Absolute vs relative URLs
I would like to know the differences between these two types of URLs: relative URLs (for pictures, CSS files, JS files, etc.) and absolute URLs.
In addition, which one is better to use?
Should I use absolute or relative URLs?
If by absolute URLs you mean URLs including scheme (e.g. http / https) and the hostname (e.g. yourdomain.com) don't ever do that (for local resources) because it will be terrible to maintain and debug.
Let's say you have used absolute URL everywhere in your code like <img src="http://yourdomain.com/images/example.png">. Now what will happen when you are going to:
- switch to another scheme (e.g. http -> https)
- switch domain names (test.yourdomain.com -> yourdomain.com)
In the first example what will happen is that you will get warnings about unsafe content being requested on the page. Because all your URLs are hardcoded to use http(://yourdomain.com/images/example.png). And when running your pages over https the browser expects all resources to be loaded over https to prevent leaking of information.
In the second example when putting your site live from the test environment it would mean all resources are still pointing to your test domain instead of your live domain.
So to answer your question about whether to use absolute or relative URLs: always use relative URLs (for local resources).
What are the differences between the different URLs?
First lets have a look at the different types of urls that we can use:
http://yourdomain.com/images/example.png//yourdomain.com/images/example.png/images/example.pngimages/example.png
What resources do these URLs try to access on the server?
In the examples below I assume the website is running from the following location on the server /var/www/mywebsite.
http://yourdomain.com/images/example.png
The above (absolute) URL tries to access the resource /var/www/website/images/example.png. This type of URL is something you would always want to avoid for requesting resources from your own website for reason outlined above. However it does have its place. For example if you have a website http://yourdomain.com and you want to request a resource from an external domain over https you should use this. E.g. https://externalsite.com/path/to/image.png.
//yourdomain.com/images/example.png
This URL is relative based on the current scheme used and should almost always be used when including external resources (images, javascripts etc).
What this type of URL does is use the current scheme of the page it is on. This means that you are on the page http://yourdomain.com and on that page is an image tag <img src="//yourdomain.com/images/example.png"> the URL of the image would resolve in http://yourdomain.com/images/example.png.
When you would have been on the page http**s**://yourdomain.com and on that page is an image tag <img src="//yourdomain.com/images/example.png"> the URL of the image would resolve in https://yourdomain.com/images/example.png.
This prevent loading resources over https when it is not needed and automatically makes sure the resource is requested over https when it is needed.
The above URL resolves in the same manner on the server side as the previous URL:
The above (absolute) URL tries to access the resource
/var/www/website/images/example.png.
/images/example.png
For local resources this is the prefered way of referencing them. This is a relative URL based on the document root (/var/www/mywebsite) of your website. This means when you have <img src="/images/example.png"> it will always resolve to /var/www/mywebsite/images/example.png.
If at some point you decide to switch domain it will still work because it is relative.
images/example.png
This is also a relative URL although a bit different than the previous one. This URL is relative to the current path. What this means is that it will resolve to different paths depending on where you are in the site.
For example when you are on the page http://yourdomain.com and you use <img src="images/example.png"> it would resolve on the server to /var/www/mywebsite/images/example.png as expected, however when your are on the page http://yourdomain.com/some/path and you use the exact same image tag it suddenly will resolve to /var/www/mywebsite/some/path/images/example.png.
When to use what?
When requesting external resources you most likely want to use an URL relative to the scheme (unless you want to force a different scheme) and when dealing with local resources you want to use relative URLs based on the document root.
An example document:
<!DOCTYPE html>
<html>
<head>
<title>Example</title>
<link href='//fonts.googleapis.com/css?family=Lato:300italic,700italic,300,700' rel='stylesheet' type='text/css'>
<link href="/style/style.css" rel="stylesheet" type="text/css" media="screen"></style>
</head>
<body>
<img src="/images/some/localimage.png" alt="">
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js" ></script>
</body>
</html>
Some (kinda) duplicates
- Safe way to write URLs that transfer across environments
- what is the correct way to link image in website?
In general, it is considered best-practice to use relative URLs, so that your website will not be bound to the base URL of where it is currently deployed. For example, it will be able to work on localhost, as well as on your public domain, without modifications.
See this: http://en.wikipedia.org/wiki/URI_scheme#Generic_syntax
foo://username:[email protected]:8042/over/there/index.dtb;type=animal?name=ferret#nose
\ / \________________/\_________/ \__/ \___/ \_/ \_________/ \_________/ \__/
| | | | | | | | |
| userinfo hostname port | | parameter query fragment
| \_______________________________/ \_____________|____|____________/
scheme | | | |
| authority |path|
| | |
| path interpretable as filename
| ___________|____________ |
/ \ / \ |
urn:example:animal:ferret:nose interpretable as extension
An absolute URL includes the parts before the "path" part - in other words, it includes the scheme (the http in http://foo/bar/baz) and the hostname (the foo in http://foo/bar/baz) (and optionally port, userinfo and port).
Relative URLs start with a path.
Absolute URLs are, well, absolute: the location of the resource can be resolved looking only at the URL itself. A relative URL is in a sense incomplete: to resolve it, you need the scheme and hostname, and these are typically taken from the current context. For example, in a web page at
http://myhost/mypath/myresource1.html
you could put a link like so
<a href="pages/page1">click me</a>
In the href attribute of the link, a relative URLs used, and if it is clicked, it has to be resolved in order to follow it. In this case, the current context is
http://myhost/mypath/myresource1.html
so the schema, hostname, and leading path of these are taken and prepended to pages/page1, yielding
http://myhost/mypath/pages/page1
If the link would have been:
<a href="/pages/page1">click me</a>
(note the / appearing at the start of the URL) then it would have been resolved as
http://myhost/pages/page1
because the leading / indicates the root of the host.
