Restarting Linked Data from scratch, part 2

Friday 12 Mar 2010 by nathan | No Comments
Filed under: linked data Tags: , , , , , , , , , , , , , , , , , , , , , ,

This post is part of a series, following on from my earlier post Restarting Linked Data from scratch, part 1. In this post I'm going to take the first step by trying to approach publishing and exposing linked data RESTfully.

I'm assuming that if you are reading this, you know what linked data is, and REST as per the dissertation of Roy T. Fielding. If not go do some reading :)

Interface Constraints

REST is defined by four interface constraints:

  1. identification of resources
  2. manipulation of resources through representations
  3. self-descriptive messages
  4. hypermedia as the engine of application state.

From here I'll look at each of these four constraints and build up the approach as I go.

What a resource is

Quoting extensively from REST 5.2.1.1 Resources and Resource Identifiers:

The key abstraction of information in REST is a resource. Any information that can be named can be a resource: a document or image, a temporal service (e.g. "today's weather in Los Angeles"), a collection of other resources, a non-virtual object (e.g. a person), and so on. In other words, any concept that might be the target of an author's hypertext reference must fit within the definition of a resource...

A resource is a conceptual mapping to a set of entities, not the entity that corresponds to the mapping at any particular point in time...

The values in the set may be resource representations and/or resource identifiers...

A resource can map to the empty set, which allows references to be made to a concept before any realization of that concept exists...

The only thing that is required to be static for a resource is the semantics of the mapping, since the semantics is what distinguishes one resource from another...

What a representation is

Again, quoting extensively from REST 5.2.1.2 Representations:

... A representation is a sequence of bytes, plus representation metadata to describe those bytes. Other commonly used but less precise names for a representation include: document, file, and HTTP message entity, instance, or variant...

If the value set of a resource at a given time consists of multiple representations, content negotiation may be used to select the best representation for inclusion in a given message...

The data format of a representation is known as a media type

Identification of resources

To do this properly I need to identify some resources, so for this I'm going to work with "Something" :)

  • "Something" - a resource, a non-virtual object

At any point in time I have a description of Something which has multiple representations in different mediatypes, all semantically matching or equivalent:

  • "something.rdf" - representation of Something with mediatype RDF+XML
  • "something.n3" - representation of Something with mediatype RDF+N3
  • "something.en.html" - representation of Something, in english, with mediatype text/html
  • "something.de.html" representation of Something, in german, with mediatype text/html

Each one of those representations is also a resource because they can be the target of a hyperlink. Of course by resource I mean a conceptual mapping to each of the things listed, and I haven't assigned URIs but will..

To be able to make this set of representations manageable and to indicate they are in a set, I'm going to add in another resource which is a collection of resources, which can be considered a set of these equivalent representations of Something at a fixed point in time. For the purpose of this exercise, that point in time is today.

  • "Something-20100311" - a resource which is a collection of equivalent representations of Something on the 11th March 2010.

Additionally, for the sake of argument, I'm going to say that a new set of representations (or version) is added every day - to handle this I then need one more resource, a collection of resources, where each resource in the collection is itself a collection of resources (one of the aforementioned and including the example "Something-20100311"). This will give me a conceptual mapping which covers time, and therefore everything I could need.

  • "Somethings" - a resource which is a collection of resources, see above for full description!

Finally, I'm going to add in two shortcut resources which have no representation and are simply conceptual maps to the first and most current sets of representations.

  • "first" - a resource which always maps to the first collection of representations of Something.
  • "latest" - a resource which maps to the most recent collection of representations of Something.

Giving the resources URIs

Now to assign some URIs for this use case, there is no set structure and I'm not going to define one because it is up to each server (or manager of) to control it's own URI space, but for the sake of this exercise I'll define mine as follows:


base: http://data.webr3.org
...
/d/Something
/rg/Somethings
/rg/Somethings/first
/rg/Somethings/latest
/rg/Somethings/Something-20100311
/rg/Somethings/Something-20100311/something.rdf
/rg/Somethings/Something-20100311/something.n3
/rg/Somethings/Something-20100311/something.en.html
/rg/Somethings/Something-20100311/something.de.html
...
/rg/Somethings/Something-20100305
/rg/Somethings/Something-20100305/something.rdf
/rg/Somethings/Something-20100305/something.n3
/rg/Somethings/Something-20100305/something.en.html
/rg/Somethings/Something-20100305/something.de.html
...

