RESTful URIs, Unicorns, and Pixie Dust

It often pains me to hear people talking about  so-called “RESTful URLs”. If you’re using that term, or your spending the majority of you application planning designing URI structures rather than your media types, then chances are you don’t really get the concepts in REST.

Frequently, I see developers sit down and start doodling a “REST API” by mapping out a bunch of URI templates like so:

/users/{userId}

/users/{userId}/stuff

/users/{userId}/stuff/{stuffId}

I’ll admit that I’ve been guilty of taking this approach myself. For one thing, it’s easy to communicate on paper. Most folks in business roles are used to seeing site maps where the content layout and URL structure are usually one in the same. By laying URI templates, you’ve kind of accomplished the same thing. Folks can visualize a high-level structure of your application, but you end up backing yourself into a corner that is difficult to get out of. Stu Charlton perhaps summed it up best in one of his more insightful posts:

If one is thinking of “how to methods map to my URI structures”, we’ve basically lost the plot as to why we have HTTP, URIs and media types in the first place. The goal is to *eliminate* the API as we normally think of it. (It’s not about a library of calls over a network).

The problem is that in approaching application design with the URI structure first is that you’re doing things bassackwards. Some people do this because they’ve followed some debatable advice and identified all of the “nouns” their application and started to work out a series of URL patterns that map to these nouns. As they create these URLs, they’ve followed some questionable advice as to what constitutes a “RESTful URL.” Subbu has another nice post dispelling some of those claims, so I won’t get into it here. The problem with doing all of the URI structure work up front is that you end up create a set of type resource URLs end up becoming fixed. Clients now end up coding to a specific set of URI patterns and/or conventions that are only discoverable from your documentation. The URI says nothing about what the data looks like or how the client should interpret it.

Imagine for a moment that you’re a DBA and you’re designing a set of database tables. Which of the two activities are you likely to spend more time on:

  1. The structure of your primary keys
  2. The schema of your database tables

A few of us would opt for something like MySQL’s auto-increment function and we’d be spending the majority of our time on describing the schema. In designing a RESTful application, you should be focusing on the design of your media types rather than what your URLs look like. To be more blunt: you must be focusing your efforts on what the hell your data looks like to consumers of your application. In addition to that, you need to think about how you are expressing links to other resources within your application or resources that are external to it.

This isn’t to say that URL design should be arbitrary and delegated to your web framework of choice. Of course not, you should still have URI strategy. The point is that the specific URI structures are not what consumers of your API will have to deal with directly. It is bad form to make a client rely on “out-of-band” information to construct a URI in your own special little way in order to get into your application. Take this blog post for example. You likely followed a link posted somewhere else. It could have been from an Atom feed, a search result in Google, or a Bit.ly link off of Twitter. It is highly unlikely that you had to type in the URL yourself and figured out the WordPress permalink structure that I have enabled on this site. If you did do that, well then, you’re awesome!

The fact is that clients will enter your application from some entry point or bookmark you’ve defined (.well-known is looking promising), or somewhere else on the web. Unless you’re a major player like Amazon, Facebook, or others, these clients won’t know that you have documentation that painfully detail your APIs URI structures. These clients will simply follow a link into your application. They didn’t type it in according to your fancy-schmancy URI template scheme that is only found in your documentation.  The URL is only a means to identify an locate a resource on the web, it does not define how the resource is represented or make suggestions as to what it’s about (remember that URIs are opaque?). At the end of the day, the client is going to have to be able to understand the media type that is retuned by requests made that URI. If you spend all of your time up front mapping methods to URI structures, you’ll end up introducing a coupling that you can’t easily break free from.

RESTful URIs, Unicorns, and Pixie Dust

URL vs. URI: URLs as Queries and URIs as Identities

Continuing my with my ranting about the URL vs. URI bit, I thought I’d continue on given my renewed interest in this topic thanks to Ora. In our LEDP position paper, we made the observation that URLs represent queries while URIs are identifiers. If you’re wondering why you should care about this subtle distinction, please read on.

