(To read this article with its formatting and typography intact, click here)Update, summer 2011: The Firefox team has decided to pursue these types of improvements via SPDY and HTTP Pipelining instead, but I will keep this original article about Resource Packages online.
What if there was a backwards compatible way to transfer all of the resources that are used on every single page in your site — CSS, JS, images, anything else — in a single HTTP request at the start of the first visit to the page? This is what Resource Package support in browsers will let you do.
Update: There's now a proper Resource Package specification, and any details there override whatever is written here. This article is a bit more conversational, and is useful to understand what we’re trying to do. If you’re looking for specifics — like what formats we support and how we parse — please refer to the formal specification instead of this article.
When it comes to browser performance, it’s widely known that a lot of the time is spent waiting for HTTP requests. You are probably familiar with the issue; a well-known optimization technique is to reduce the number of HTTP requests that are done for a given web site, since browsers only do 2–6 requests in parallel. This is why techniques like image spriting exist.
There are problems with image spriting, though. In addition to potentially severe memory penalties, they obfuscate the code — “What is this icon at 704px, exactly?” — and every time you add a new icon, you have to update the sprite file, which adds to the maintenance burden.
Some images can’t be sprited (think about YouTube, which easily serves up 40 JPEG thumbnails on a given page), and there’s also other resources like JavaScript & CSS, which — while possible to combine — at the very least need one file each. You can see how this quickly saturates the available parallel HTTP pipelines.
Even if bandwidth is getting better, and the internet is getting faster, latency are actually getting worse in a lot of cases. With mobile internet browsing, and to some extent US domestic cable internet and DSL, the round-trip time for a single request can be slow, even if it downloads relatively fast once the transfer starts.
While there are lots of workarounds to solve this class of problems, we suggest a standard approach that all browsers makers can easily implement, and that is backwards compatible with browsers that do not support it. We also want a solution that works for all types of resources, not only image bitmaps, and one that doesn’t require any new tools, file types or protocols.
This proposal has the following goals:
- Make it possible to serve all the resources (images, stylesheets, javascript) required by a page in a single HTTP request, freeing up the other parallel requests to fetch resources that are page-specific.
- Be as simple to implement as possible, so anyone with a passing familiarity with HTML should be able to perform the optimization.
- Be entirely transparent to browsers that do not support it.
- Avoid retransmission of existing resources.
- Use existing tools that are widely used on all platforms.
- Support the “80% use case” over adding a lot of complexity to the spec.
Some explicit non-goals of this proposal:
- Invent new file formats
- Invent new compression formats
Our goal is to be compression-format independent, but the obvious candidates for support are:
tar.gz, aka. .tgz files — MIME type: application/x-tar-gzzip files — MIME type: application/zip
While both zip and tar.gz files do not have not the most elegant or efficient packing format out there, they have the following very desirable traits:
- Easily available reference implementations.
- Can be unpacked even in partial state — which means that we can stream the file, and put CSS and JavaScript first in the archive, and they will unpacked and made available before the entire file has been downloaded.
- Excellent toolchain support, available on all major platforms, so it’s easy for web developers to use.
Other formats, like bzip may be more efficient in terms of file size, but they aren’t streamable and can’t be unpacked as partial files — so they are considered unfit for this particular purpose.
We propose this markup to signal a zipped resource package:
<link rel="resource-package"
type="application/zip"
href="site-resources.zip" />
The default MIME type for a resource package will be application/x-tar-gz, and you can omit it in documents where it is valid, like in HTML5, where an equivalent would be:
<link rel="resource-package"
href="site-resources.tar.gz" />
This will tell the browser to download this file first, and use the resources contained in the file instead of the referenced images, style sheets and javascript files — or for that matter, any other file. Browsers should prefer the files in the resource package, and do individual requests for images that are not contained in the package.
A given browser will probably block downloading any resources until the lists of files that are available in resource packages have been accounted for — or there may be a way to do opportunistic requests or similar, we leave this up to the browser vendor unless there’s a compelling reason to specify how this should work.
Older browsers that do not support resource packages will simply ignore this tag, and fetch the files normally, with one HTTP request for each.
Paths will be rendered relative to where the resource package is located, so you can supply additional directories inside the resource package to mimic existing site structure.
The resource package is referenced as follows in a page somewhere on the site www.example.com:
<link rel="resource-package"
type="application/zip"
href="/static/site-resources.zip" />
The Zip file has this internal structure:
javascript/jquery.js
css/reset.css
css/grid.css
css/main.css
images/save.png
images/info.png
In this example, the resolved path to the main.css file would be http://www.example.com/static/css/main.css. Notice how the path inside the resource package is added to the path where the actual file is located.
