Prevent unnecessary network requests with the HTTP Cache
Fetching resources over the network is both slow 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 of their money.
How can you avoid unnecessary network requests? The browser's HTTP Cache is your first line of defense. It's not necessarily the most powerful or flexible approach, and you have limited control over the lifetime of cached responses, but it's effective, it's 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
There isn't actually a single API called the HTTP Cache. It's the general name for a collection of web platform APIs. Those APIs are supported in all browsers:
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 can be used to fulfill the request. If there's a 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 have control over both the code for your web application (which will determine the request headers) and your web server's configuration (which will determine the response headers).
Check out MDN's 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
web app's outgoing requests, the browser almost always takes care of setting
them on your behalf when it makes requests. Request headers that affect checking
for freshness, like If-None-Match and
If-Modified-Since just appear based on the browser's
understanding of the current values in the HTTP Cache.
This is good news—it means that you can continue including tags like <img src="my-image.png"> in your HTML, and the browser automatically takes care of
HTTP caching for you, without extra effort.
Developers who do need more control over the HTTP Cache in their web application
have an alternative—you can "drop down" a level, and manually use the Fetch
API, passing it
Request objects
with specific
cache
overrides set. That's beyond the scope of this guide, though!
Response headers: configure your web server
The part of the HTTP caching setup that matters the most is the headers that your web server adds to each outgoing response. The following headers all factor into effective caching behavior:
Cache-Control. The server can return aCache-Controldirective 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 small token (usually a hash of the file's contents) to the server to check if the file has changed. If the server returns the same token, then the file is the same, and there's no need to re-download it.Last-Modified. This header serves the same purpose asETag, but uses a time-based strategy to determine if a resource has changed, as opposed to the content-based strategy ofETag.
Some web servers have built-in support for setting those headers by default, while others leave the headers out entirely unless you explicitly configure them. The specific details of how to configure headers varies greatly depending on which web server you use, and you should consult your server's documentation to get the most accurate details.
To save you some searching, here are instructions on configuring a few popular web servers:
Leaving out the Cache-Control response header does not disable HTTP caching!
Instead, browsers effectively
guess
what type of caching behavior makes the most sense for a given type of content.
Chances are you want more control than that offers, so take the time to
configure your response headers.
Which response header values should you use?
There are two important scenarios that you should cover when configuring your web server's response headers.
Long-lived caching for versioned URLs
How versioned URLs can help your caching strategy
Versioned URLs are a good practice because they make it easier to invalidate cached responses.
Suppose your server instructs browsers to cache a CSS file
for 1 year (Cache-Control: max-age=31536000) but your designer just made an
emergency update that you need to roll out immediately. How do you notify browsers
to update the "stale" cached copy of the file?
You can't, at least not without changing the URL of the resource. After the
browser caches the response, the cached version is used until it'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 end up using different 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 (but still valid) copy use an older
version of its response. How do you get the best of both worlds: client-side
caching and quick updates? You change the URL of the resource and force the user
to download the new response whenever its content changes. Typically, you do
this by embedding a fingerprint of the file, or a version number, in its
filename—for example, style.x234dff.css.
When responding to requests for URLs that contain
"fingerprint" or
versioning information, and whose contents are never meant to change, add
Cache-Control: max-age=31536000 to your responses.
Setting this value tells the browser that when it needs to load the same URL anytime over the next one year (31,536,000 seconds; the maximum supported value), it can immediately use the value in the HTTP Cache, without having to make a network request to your web server at all. That's great—you've immediately gained the reliability and speed that comes from avoiding the network!
Build tools like webpack can automate the process of assigning hash fingerprints to your asset URLs.
You can also add the immutable
property
to your Cache-Control header as a further optimization, though it will be
ignored in
some browsers.