From the above you can see that every possible representation has its own URI, in addition every collection of equivalent representations has its own URI, as does the collection of all those collections; and so does "Something" our non virtual object.

Also we've exposed multiple resources which could also be RESTful CRUD access points operating on an atompub style protocol. Small sentence, big potential, will cover approaches and protocols in later posts.

The Key resource

The most important thing, which I haven't yet covered, is that we've exposed a key resource, namely /rg/Somethings. This is a resource at the top of the representation chain which can be used to expose content negotiation, be it server or agent driven (or a mix of both), and regardless of the mappings and levels of collection further down the line this can always be a single point of entry to get representations.

I'll cover just how in a moment, but for now something important.

Important

I've had to give a fixed example just to make some progress, but we have to remember that every system has different needs, in some cases it may be that there is only a single fixed representation for a resource, whilst in others each strand of representation (like something.de.html) may take it's own versioning / temporal path. This could indicate that a structure such as the following may be in order:

...
/d/Something
/rg/Somethings
/rg/Somethings/first
/rg/Somethings/latest
/rg/Somethings/Something-20100311
/rg/Somethings/Something-20100305
/rg/Somethings/Something-rdf
/rg/Somethings/Something-rdf/20100311.rdf
/rg/Somethings/Something-rdf/20100305.rdf
/rg/Somethings/Something-html-en
/rg/Somethings/Something-html-en/20100311.html
/rg/Somethings/Something-html-en/20100305.html
/rg/Somethings/Something-html-de
/rg/Somethings/Something-html-de/20100308.html
/rg/Somethings/Something-html-de/20100303.html

The above highlights that whilst we may have added more resources, the core resources are still the same; remember that they are "conceptual maps", meaning that Something-20100311 may "map" to the version of en-html on the 11th and de-html on the 8th, because the de version was written first, then translated to english and from there rdf and so forth, but they are all semantically equivalent, containing the same information even though they were created at different times.

The Conceptual Maps are as follows, from what I can tell this should always cover any use-case, no matter how complex.


Thing 1-1 CollectionOfCollections
CollectionOfCollections 1-* CollectionOfEquivalentRepresentations
CollectionOfEquivalentRepresentations 1-* Representation

aside:At times like this I wish I'd had a chance to study computer science so that I could express these things formally, so you'll have to make sense of it as best you can :( sorry.

Exposing via Content Negotiation

In my research so far, I've been able to figure out how to expose all of the aforementioned via HTTP, RESTfully using content negotiation in a manner which seems to be transparent to existing web browsers, but exposes all the information needed in a manner that is visible to machines; without using any additional extensions headers. As follows:

1 The client does a normal GET request on our "Something", notice that no content negotiation is happening yet, we are simply asserting via a 303 "that the requested resource does not have a representation of its own that can be transferred by the server over HTTP."

#Request
GET /d/Something HTTP/1.1
Host: data.webr3.org
Accept: text/html;q=0.5, application/rdf+xml


#Response
HTTP/1.1 303 See Other
Location: http://data.webr3.org/rg/Somethings

2The client does a GET on the URI we specified in the Location field, namely to our key resource that can be used for content negotiation over all the representations.

#Request
GET /rg/Somethings HTTP/1.1
Host: data.webr3.org
Accept: text/html;q=0.5, application/rdf+xml


#Response
HTTP/1.1 300 Multiple Choices
Location: http://data.webr3.org/rg/Somethings/latest
Content-Type: application/xhtml+xml
Content-Length: 17400


<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.0//EN"
"http://www.w3.org/MarkUp/DTD/xhtml-rdfa-1.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:foaf="http://xmlns.com/foaf/0.1/"
xmlns:dc="http://purl.org/dc/elements/1.1/"
version="XHTML+RDFa 1.0" xml:lang="en">
...

Here's where it gets interesting and clients can take different routes; first the route of the typical user agent:

User Agent Route


#Request
GET /rg/Somethings/latest HTTP/1.1
Host: data.webr3.org
Accept: text/html;q=0.5, application/rdf+xml


#Response
HTTP/1.1 307 Temporary Redirect
Location: http://data.webr3.org/rg/Somethings/Something-20100311


#Request
GET /rg/Somethings/Something-20100311 HTTP/1.1
Host: data.webr3.org
Accept: text/html;q=0.5, application/rdf+xml


#Response
HTTP/1.1 302 Found
Vary: Accept
ETag: W/"xyzzy"
Last-Modified: Wed, 11 Mar 2010 12:45:26 GMT
Content-Type: application/xhtml+xml
Content-Length: 17400
Content-Language: en
Content-Location: http://data.webr3.org/rg/Somethings/Something-20100311/something.en.html


<!DOCTYPE html...