The most efficient way to declare what’s in the resource package without blocking to wait for the first part of it to load, is to do it inline in the resource package <link> tag itself:
<link rel="resource-package"
type="application/zip"
href="/static/site-resources.zip"
content="javascript/jquery.js;
css/reset.css;
css/grid.css;
css/main.css;
images/save.png;
images/info.png" />
The only problem with the above is that it will break validation for existing specifications like HTML 4 and XHTML 1.x, so we also support an alternate syntax, (ab)using the title attribute to get the same result:
<link rel="resource-package"
type="application/zip"
href="/static/site-resources.zip"
title="javascript/jquery.js;
css/reset.css;
css/grid.css;
css/main.css;
images/save.png;
images/info.png" />
This makes the file work with the existing validators and older standards.
There should be no compatibility issues with old browsers, as they will just load the individual files instead of the resource package.
Browsers that don’t implement this will seem slow in comparison to other browsers. Luckily, it should be a simple addition to any of the modern browsers.
You can also give the browser the ability to know what files are in the resource package file without reading the entire file first, by adding a manifest file that can contain this information. This file can be supplied as a separate file (useful if combining with Offline Resources), or as the first file in the file itself. Of course, this will be slower to get than defining the contents inline in the HTML, but can be easier and cleaner to implement, especially if you're using offline resource support already.
Example manifest.txt file:
javascript/jquery.js
styles/reset.css
styles/grid.css
styles/main.css
images/save.png
images/info.png
- This file must be the first file in the archive.
- This file must be named
manifest.txt when supplied as part of the archive.
If this simple format looks familiar, that’s not a coincidence. Initially, we were looking at using either an XML or JSON format to specify this, but we believe it’s easier to add a couple of new abilities to the offline resource specification instead. When using resource packages with Offline Resources (which are also part of the HTML5 spec), we’d like it to be easy to extend the rules, so the offline manifest with resource package support could look like this:
CACHE: resources=/static/siteresources.zip
javascript/jquery.js
styles/reset.css
styles/grid.css
styles/main.css
images/save.png
images/info.png
# The above section lists the files in siteresources.zip.
# To start a new section, do:
CACHE:
images/outside-package.png
The only thing we’d need to add to the HTML5 offline spec is that it should ignore anything on the same line after CACHE: if it doesn’t know how to handle it. This means that you could potentially put the resource package definitions in your Offline Resources manifest — we would also support doing it the other way around, and put the Offline Resources manifest inside the resource package.
This isn’t a requirement for implementing the initial version of resource packages, however — but could be an easy way to add support for it to the offline resource specification. If there’s a better way to do it, let me know.
If a resource is defined twice on the same path — e.g. using multiple resource-packages — the file defined last takes priority. This enables an application with offline mode to synchronize changes without downloading the entire resource package again.
For example, on the first sync, the browser sees a resource package like this:
<link rel="resource-package" href="/documents/offline-0.zip" />
offline-0.zip contains:
manifest.txt
doc1.html
doc2.html
Offline the application would be able to access documents at
/documents/doc1.html and /documents/doc2.html.
The user then performs another sync and the browser now sees:
<link rel="resource-package" href="/documents/offline-0.zip" />
<link rel="resource-package" href="/documents/offline-1.zip" />
offline-1.zip contains:
manifest.txt
doc1.html
doc3.html
doc1.html and doc3.html would now be read from the offline-1.zip, doc2.html
would continue to be read from offline-0.zip.
Even for use outside of the offline applications space, it allows pages to easily override the site look and feel with section-specific images and styling. A page could serve up:
<link rel="resource-package" href="/static/main-theme.zip" />
<link rel="resource-package" href="/static/section-theme.zip" />
Resources in section-theme.zip would take precedence over the content in main-theme.zip — making it easy to do overrides without replacing the entire resource package.
It’s not hard to see where Resource Packages could be useful in existing sites, two obvious categories would be:
- Supply the core layout & functionality of a site
- Typically, you would ship over all the CSS, JS and images that are used on every page in the site. These could be cached quite aggressively, and even use ETags to invalidate the resource package when needed.
- The thumbnail search result case
- Consider a typical YouTube search results page. It contains 20-40 thumbnails of videos, and there’s no easy way to add all these images into an image sprite, since the long tail of search results would vary a lot. Resource Packages would let you build a zip of search result thumbnails on the fly, and ship them all over in one HTTP request. It would require some CPU power, but would be much faster for the end-user. This wouldn’t have to be cached, or could be cached on a per-search basis.
There are several other approaches that could solve parts of this problem, but they all have issues with current browsers and graceful degradation and/or are trying to solve a slightly different problem:
- HTTP pipelining
- This is a more aggressive way of utilizing the HTTP keep-alive mode, but is not implemented correctly by all web servers. Proxies have a hard time with it, some browsers also do, so it’s not really working unless you want to be aggressive and/or whitelist/blacklist certain servers.