In a webapplication, I would advise to use relative URLs for all resources that belong to your app. That way, if you change the location of the pages, everything will continue to work. Any external resources (could be pages completely outside your application, but also static content that you deliver through a content delivery network) should always be pointed to using absolute URLs: if you don't there simply is no way to locate them, because they reside on a different server.
Assume we are creating a subsite whose files are in the folder http://site.ru/shop.
1. Absolute URL
Link to home page
href="http://sites.ru/shop/"
Link to the product page
href="http://sites.ru/shop/t-shirts/t-shirt-life-is-good/"
2. Relative URL
Link from home page to product page
href="t-shirts/t-shirt-life-is-good/"
Link from product page to home page
href="../../"
Although relative URL look shorter than absolute one, but the absolute URLs are more preferable, since a link can be used unchanged on any page of site.
Intermediate cases
We have considered two extreme cases: "absolutely" absolute and "absolutely" relative URLs. But everything is relative in this world. This also applies to URLs. Every time you say about absolute URL, you should always specify relative to what.
3. Protocol-relative URL
Link to home page
href="//sites.ru/shop/"
Link to product page
href="//sites.ru/shop/t-shirts/t-shirt-life-is-good/"
Google recommends such URL. Now, however, it is generally considered that http:// and https:// are different sites.
4. Root-relative URL
I.e. relative to the root folder of the domain.
Link to home page
href="/shop/"
Link to product page
href="/shop/t-shirts/t-shirt-life-is-good/"
It is a good choice if all pages are within the same domain. When you move your site to another domain, you don't have to do a mass replacements of the domain name in the URLs.
5. Base-relative URL (home-page-relative)
The tag <base> specifies the base URL, which is automatically added to all relative links and anchors. The base tag does not affect absolute links. As a base URL we'll specify the home page: <base href="http://sites.ru/shop/">.
Link to home page
href=""
Link to product page
href="t-shirts/t-shirt-life-is-good/"
Now you can move your site not only to any domain, but in any subfolder. Just keep in mind that, although URLs look like relative, in fact they are absolute. Especially pay attention to anchors. To navigate within the current page we have to write href="t-shirts/t-shirt-life-is-good/#comments" not href="#comments". The latter will throw on home page.
Conclusion
For internal links I use base-relative URLs (5). For external links and newsletters I use absolute URLs (1).
There are really three types that should be discussed explicitly. In practice though URLs have been abstracted to be handled at a lower level and I would go as far as to say that developers could go through their entire lives without writing a single URL by hand.
Absolute
Absolute URLs tie your code to the protocol and domain. This can be overcome with dynamic URLs.
<a href=“https://dev.example.com/a.html?q=”>https://dev.example.com/a.html?q=</a>
Absolute Pros:
Control - The subdomain and protocol can be controlled. People that enter through an obscure subdomain will be funneled into the proper subdomain. You can hop back and forth between secure and non-secure as appropriate.
Configurable - Developers love things to be absolute. You can design neat algorithms when using absolute URLs. URLs can be made configurable so that a URL can be updated site-wide with a single change in a single configuration file.
Clairvoyance - You can search for the people scraping your site or maybe pick up some extra external links.
Root Relative
Root Relative URLs tie your code to the base url. This can be overcome with dynamic URLs and/or base tags.
<a href=“/index.php?q=”>.example.com/index.php?q=</a>
Root Relative Pros:
- Configurable - The base tag makes them relative to any root you choose making switching domains and implementing templates easy.
Relative
Relative URLs tie your code to the directory structure. There is no way to overcome this. Relative URLs are only useful in file systems for traversing directories or as a shortcut for a menial task.
<a href=“index.php?q=”>index.php?q=</a>
<link src=“../.././../css/default.css” />
Relative Cons:
CONFUSING - How many dots is that? how many folders is that? Where is the file? Why isn't it working?
MAINTENANCE - If a file is accidentally moved resources quit loading, links send the user to the wrong pages, form data might be sent to the incorrect page. If a file NEEDS to be moved all the resources that are going to quit loading and all the links that are going to be incorrect need to be updated.
DOES NOT SCALE - When webpages become more complex and views start getting reused across multiple pages the relative links will be relative to the file that they were included into. If you have a navigation snippet of HTML that is going to be on every page then relative will be relative to a lot of different places. The first thing people realize when they start creating a template is that they need a way to manage the URLs.
COMPUTED - They are implemented by your browser (hopefully according to RFC). See chapter 5 in RFC3986.
OOPS! - Errors or typos can result in spider traps.
The Evolution of Routes
Developers have stopped writing URLs in the sense being discussed here. All requests are for a website's index file and contain a query string, aka a route. The route can be thought of as a mini URL that tells your application the content to be generated.
<a href="<?=Route::url('named_url', array('first' => 'my', 'last' => 'whacky'))?>">
http://dev.example.com/index.php/my:whacky:url
</a>
Routes Pros:
- All the advantages of absolute urls.
- Use of any character in URL.
- More control (Good for SEO).
- Ability to algorithmically generate URLs. This allows the URLs to be configurable. Altering the URL is a single change in a single file.
- No need for 404 not founds. Fallback routes can display a site map or error page.
- Convenient security of indirect access to application files. Guard statements can make sure that everybody is arriving through the proper channels.
- Practicality in MVC approach.
My Take
Most people will make use of all three forms in their projects in some way or another. The key is to understand them and to choose the one best suited for the task.