First you can see that the user agent simply goes straight through to the most recent content and what they expect to see; which is nice, with additional Server driven content negotiation.

Further, we can see that full cache control is in there which as we know speeds up the net, and further still we have a rather nifty "weak" entity tag; this entity tag is shared by all representations which are semantically equal, and asserts they are equal via the entity tag. It's also worth noting that you could add this entity tag to your RDF graphs and further assert provenance which could come in very handy down the line for POST and PUT implementations.

To recap, common user agents just go straight through to the expected resource via server driven content negotiation and can take full advantage of cache / control data.

The Machine Route

Back at 2 the server returned a 300 Multiple Choices as soon as /rg/Somethings was requested. All important was that the entity returned was XHTML+RDFa (although this could have been Atom or similar..), which means we can give both a human and machine readable list of all our various representations, the "machine" can then select which one it finds most fitting. The choices could be expressed using any suitable ontology; and further both Alternative and Link headers could be added if publishers wished.

I think that covers it all, if there are any errors or things I've missed please do let me know asap; but for now that'll do me - it's verbose, but I like verbose - prove it works then optimise it later :)


Restarting Linked Data from scratch, part 1

Thursday 11 Mar 2010 by nathan | No Comments
Filed under: linked data Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

I'm going out on a limb and starting my whole journey through Linked Data and "Web 3.0" again - in order to address the challenges many in the community are facing, and which are "blocking" me. I'm going to take everything I've learned so far and go right back to grass roots with linked data.

I'm primarily documenting this journey for my own benefit, for reference and to unload it from my brain; but hopefully it'll be of use to the wider community and any feedback will be massively appreciated.

Here goes, I'll start by analysing the web thus far:

The Web till now

The power and the success of the web so far, in my opinion, has come from four main things:

  • The URI
  • Hyperlink
  • The Resource
  • HTTP and it's RESTful design.

More importantly it's the combination of the four working together that makes the web so great, because, they let you cut out everything else and go straight to the resource you want. This is a point that we need to concentrate on for a minute.

Going straight to the resource

Back at the start of the web, this allowed people to (for the first time) jump from a resource in the bowels of one companies hierarchy straight to another resource in a different companies hierarchy - very much like reading a book, looking at the references and suddenly having the book or paper it references right there in your lap - amazing to say the least.

Skip forwards a few years and we have the search engines, suddenly simply by typing a few keywords you can jump right to a resource (page/image/...) anywhere on the web. Fast forward some more and we get to web 2.0 where again people are amazed every time direct access to a resource is exposed. Yup.. all your web 2.0 is just this simple principal..

An RSS feed, well it lets you read a resource (a post) outside the context of the website and inside lets say google reader. You can rip a resource (video) out of youtube and embed it anywhere you like. You can interact with a web application super fast thanks to targeting a resource directly with say ajax and only updating that resource rather than updating the entire view (page). You can interact with web applications using desktop clients because they let you access one resource at a time; and so on and so forth, virtually every improvement you see on the web comes down to that one thing, directly accessing a resource (and creating resources at a more granular level).

How we made the web faster

Negating the rather obvious upgrades in technology over the years, there is one primary thing that speeds up the web and virtually everything computer oriented, the cache.

Before all the web 2.0 stuff, resource caching was at an all time high and was making the web faster for all of us; caching at the resource level is enabled by HTTP and its RESTful design. Control Data allows us to limit how much information needs transferred over the web, request an image once and it gets transferred, request it a second time and thanks to caching and HTTP odds are very high that it won't get transferred again. When you consider that the average web page can easily have 30, 50, 300 images and static files embedded in it this is a huge speed increase, and frankly one we could not live without.

Skipping forwards to web 2.0 and the present day again, we've gone wild with caching; anybody who's been involved with a high traffic site will tell you that the only way to do it is to cache everything you can; from data in memory, through to code and op code caches. But this is only half the story.. a strange this has happened..

How we made the web slower

Simply, we forgot HTTP and a RESTful web somehow - that all important web caching whether it be at intermediate servers or in a web browser, it's forgotten.