URLs as Queries

We’ve stated that URLs are queries, but what does that really mean? Those of you familiar with blog software such as WordPress, know that the default URL pattern might go something like this:

http://www.damnhandy.com/?p=399

Here, the URL forms a query for a blog post using its internal identifier. In this case, the URL is asking the WordPress database for a post and related items using the primary key of a row that represents the post. For most, it’s pretty obvious that the query parameter “p” refers to the internal identity of the post.

As we mention in the paper, there are many other ways to construct URLs to the same post. For example, we can embed the ID into a path segment:

http://damnhandy.com/archives/399

In all of these cases, the server application is interpreting the URL and using elements of the URL to internally resolve the information the client requested. While I’ve only singled out WordPress, this pattern is quite common among several web application frameworks.

Internal Identity vs. Global Identity

When folks put information on the web, the content they publish usually has two identities:

  1. An Internal or local identity. This maybe the name of a file (i.e. “me.jpg”) or a the primary key of a row in a database.
  2. An external identity which is the ID of the information you’ve published. On the web, this is the “global identity” exposed by the URL of the content

Often, people don’t tend to think about either much. The global identity of a resource is usually an after thought and is determined by the underlying framework driving the application. With web servers serving up documents, we’re usually exposing the the local file name of the of the document. With database driven applictaions, we’re exposing the primary key, or some alternate key, of a row in a database. Quite often when web applictaion changes frameworks, we see the global identity change too and the URL patterns change (i.e. .NET to Java’s JSP, to Ruby, etc.).

Using the previous WordPress example, we know that the internal blog post ID is “399”, but this internal ID really isn’t suitable as a globally unique, unambiguous identifier. Another blog using WordPress, running the same exact version of the software, could also have a blog post ID of “399”. This does not means that the two sites have the same content, it only means that the two instances happen to have a post with an internal ID of “399”. As you might have noticed, the value “399” isn’t a suitable web-scale identifier. We need something else.

Globally Identifiers

Subbu Allamaraju had an interesting post a while back about Resource Identity and Cool URIs. Subbu asserts that there is a distintion between the identity of a resource and its location and his key point is spot on:

URIs uniquely identify resources but a URI used to fetch something is not always a good candidate to serve as a unique identifier in client applications.

And this is where I feel that the core confusion with URLs and URIs: identity vs. location. If we look at his initial example, he desribes the following:


  
      AZA12093
      
      ...
  
  
      ADK31242
      
      ...
  

In his example, the internal identity of each account is being expressed through a path segment in the href attribute of the link element. This approach is functional and is similar to that of the previous WordPress example.

The problem with this approach is that the ID values are only unique within the domain “bank.org”. There’s no reliable way to assert that two sites are referring to the same account if we have to rely on the value of a path segment or query parameter. As stated earlier, if we take WordPress as an exmaple again, blog post “399” might talk about the Kardashians, or something else. There’s no gurantee that the two URLs refer to the same information if they share the same internal identity. Most likely, they don’t.

We could make the link and the ID one in the same:


  
      http://bank.org/account/AZA12093
      
      ...
  
  
      http://bank.org/account/ADK31242
      
      ...
  

You might wonder what the hell is going on here since it looks pretty much the same as the first example. The difference is that we’re saying that the ID and the link are identical. That is, the identity of the account is the URI. That URI also happens to be a URL that can be dereferenced. This works, and it’s considered basic principle of Linked Data.

However, there are some problems with this approach too. As Subbu rightly points out, URIs are not always cool URIs. We are all aware that URIs do change at some point. If Bank.org is acquired by BiggerBank.com, what happens to the ID since we tied the ID to a host name that is likely to be retired soon?

One solution is to follow good web practices and maintain the bank.org domain and either redirect requests to the older URLs to the new ones. Adobe does this with links to FurtureSplash and Macromedia Flash locations. These URLs all resolve to the Adobe Flash product pages.

This strategy allows us to keep the original identity but the link is changed to accommodate the new domain. We can expand on this strategy and change the value of the link:


  
      http://bank.org/account/AZA12093
      
      ...
  
  
      http://bank.org/account/ADK312423
      
      ...
  

