many roundtrips between the browser and the server.
• Your page won't load until all of its critical resources have downloaded completely.
• If a person is accessing your site with a limited mobile data plan, every unnecessary network request is a waste material of their money.

How can you avoid unnecessary network requests? The browser's HTTP Cache is your outset line of defense. It's not necessarily the virtually powerful or flexible arroyo, and you have limited control over the lifetime of cached responses, but it'south effective, it'southward supported in all browsers, and it doesn't require much work.

This guide shows you the basics of an effective HTTP caching implementation.

Browser compatibility #

In that location isn't actually a single API chosen the HTTP Cache. Information technology's the general proper name for a drove of web platform APIs. Those APIs are supported in all browsers:

• Cache-Control
• ETag
• Concluding-Modified

How the HTTP Cache works #

All HTTP requests that the browser makes are first routed to the browser cache to check whether there is a valid cached response that tin can be used to fulfill the request. If there's a friction match, the response is read from the cache, which eliminates both the network latency and the data costs that the transfer incurs.

The HTTP Cache's behavior is controlled by a combination of request headers and response headers. In an ideal scenario, you'll accept control over both the lawmaking for your web application (which volition determine the request headers) and your web server's configuration (which will determine the response headers).

Cheque out MDN'southward HTTP Caching article for a more in-depth conceptual overview.

Request headers: stick with the defaults (usually) #

While there are a number of important headers that should be included in your spider web app'due south outgoing requests, the browser almost always takes care of setting them on your behalf when it makes requests. Request headers that touch checking for freshness, like If-None-Lucifer and If-Modified-Since only appear based on the browser'due south understanding of the electric current values in the HTTP Cache.

This is skillful news—it ways that yous can continue including tags like <img src="my-image.png"> in your HTML, and the browser automatically takes intendance of HTTP caching for you, without actress endeavour.

Response headers: configure your web server #

The part of the HTTP caching setup that matters the about is the headers that your web server adds to each outgoing response. The following headers all factor into effective caching behavior:

• Cache-Command. The server can render a Cache-Control directive to specify how, and for how long, the browser and other intermediate caches should cache the individual response.
• ETag. When the browser finds an expired cached response, it can send a pocket-sized token (ordinarily a hash of the file'southward contents) to the server to check if the file has changed. If the server returns the aforementioned token, then the file is the same, and at that place's no demand to re-download it.
• Final-Modified. This header serves the same purpose equally ETag, but uses a fourth dimension-based strategy to decide if a resource has changed, as opposed to the content-based strategy of ETag.

Some web servers have built-in support for setting those headers by default, while others exit the headers out entirely unless you explicitly configure them. The specific details of how to configure headers varies profoundly depending on which web server you use, and yous should consult your server'southward documentation to become the almost accurate details.

To salve you some searching, here are instructions on configuring a few popular spider web servers:

• Express
• Apache
• nginx
• Firebase Hosting
• Netlify

Leaving out the Cache-Control response header does not disable HTTP caching! Instead, browsers finer guess what type of caching behavior makes the most sense for a given blazon of content. Chances are you want more command than that offers, so accept the time to configure your response headers.

Which response header values should yous use? #

There are two important scenarios that you should comprehend when configuring your web server's response headers.

Long-lived caching for versioned URLs #

How versioned URLs tin assist your caching strategy

Versioned URLs are a good practice because they get in easier to invalidate cached responses.

Suppose your server instructs browsers to cache a CSS file for 1 year (Enshroud-Control: max-age=31536000) but your designer just made an emergency update that you need to scroll out immediately. How practise you notify browsers to update the "stale" buried copy of the file? You lot can't, at least not without changing the URL of the resource. After the browser caches the response, the buried version is used until information technology's no longer fresh, as determined by max-age or expires, or until it is evicted from the cache for some other reason—for example, the user clearing their browser cache. As a result, different users might stop up using dissimilar versions of the file when the page is constructed: users who just fetched the resource use the new version, while users who cached an earlier (merely still valid) copy use an older version of its response. How do you go the all-time of both worlds: client-side caching and quick updates? You modify the URL of the resource and force the user to download the new response whenever its content changes. Typically, you do this past embedding a fingerprint of the file, or a version number, in its filename—for case, style.x234dff.css.

When responding to requests for URLs that contain "fingerprint" or versioning information, and whose contents are never meant to change, add Enshroud-Command: max-age=31536000 to your responses.

Setting this value tells the browser that when it needs to load the aforementioned URL someday over the adjacent ane year (31,536,000 seconds; the maximum supported value), it tin immediately employ the value in the HTTP Cache, without having to make a network request to your web server at all. That's corking—you've immediately gained the reliability and speed that comes from avoiding the network!

Build tools similar webpack tin can automate the procedure of assigning hash fingerprints to your asset URLs.

Server revalidation for unversioned URLs #

Unfortunately, non all of the URLs you load are versioned. Maybe you're not able to include a build step prior to deploying your spider web app, so you tin't add together hashes to your asset URLs. And every web awarding needs HTML files—those files are (almost!) never going to include versioning information, since no 1 volition bother to use your web app if they demand to remember that the URL to visit is https://example.com/index.34def12.html. Then what can y'all practice for those URLs?