To illustrate, if you view an image and then view it again, it'll be there instantly - why? because last-modified, etag and other control data is sent by great web servers like apache for static files, until you force a refresh or the file changes on the server you'll simply get a 304 response telling whatever cache down the line to use it's own copy instead. Now, try jumping on to a web page, even this one and you'll find the whole thing is reloaded, every time. I'd estimate that circa 80% of all pages you visit are fully reloaded every single time you see them, if not more.

Here's the reason - most pages are generated by scripts now, and something that goes unnoticed by most developers is that the web server (like apache) hand over *full* control to the language runtimes, and in turn to the developer. In other words, unless developers are calculating, receiving and sending control data for each response, and validating every http message in to their scripts, then most of the benefits of HTTP and RESTful design are completely lost; especially caching.

Here's a fact, whether you agree with it or not, to me it is a fact: the web has to be RESTful for it to work properly, whether it's a web of documents, or a web of data, or both.

Looking at the current state of Linked Data

Linked Data is amazing because it takes the big four I mentioned earlier (URI, Hyperlink, Resource, HTTP) to a new level; we create resources at the most granular level possible, assign them URIs, link them together with typed hyperlinks then expose them via HTTP.

Notice I didn't mention REST in there? that's because I (and I'm not the only one) don't feel that Linked Data is currently RESTful. And as we can learn from web 2.0, unless this is addressed we'll face major problems down the line. In addition, because of this lack of RESTful-ness I feel like the data isn't linked; simply using URIs from different datasets on both sides of a triple does not link those datasets, well not from a client perspective anyway.

To expand and refining the issues:

SPARQL Silos

Issue one, is that SPARQL and the servers with RDF stores which power it are positioned at the wrong side of the client / server relationship imho. Because each major dataset effectively has it's own server and access point (SPARQL interface) it means that when you query it, it can only return the Linked Data which it stores. This leaves us three options at the minute:

  • let that server pull in remote Linked Data and store it too (which makes the server fill up and slow down, and turns it in to a silo).
  • use one great big server that tries to store all the linked data (which feels like a silo all over again to me, not distributed at all).
  • Run our own server and only store limited data in it (limited.. and again a silo I guess)

If we moved SPARQL to the client side however, then all it would need is a starting point from which it could traverse the web of data, only pulling in what it needed for a query. This may sound slow but if all data was exposed as resources like it should be, and with control data so it could be cached, this slow down would soon disappear; lesson from web 1 and 2!

With regards the caching, this could happen at traditional intermediate caches within the internet and at ISPs, locally in client side triples stores (like a browsers cache) or the existing big servers that attempt to store all the linked data could be repositioned as linked data caches.

For example a small RDF document could simply delegate seeAlso http://datacac.he/http://subject.uri and that linked data cache could return back all the information it knows about the subject by returning the RDF results of a SPARQL describe. This alone would be a HUGE speed up, prevent silos and create a real web of data.

In addition, this would keep all linked data transferred through the web in RDF format, and thus machine readable and typed. At the minute we have lots and lots of SPARQL queries, which essentially are just untyped junk data that a machine couldn't possibly understand - SPARQL results remove all the goodness from RDF and give us something that is domain and developer specific, not re-usable. Think about that for a moment..

Clarification: I'm not saying SPARQL + RDF stores shouldn't be on the server side, they should as they are needed in most cases, I'm simply saying that the primary interface to linked data shouldn't be SPARQL over HTTP to a remote SPARQL endpoint. Rather we should be accessing RDF documents, or entities if you like via HTTP.

RESTful RDF

Issue two, the focus has been on getting data on the web, finding ways to link it, access it, store and query these vast datasets; and the work done thus far is amazing! But now that's handled it's time to go back to basics and find ways of both getting and publishing Linked Data RESTfully, at a granular per resource level.

This means handling RDF like ATOM, and essentially making atompub all over for RDF (as many are thinking and working on). I feel that regardless of what's implemented behind the interface, and whether triples stores and SPARUL are used, we still need to manage RDF / Linked Data in terms of documents and entities for it to be RESTful.

An additional issue raised by this is loosing the notion of a quad, g s p o, Named Graphs are vastly important to linked data, but we need to get named graphs in to triples and out of quads so we are always working with RDF through HTTP.

Also worth noting that temporal, provenance, multi-language, multiple representations etc will all need handled too; without using any HTTP extensions; no point half baking it or making the solution dependant on drafts - needs to work with the Universal Interface!