- It also causes a “head-of-line blocking” issue, where if you request X and Y
on the same connection and X is slow, then Y is slowed too.
- Multipart MIME
- Hard for integrators, requires special packing that isn’t trivial to do, and has poor browser support.
- JAR files or anything using data: URLs
- No reasonable fallback mode, as the file name is embedded in the href/src link, and browsers that doesn’t support it just won’t render it.
- SPDY
- While this effort from Google aims to make everything faster, it is largely orthogonal to what we’re trying to do with Resource Packages. It also requires you to retrofit both web browsers and web servers to make it work, which means it will take quite a while before this will be in common use. Resource Packages work without any changes to the web server software, and will work as soon as any browser supports it — with no adverse effects to the browsers that don’t.
- The Zip format doesn’t have MIME type support, so this will have to be solved by the browser based on filename extensions or other heuristics. We don’t believe this to be a problem, since browsers already have to do this.
- All the resources in the package will have the same headers (expiry, ETags, etc.) as the resource package itself. If you need different expiry dates or other caching settings, you should specify multiple resource files with different cache headers.
- You can specify a charset in the resource package definition. If unspecified, it is assumed that any non-binary files inside are UTF-8.
We have sent this out to the major browser vendors for feedback, and we will be implementing this in the next upcoming release of Firefox — which tentatively has the version number 3.7, but this may change.
- Does zipping up multiple optimized PNGs or other files work with zip? Can it potentially increase file size or lead to a high unpacking CPU overhead?
- Zip automatically chooses the best of deflation or no compression. Images will usually not be compressed, since they already are, text files like CSS/JS will be. In general, CPU impact from unzipping is negligible, even on slow devices.
- How does this affect mobile devices, which have limited CPUs?
- More realistic concerns are cache ability and bandwidth — as well as the latency on mobile networks — and memory. A lot of mobile browsers only keeps things in the browser cache at all if the individual file is something like 20kB or less. For returning visitors, you suddenly need to download one large file again, instead of having multiple small files locally.
- In general, mobile browsers clear their caches quite aggressively — although with the resource package spec, one would hope that they would implement more optimal handling and prioritize caching these, since they more likely to be valuable for browsing performance than another random image/CSS/JS file in a site.
- How would Resource Packages work with CDNs?
- There would be no special handling, these mirrors would just carry the resource package file like any other file they are supplying.
- How would you manage priority in the resource package? It would be useful to decide which files get downloaded first.
- The priority is managed by the order they are added to the resource package. If you want a specific order, it’s trivial to specify this on the command line, so we aren’t adding any special syntax for this. Also, we have to do it this way to take advantages of the ability to unpack and display resources while the file is still downloading.
- I worry about reduced parallelism. Lots of sites make
heavy use of resource sharding across many hostnames to take advantage
of multiple connections. Won’t this be a problem?
- Sites usually use multiple hostnames to get around per-host connection limits, which are almost entirely a latency issue — not a bandwidth issue. Resource packages make multiple hostnames unnecessary, because it solves the latency issue a different way.
If you have any suggestions on how to improve this proposal, send me an email at limi@mozilla.com or even better, comment in the open thread over at Mozilla’s dev.platform forum. It has been filed as bug #529208 in Bugzilla for those of you that want to monitor its progress.
Proposal State: Ready for prototype implementation
Revision 7: Feb 22nd, 2010 — Added new information on compression formats, both zip and tar.gz files are now part of the spec.
Revision 6: Feb 12th, 2010 — Added a defined behavior for what happens when resources are defined twice.
Revision 5: January 10th, 2010 — added inline definition of resource package content in a manner that is compatible with HTML4/XHTML validators.
Revision 4: Nov 16th, 2009 — first published & widely circulated version, added Offline Resources support
Revision 3: Nov 10th, 2009
Revision 2: Sep 1st, 2009
Revision 1: Jun 15th, 2009
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App tabs: If you use web apps like Pandora, Gmail, Twitter, Facebook or any other site that you keep open all day, you’ll love our new app tabs. With a simple right click on any tab, you can pin it to the left side of tabs, which makes it always available, and takes up less space. It will also notify you with a discreet light bulb when there’s new content in the app tab.
Install add-ons without restarting: Tired of having to restart when you install a new add-on? Firefox 4 adds the capability for add-ons to avoid this restart, and as they are updated to make use of this, you shouldn’t have to restart your browser much anymore.
— this shows that the download is 50% complete. I believe we are in the process of landing something similar on Windows Vista/7 in the nightly Windows builds of Firefox too.
We’d like to introduce indicators in the cursor/pointer to give you a bit of warning before you click things that aren’t actually web pages. Showing a small envelope icon for 