This is i scenario in which y'all need to acknowledge defeat. HTTP caching alone isn't powerful plenty to avoid the network completely. (Don't worry—yous'll soon larn well-nigh service workers, which will provide the support we need to swing the battle back in your favor.) Simply there are a few steps you can take to make sure that network requests are as quick and efficient as possible.

The following Enshroud-Control values tin can help you fine-tune where and how unversioned URLs are cached:

• no-cache. This instructs the browser that it must revalidate with the server every time before using a cached version of the URL.
• no-store. This instructs the browser and other intermediate caches (like CDNs) to never store whatsoever version of the file.
• private. Browsers can enshroud the file merely intermediate caches cannot.
• public. The response can be stored by whatsoever cache.

Check out Appendix: Cache-Control flowchart to visualize the process of deciding which Cache-Command value(due south) to use. Note also that Cache-Control tin can have a comma-separated list of directives. Run into Appendix: Cache-Control examples.

Along with that, setting one of 2 additional response headers can as well help: either ETag or Final-Modified. Equally mentioned in Response headers, ETag and Last-Modified both serve the aforementioned purpose: determining if the browser needs to re-download a cached file that has expired. ETag is the recommended arroyo considering information technology's more than accurate.

ETag example

Presume that 120 seconds have passed since the initial fetch and the browser has initiated a new request for the same resource. First, the browser checks the HTTP Cache and finds the previous response. Unfortunately, the browser can't use the previous response because the response has now expired. At this point, the browser could acceleration a new request and fetch the new full response. However, that's inefficient because if the resources hasn't changed, then there's no reason to download the same information that's already in the enshroud! That's the problem that validation tokens, equally specified in the ETag header, are designed to solve. The server generates and returns an arbitrary token, which is typically a hash or some other fingerprint of the contents of the file. The browser doesn't demand to know how the fingerprint is generated; it only needs to send it to the server on the side by side request. If the fingerprint is still the same, then the resource hasn't changed and the browser tin skip the download.

By setting ETag or Last-Modified, you'll end up making the revalidation request much more than efficient. They end upwards triggering the If-Modified-Since or If-None-Match request headers that were mentioned in Request headers.

When a properly configured web server sees those incoming request headers, it can confirm whether the version of the resource that the browser already has in its HTTP Cache matches the latest version on the spider web server. If at that place's a match, then the server can respond with a 304 Non Modified HTTP response, which is the equivalent of "Hey, keep using what you lot've already got!" There'due south very little data to transfer when sending this type of response, so it'southward usually much faster than having to actually send back a copy of the bodily resources existence requested.

The browser requests /file from the server and includes the If-None-Match header to instruct the server to only render the full file if the ETag of the file on the server doesn't match the browser's If-None-Match value. In this case, the 2 values did match, so the server returns a 304 Not Modified response with instructions on how much longer the file should exist cached (Enshroud-Control: max-historic period=120).

Summary #

The HTTP Enshroud is an effective way to meliorate load performance considering it reduces unnecessary network requests. It's supported in all browsers and doesn't take besides much work to set up.

The following Enshroud-Control configurations are a good start:

• Cache-Control: no-enshroud for resource that should be revalidated with the server before every use.
• Cache-Control: no-store for resources that should never be buried.
• Cache-Control: max-historic period=31536000 for versioned resource.

And the ETag or Last-Modified header tin can help yous revalidate expired cache resources more efficiently.

Learn more than #

If yous're looking to go beyond the basics of using the Cache-Control header, bank check out Jake Archibald's Caching best practices & max-age gotchas guide.

See Love your cache for guidance on how to optimize your enshroud usage for return visitors.

Appendix: More than tips #

If y'all take more time, here are farther ways that you tin optimize your usage of the HTTP Cache:

• Use consequent URLs. If you serve the same content on different URLs, and so that content will be fetched and stored multiple times.
• Minimize churn. If part of a resources (such as a CSS file) updates often, whereas the remainder of the file does not (such as library code), consider splitting the oftentimes updating code into a dissever file and using a brusk duration caching strategy for the ofttimes updating code and a long caching elapsing strategy for the code that doesn't alter oftentimes.
• Bank check out the new stale-while-revalidate directive if some degree of staleness is acceptable in your Cache-Control policy.

Appendix: Enshroud-Control flowchart #

Appendix: Cache-Command examples #

Cache-Control value	Caption
max-historic period=86400	The response can be buried by browsers and intermediary caches for up to ane day (60 seconds x lx minutes x 24 hours).
private, max-age=600	The response can be cached past the browser (merely non intermediary caches) for up to 10 minutes (60 seconds 10 ten minutes).
public, max-age=31536000	The response can be stored by any enshroud for i year.
no-store	The response is not allowed to be buried and must be fetched in total on every request.

Last updated: Apr 17, 2020 — Ameliorate article

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Source: https://web.dev/http-cache/

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