First Step

The above means one of the first challenges and things I'll try to tackle, is to find a way to fit a RESTful RDF publishing and exposing protocol in to the shared http space on a web server; taking in to account things like content negotiation, multiple representations, versions / time varying representations, and backwards compatibility with the current web.

note: I'm not going to define the protocols, plenty of more intelligent people than me are working on these things, just leverage a space where a full RESTful protocol can work in unison with the way we currently do things so that it's transparent to browsers and visible to linked data clients. This should then allow a stable test environment to try out different ways of doing things and test that the current web doesn't break.

To be continued.. often and frequently. I'm blocked on my current, v important project, and need to address these things.


Reading List : Web, Linked Data, REST, Semantic Web

Wednesday 10 Mar 2010 by nathan | No Comments
Filed under: internet Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

Personally, I have two types of reading, the posts etc that I "tweet" and then the heavier reading I do over time; this is a list of the latter for the past month - hopefully it'll help somebody who's looking for the same kind of info I have been.

I've grouped all the links in to two main sections, and then sub-grouped by how they make sense in my head! :)

Web, HTTP and REST

Roy T. Fielding Dissertation - Architectural Styles and the Design of Network-based Software Architectures Of particular relevance and note are chapters 4-6 (many only ever read chapter 5 and miss the context + summary *needed* in chapters 4 and 6!)

Roy T. Fielding - REST APIs must be hypertext-driven
Discussion on HTML5 and RESTful HTTP in browsers
Discussion on URIs Resources and Switching content types w/ REST angle (v good)

RFC 2616 HTTP/1.1 and the HTTPbis Working Group HTTP/1.1 update in parts:

  1. Messaging
  2. Semantics
  3. Payload
  4. Conditional
  5. Range
  6. Cache
  7. Authentication

Linked Data and the Semantic Web

Linking Open Data Community Project
Linked Data Applications
Equivalence Mining and Matching Frameworks
Linked Data Browsers, Mashups and other Client Applications

Dataset Dynamics - On the Dynamics of Linked Datasets
Realizing a write-enabled Web of Data
Web Access Control (WAC) - a decentralized system for allowing different users and groups various forms of access to resources where users and groups are identified by HTTP URIs.
Discussion of the WAC vocabulary
Socially Aware Cloud Storage Design Note
Distributed Social Networking through Socially Aware Cloud Storage from TimBL
AWWSW - "Architecture of the World Wide Semantic Web" Task Force

SPARQL 1.1 Uniform HTTP Protocol for Managing RDF Graphs
A Linked Data Frontend for SPARQL Endpoints
RAP - RDF API for PHP V0.9.6
Inav the Terrible - An idea for posting RDF through HTTP.

Talis Changesets
Triplify Update Vocabulary

skos as atom

RFC 4287 - The Atom Syndication Format
RFC 5023 - The Atom Publishing Protocol
AtomPub Tombstones - The Atom "deleted-entry" Element
RFC 5005 - Feed Paging and Archiving
Versioning Link Relations - Link Relation Types for Simple Version Navigation between Web Resources

Named Graphs, Provenance and Trust
Accessing Site-Specific APIs Through Write-Wrappers From The Web of Data
Provenance Information in the Web of Data - LDOW 2009 paper
Using Reification To Extend RDF (historical reification approach)
RDF Policy-based URI Access Control for Content Authoring
The Open Provenance Model Core Specification (v1.1)
W3C Provenance Incubator Group

History of the Web 1945, 1980 through 1997 on W3
LEIRI - Legacy extended IRIs for XML resource identification The type of "URI" used in xml:base
CSHALS 2010 W3C Semanic Web Tutorial
Mindswap online RDF Converter
W3 online RDF Validator


Human Flesh Search a catalyst for Linked Data generation?

Monday 8 Mar 2010 by nathan | No Comments
Filed under: general, semantic web

Earlier today I saw an interesting slideshow from James A Hendler which had some focus on Human Flesh Search.

Personally I find this somewhat inspiring, out of all of it's features the thing that I find most interesting is using human power to rapidly "link" data together.

Here's a very quick look at the human process involved:

  1. Assign the subject of the Human Flesh Search.
  2. Send out humans to find data about that subject, on both the web and in the physical world
    • When the information is on the web link to it and perhaps highlight some key points in an abstract
    • When the information is in the physical world identify it, and describe it.
  3. Correlate all data together and reference it to the subject.
  4. Manually cleanse the data to weed out incorrect / wrongly placed data

If this isn't a huge human powered linked data generating and cleansing machine then what is?