Yes, it looks ugly and weird, but it’s valid and it works. If you look closely, it’s not much different from the initial WordPress URL. The only difference is that we’ve replaced a numeric identifier for a URI. It’s a URI that references another URI, but it is valid. For some reason, people just don’t like URLs that look like this.DBPedia does this since they’re describing data on Wikipedia:

http://dbpedia.org/snorql/?describe=http%3A//en.wikipedia.org/wiki/BMW_7_Series_%2528E23%2529

The big difference with this approach is that it’s clear that the identifier is globally unique. There’s significantly less ambigutity about the ID: http://en.wikipedia.org/wiki/BMW_7_Series_(E23) than the ID: BMW_7_Series_(E23). Because no one else can mint valid URLs within the Wikipedia domain, you can have greater confidence that multiple applications are referring to the same thing. URIs as identifiers are globally unique.

The global identity of information resources shouldn’t change as frequently as it does. It drives my wife apeshit that all of her recipe bookmarks change everytime MarthaStewart.com updates their site. Part of the problem I believe is that most folks doing Information Architecture don’t take identity into consideration and that a fair number of web frameworks do very little to assist in quality URI/URL design. But this post is long enough, so I’ll save that for another post.

URL vs. URI: URLs as Queries and URIs as Identities

W3C Linked Enterprise Data Workshop and more about URLs vs. URIs

Last week, I had the pleasure of attending the W3C workshop on Linked Enterprise Data Patterns. My colleague Ora Lassila and I gave a presentation on the isues of identity in Linked Data. You can read our position paper and view our presentaion here:

It would seem that many share the same frustrations that Ora and I have with the confusion between URLs and URIs, and the idea that URLs are queries and URIs are identifiers. Readers of this blog will note that over the past few years I have been trying to clarify the difference between a URL and a URI. I think it’s safe to say that even after multiple, detailed posts on the topic, the distinction between the two remains murky for some. Hopefully we can start to clear things up.

W3C Linked Enterprise Data Workshop and more about URLs vs. URIs

Amazon Makes the Kindle Fire an Easy Target for a Thief

Having recieved a Kindle Fire this year, I was really surprised at the unpacking process and minimal security. While the device is very nice, the whole unpacking process made me realize that the Kindle Fire is a package thief’s dream. Here’s a few reason why:

Packaging
One thing that makes stealing a Kinlde Fire easy is the packaging, Amazon makes it quite clear what is in the box. When it arrives to your door, or your bulding’s mail room, the package is clearly marked with the words “Kindle Fire” all over it. In my case, the package sat in our building lobby clearly advertising what’s inside:

This makes it really easy to identify which package has the 7-inch tablet inside.

Automatic Sign-In
When I first opened the package and turned on the Kindle for the 1st time, it skipped past the sign-in screen since my wife’s credentials where already entered and she was automatically logged into Amazon. At first, this seemed like a nice touch for folks like my grandmother who don’t quite get sign-in procedures. However, this approach has far more cons than advantages.

1-Click Enabled by Default
The real kicker is that 1-Click is enabled by default. When you purchase content, you are never prompted for a password. This gives potential gift takers an added bonus: the ability to purchase Amazon content on your dime. And if you have kids, they can snag games as often as they want.

So if you’re considering getting an Amazon Kindle for someone this year, make sure you check the box that says “this is a gift” as this will make sure the device doesn’t sign you in automatically when you get it. Even if you’re going to get it for yourself, this is probably a good thing to do. Second, if you live in an apartment building or condo, you might want to consider shipping the box to your work or someplace where the box isn’t in plain sight.

 

Amazon Makes the Kindle Fire an Easy Target for a Thief

URL vs. URI vs. URN: The Confusion Continues

A year has passsed since my last post on URIs and URLs and it would seem that some of the concepts are still lost on some folks. With that said, I figured I’d throw up another post that I could try and address some of the questions raised in the comments of both posts.

URLs and URNs are both URIs

This is one point that can’t be stated enough. A URL is a URI and a URN is a URI, plain and simple. It’s really quite challenging to phrase it any other way. It’s like trying to explain that a Human is a Mammal but a Mammal is not always a Human.