Server revalidation for unversioned URLs
Unfortunately, not all of the URLs you load are versioned. Maybe you're not able
to include a build step prior to deploying your web app, so you can't add hashes
to your asset URLs. And every web application needs HTML files—those files are
(almost!) never going to include versioning information, since no one will
bother to use your web app if they need to remember that the URL to visit is
https://example.com/index.34def12.html. So what can you do for those URLs?
This is one scenario in which you need to admit defeat. HTTP caching alone isn't powerful enough to avoid the network completely. (Don't worry—you'll soon learn about service workers, which will provide the support we need to swing the battle back in your favor.) But there are a few steps you can take to make sure that network requests are as quick and efficient as possible.
The following Cache-Control values 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 any version of the file.private. Browsers can cache the file but intermediate caches cannot.public. The response can be stored by any cache.
Check out Appendix: Cache-Control flowchart to visualize the process
of deciding which Cache-Control value(s) to use. Note also that Cache-Control can
accept a comma-separated list of directives. See Appendix: Cache-Control examples.
Along with that, setting one of two additional response headers can also help:
either ETag or Last-Modified. As mentioned in
Response headers, ETag and Last-Modified both serve the
same purpose: determining if the browser needs to re-download a cached file
that has expired. ETag is the recommended approach because it's more accurate.
ETag example
Assume 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 dispatch a new request and fetch the new full
response. However, that's inefficient because if the resource hasn't changed,
then there's no reason to download the same information that's already in the
cache! That's the problem that validation tokens, as 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 need to know how the fingerprint is generated; it
only needs to send it to the server on the next request. If the fingerprint is
still the same, then the resource hasn't changed and the browser can skip the
download.
By setting ETag or Last-Modified, you'll end up making the
revalidation request much more efficient. They end up 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 web server. If there's a match,
then the server can respond with a 304 Not Modified HTTP response,
which is the equivalent of "Hey, keep using what you've already got!" There's
very little data to transfer when sending this type of response, so it's usually
much faster than having to actually send back a copy of the actual resource
being requested.
/file from the server and includes the If-None-Match
header to instruct the server to only return 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 be cached (Cache-Control: max-age=120).
Summary
The HTTP Cache is an effective way to improve load performance because it reduces unnecessary network requests. It's supported in all browsers and doesn't take too much work to set up.
The following Cache-Control configurations are a good start:
Cache-Control: no-cachefor resources that should be revalidated with the server before every use.Cache-Control: no-storefor resources that should never be cached.Cache-Control: max-age=31536000for versioned resources.
And the ETag or Last-Modified header can help you revalidate expired cache resources more efficiently.
Try it!
Try the HTTP Cache codelab to get hands-on experience with Cache-Control
and ETag in Express.
Learn more
If you're looking to go beyond the basics of using the Cache-Control header,
check out Jake Archibald's Caching best practices & max-age
gotchas guide.
Appendix: More tips
If you have more time, here are further ways that you can optimize your usage of the HTTP Cache:
- Use consistent URLs. If you serve the same content on different URLs, then that content will be fetched and stored multiple times.
- Minimize churn. If part of a resource (such as a CSS file) updates frequently, whereas the rest of the file does not (such as library code), consider splitting the frequently updating code into a separate file and using a short duration caching strategy for the frequently updating code and a long caching duration strategy for the code that doesn't change often.
- Check out the new
stale-while-revalidatedirective if some degree of staleness is acceptable in yourCache-Controlpolicy.
Appendix: Cache-Control flowchart
Appendix: Cache-Control examples
Cache-Control value |
Explanation |
|---|---|
max-age=86400 |
The response can be cached by browsers and intermediary caches for up to 1 day (60 seconds x 60 minutes x 24 hours). |
private, max-age=600 |
The response can be cached by the browser (but not intermediary caches) for up to 10 minutes (60 seconds x 10 minutes). |
public, max-age=31536000 |
The response can be stored by any cache for 1 year. |
no-store |
The response is not allowed to be cached and must be fetched in full on every request. |