IMHO an implementation to cater for this (in early stages) would be extremely simple, for example consider the following:

  • The subject of the flesh search is assigned an Identifier (perhaps under the guise of a tag - eg flesh:the_subject)
  • A simple interface is made, perhaps a tiny browser extension or toolbar button, which when clicked associates the current web page with the flesh:subject (no doublt by quickly sending the URI of the page as a get parameter to a server, at which point a single triple flesh:the_subject predicate uri is stored)
  • For offline resources a simple UI is made to enter in some data (back end obviously stores data cleansed and associated w/ flesh:the_subject)
  • Another simple UI to expand or remove information and "links".

Certainly running this as social style web app would be in order, but would require minimal work from what I can see, and serve as a model for a human powered mass linked data generating machine.

nb: you could go wild with this, sponging data and uri's submitted through various web services, now is a good time to stop I think.

This may be overly obvious, but just in-case I thought it needed said :)


Are GET and PUT symmetrical? (Linked Data, CONNEG)

Tuesday 2 Mar 2010 by nathan | 8 Comments
Filed under: semantic web Tags: , , , , , , , , , ,

John S. Erickson, Ph.D. left a rather interesting question on my earlier post HTTP RFC paraphrased for the Web of Data, quoted here in full:

Here's a question: are GET and PUT symmetrical? Perhaps my question is a bit out-of-band, because the context of my question is really about the symmetry of content negotiation. Implicit in RFC 2616 section 14 is the existence of a (potentially) wide variety of alternate representations that may be accessed via the HTTP/1.1 request header fields; in the world of linked data these are useful for differentiating between text/html, application/rdf+xml, text/rdf+n3, etc.

So maybe the heart of my question is this: in a linked data world where conneg is alive and well (and assumed), what should be the correct interpretation of RFC 2616's (if) the Request-URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server? If I can GET a resource to the resolution of a MIME type, should I not expect to PUT to that same resolution as well? Or, if during a GET the server responds with a list of Alternates, shouldn't I expect to PUT using any of those choices?

Readers familiar with MIME-typed disseminators in the digital repository world (think: Fedora) might think of this as a question about MIME-typed "ingestors"; it is a similar issue: instead of requesting a particular manifestation of the object, this is about updating a specific manifestation (possibly updating the whole).

It's a very good question, and I'm perhaps a bit out of my rank in answering, but to me it seems easy (perhaps too easy) to answer... here go's:

You can request a GET on anything with an HTTP URI.

HTTP URIs can identify anything, however for the context of this let's limit URIs to identifying three distinct things:

  1. Stored Hypermedia (digital documents, audio, video, text files ... any "file" physically stored on the server and web accessible.)
  2. Web Services (a data-accepting process, a gateway to some other protocol, a separate entity that accepts annotations ... Nota bene if you are still reading then you know what I'm referring to so no need to be pedantic on my choice of terminology)
  3. Things (the sky, dave's car, me ... and in this context not hypermedia or web application endpoints)

Let's look at what we can successfully PUT. RFC 2616 section 9.6 tells us:

The PUT method requests that the enclosed entity be stored under the supplied Request-URI ... the URI in a PUT request identifies the entity enclosed with the request ...

We can derive from the above that only "Stored Hypermedia" can successfully be PUT; and thus we cannot successfully PUT a "Web Service" or a "Thing".

Nota bene I'm assuming here that you (the reader) already knows all about CONNEG, Linked Data and the multiple questions which have arisen on how to handle content negotiation, especially with regards RDF and alternative representations.

In the Linked Data world we use the RDF data model extensively. We describe things with statements, each statement is a subject-object-predicate expression which we refer to as a triple; strap enough of these triples together and you have a description of a thing. This is our RDF.

To pass these descriptions around we can serialize our RDF statements in a variety of machine readable formats application/rdf+xml and text/rdf+n3 amongst others.

An RDF Document is where we persist one of these serializations in document, id est Stored Hypermedia.

So to store an RDF document we would logically use PUT, and then GET it later, or perhaps DELETE it. We certainly wouldn't POST to an RDF Document, anymore than we would POST to a GIF image! When thinking about an RDF Document think of it as Hypermedia, exempli gratia an AVI, JPEG or WAV, and treat it as such; a single document, in a single format.