Examples of URLs and URNs:

People have also suggested that these posts could have been more helpful if I had provided some examples that illustrate the difference between a URL and a URI. Based on the previous point, one should able conclude that a URL is a URI and therefore there’s no reasonable way to present an example that distingushes the two. However, we can provide examples that distingush a URL from a URN:

All of these Examples are URIs:
Examples of URLs: mailto:someone@example.com
http://www.damnhandy.com/
https://github.com/afs/TDB-BDB.git
file:///home/someuser/somefile.txt
Examples of URNs: urn:mpeg:mpeg7:schema:2001urn:isbn:0451450523
urn:sha1:YNCKHTQCWBTRNJIV4WNAE52SJUQCZO5C
urn:uuid:6e8bc430-9c3a-11d9-9669-0800200c9a66

Again, all of the examples above are all valid examples of URIs. You’ll note of course that all of the URNs are prefixed with “urn:”.
URIs are Opaque Identifiers

There’s a very informative page by Tim Berners-Lee that provides a lot of good deails on Uniform Resource Identifiers. One very important point is the notion of URI opacity, which states:

“The only thing you can use an identifier for is to refer to an object. When you are not dereferencing, you should not look at the contents of the URI string to gain other information.”

When you followed the link to this page, you didn’t have to do anything other than clicking it. Your browser only had to look at the URI scheme and the domain in order to resolve this particular document. Everything after “damnhandy.com” is defined to be opaque to the client. This point may seem orthogonal to the original post, but it’s a very important aspect of URIs. I bring this point up because the following question was asked in the comments:

Can we say that:

“http://www.domain.tld/somepath/file.php?mykey=somevalue”

is an URI

and that the “http://www.domain.tld/somepath/file.php” part is an URL?

No. No you cannot. Both are URLs, which are also URIs. The idea behind URI Opacity is that you should not look at the string to make any inferences as to what is at the other end. The presence of a query string does not distinguish a URL from a URI. Both strings are URIs and URLs.

Another commenter also asserted the following:

URI is whole address of a resource but resource extension is not mentioned. in URL we also mention the extension.
for example:

URI: www.abc/home
URL: www.abc/home.html

This is also incorrect. Being pedantic, neither is a syntactically correct URI or URL. But that aside, the presence of a file extention also does not distinguish a URI from a URL. Again, they are both valid URLs as well as URIs. Going back to URI opacity, you also cannot conclude that the two URLs reference the same resource or the same representation. The URI is an opaque string that identifies a resource. They’re still both URIs.

HTTP URIs Can Identify Non-Document Resources

This the one point that I think hits at the crux of everyone’s confusion on the matter. Things get all meta-meta when we start using URIs to identify things that are not documents on the web. It is perfectly acceptable to use to URIs to identify concepts, or non-document resources. If a URI happens to express scheme that suggests that it can be dereferenced, it is not a requirement that a document resource is actually dereferenced by that URI.

Most people’s expectation of HTTP URIs is that they it can always be dereferenced. There is a common assumption that an HTTP URI must point to a document. If it document resources is not available at that URI, that is you get a 404 error, then the URI is somehow bad. Just because the server could not locate document or a representation for the URI, does not mean that the HTTP URI is an invalid identifier.

The finer bits of this issue are summed up in httpRange-14 as well as this article by Xiaoshu Wang. The concepts around httpRange-14 are deep , but I think it’s these types of ideas that trips people up a lot. IMHO, it’s one of those concepts that gets muddled in the great internet telephone game that causes more confusion. For example, it seems that people are under the impression that if a URL does not express a file extension, then it represents a concept and therefore is a URI, but it just doesn’t work that way.

A URL is a URI and a URI can be used to represent anything. Furthermore, each URI is unique. This is why you cannot assume that http://www.abc/home is the same as http://www.abc/home.html just by looking at the URI. These are both distinct URIs that may or may not represent the same resource. Because URIs are opaque, you as the client should not be attempting to make any decisions about the URI.

URL vs. URI vs. URN: The Confusion Continues