RDF is not one of it's serializations, and it most certainly isn't an "RDF Document".

A major factor which compounds confusion in regards to content negotiation is the Web community's fixation of thinking about the response to a GET as if it were Stored Hypermedia, especially when the mime type associated with the returned entity is something like text/html or application/rdf+xml.

In common language what you are reading is a "web page", infering that it is a Document and Stored Hypermedia.

In reality what you are viewing is an entity returned by a GET request to a Web Service, and that entity has a mime type of text/html. What you are reading is actually just some data I input in to a database via a POST to a CMS (wordpress); there are other things here too, presentation data, navigation on the right and so forth. Leave a comment and the data I entered will stay the same but the entity returned when you next GET this URI will be different. This certainly is not Stored Hypermedia (as we defined it above).

Considering the conneg questions, ask yourself "are these Alternatives Stored Hypermedia or simply entities returned by a web service?". I'm willing to take a punt here and say that your answer will be the latter. Which means that you'd be getting the data in to your system via a POST to a web service. and not a PUT.

Further, if you think about the "RDF" you would no doublt be posting to create these alternatives, it may well make sense to think of it as POSTing RDF in a serialized form which the Web Service accepts. The Web Service will then (all going well) persist this RDF, or the triples, in something like a QuadStore. This serialized RDF is not an RDF Document, it's just serialized RDF in a request entity, just like any other data. Thus think of serialized RDF like json, base64 encoded strings, the egg in ?name=egg&val=shell; it's "just" data.

All of the above is driven home when you consider HTTP URIs that identify "Things". You can't send the Thing in an entity, thus if you get any response from a server when you HTTP GET the URI for a thing, then that URI must be serviced by a Web Service (as defined above).

And now that you know it's a Web Service logic would dictate that you should POST back to it...

The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions: - Annotation of existing resources; ...

Answering the questions!
After all of that, on to the questions.

  • are GET and PUT symmetrical?
    Only in the context where the URI requested identifies Stored Hypermedia.
  • If I can GET a resource to the resolution of a MIME type, should I not expect to PUT to that same resolution as well?
    No as by this point you know that the URI/MIME combo you requested definately does not represent Stored Hypermedia (a Document).
  • Or, if during a GET the server responds with a list of Alternates, shouldn't I expect to PUT using any of those choices?
    Not expect, as you have no way of knowing whether each URI returned identifies Stored Hypermedia, a Web Service or another Thing. With every new URI the cycle starts again.

Please don't take any of this as gospel, it's all IMO. I do at least hope it makes sense though, and if it doesn't please do correct me asap :)


HTTP RFC paraphrased for the Web of Data

Tuesday 2 Mar 2010 by nathan | 3 Comments
Filed under: linked data, semantic web Tags: , , , , , , , , , , , , , , , , , , , , , , ,

This post is all about gleaning as much useful information as possible from the HTTP Protocol RFC 2616 in order to answer simple and complex Web of Data related questions.

I've chosen the rather old RFC 2616 (1999!) at this time rather than the upcoming HTTPbis because I feel it's important to know where you are coming from, and whilst many things about the Web of Data feel new, they are really age old principals and technologies which have never been used to their full potential. Further you won't be able to appreciate the refinements in HTTPbis if you don't know what it's refining.

Virtually everything from here on is just a snippet/quote or paraphrase of the RFC. Let's start with a simple one:

Why use HTTP?

HTTP is an application-level protocol for distributed, collaborative, hypermedia information systems. ... HTTP allows an open-ended set of methods and headers that indicate the purpose of a request. ... HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet systems ... HTTP allows basic hypermedia access to resources available from diverse applications. source

I do fully recommend reading the entire RFC and the new HTTPbis, most questions can be answered by returning to these documents and reading what they say (it's all in the detail); here's some more info gleaned from the RFC:

The difference between POST and PUT, URIs as Identifiers, and URIs to identify more than just documents.

The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The URI in a POST request identifies the resource that will handle the enclosed entity. That resource might be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the URI in a PUT request identifies the entity enclosed with the request -- the user agent knows what URI is intended .. source

Using POST RESTfully for more than just form data.

The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions: Annotation of existing resources; ... Extending a database through an append operation. The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI. The posted entity is subordinate to that URI in the same way that ... a record is subordinate to a database. source

What to do if something is created as a result of a POST request.

If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain an entity which describes the status of the request and refers to the new resource, and a Location header. source

When to use a PUT request?

The PUT method requests that the enclosed entity be stored under the supplied Request-URI. source

How to handle a PUT

If the Request-URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server.

If the Request-URI does not point to an existing resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create the resource with that URI. If a new resource is created, the origin server MUST inform the user agent via the 201 (Created) response. If an existing resource is modified, either the 200 (OK) or 204 (No Content) response codes SHOULD be sent. source

if extra headers were sent?

Unless otherwise specified for a particular entity-header, the entity-headers in the PUT request SHOULD be applied to the resource created or modified by the PUT. source

and what if I want to save it somewhere other than the URI specified by the client?

If the server desires that the request be applied to a different URI, it MUST send a 301 (Moved Permanently) response. source

and if the PUT can't be done..

If the resource could not be created or modified with the Request-URI, an appropriate error response SHOULD be given that reflects the nature of the problem. source

can I use PUT with server side versioning?

A single resource MAY be identified by many different URIs. For example, an article might have a URI for identifying "the current version" which is separate from the URI identifying each particular version ... a PUT request on a general URI might result in several other URIs being defined by the origin server. source

how would I let a client know I implement server side versioning when they PUT?

If an existing resource is modified, either the 200 (OK) or 204 (No Content) response codes SHOULD be sent.. source

200 indicates a message body in the response ;)

and DELETE?
well it's short so you may as well read it all..

The DELETE method requests that the origin server delete the resource identified by the Request-URI. This method MAY be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation has been carried out, even if the status code returned from the origin server indicates that the action has been completed successfully. However, the server SHOULD NOT indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible location.

A successful response SHOULD be 200 (OK) if the response includes an entity describing the status, 202 (Accepted) if the action has not yet been enacted, or 204 (No Content) if the action has been enacted but the response does not include an entity. source

note that a response can be 200, meaning you can return a response message (like i have X other versions here [list] or delete them all by clicking here [form input which POSTs to a service] ), or an RDF response that can be interpreted by a client to do the aforementioned :)

but can't I tunnel all actions through GET?

Safe Method .. GET .. SHOULD NOT have the significance of taking an action other than retrieval! source

edit: removed small section about URI vs URL! Do however see the comment from Michael which links to more information on the subject.

Thanks for reading :)
There is much more information in the RFC, but those were some nicer points I found useful and relevant to current Web of Data topics & discussions.


Why ActionScripts poor performance is a good thing

Thursday 11 Feb 2010 by nathan | No Comments
Filed under: general Tags: , , , , , , , , , , , , , , , , ,

ActionScript gives web developers an almost unique chance to grow their skills; out of the common languages in a web developers itinerary ActionScript is almost the only one where a developers code is going to be run 30-100 times per second, with common methods being run literally thousands of times per second.

Further the code isn't running on a solid server where the environment is controlled, and where you can simply throw more processors and memory at a problem to make it go away, the code is being run on differently specified machines almost every run, in a different client and in different versions of player.

To compound this unique situation the virtual machines (relatively) poor performance comes in to play.

Why is this good?

It has spurred a good percentage of ActionScript developers on to look at more advanced levels of code optimisation, to consider factors which are simply ignored and overlooked in other languages, to work with (and create) other technologies in order to push performance to the max; indeed even to hack the virtual machine to bits then create custom compilers and optimisers.

Libs and frameworks from PV3D through to Flex, projects such as haXe, Apparat and Alchemy; not to mention the wide array of technologies from AMF to RTMFP - this list goes on (and on).

There are 3 types of coders: bad coders, normal coders and ultra coders who's skillsets and level of understanding are far and beyond that of the normal coder; what I'm trying to get at here, is that the ActionScript challenges outlined above create an environment where a higher percentage of coders can grow in to ultra coders, than in any other language I know.

The Flash / ActionScript pair gives both designers and developers the notion that anything is possible - and this spurs them on to the push boundaries of what can be done; that kind of thinking sticks with people, they carry it on to other languages, platforms and communities; bringing inspiration and forward thinking to the web of humans.

Context: Really enjoyed reading this post: The Advantage Of ActionScript from Joa, in particular I'm glad that he mentioned "Poor Performance" as an advantage. The above post leads on from Joa's to give some of my own thoughts on why we all owe ActionScript and Flash a little something.

as3poor


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