In his ongoing series of niggles about Linked Data, Rob McKinnon claims that “mandating RDF [for publication of government data] may be premature and costly“. The claim is made in reference to Francis Maude’s parliamentary answer to a question from Tom Watson. Personally I see nothing in the statement from Francis Maude that implies the mandating of RDF or Linked Data, only that “Where possible we will use recognised open standards including Linked Data standards”. Note the “where possible”. However, that’s not the point of this post.

There’s nothing premature about publishing government data as Linked Data – it’s happening on a large scale in the UK, US and elsewhere. Where I do agree with Rob (perhaps for the first time ;)) is that it comes at a cost. However, this isn’t the interesting question, as the same applies to any investment in a nation’s infrastructure. The interesting questions are who bears that cost, and who benefits?

Let’s make a direct comparison between publishing a data set in raw CSV format (probably exported from a database or spreadsheet) and making the extra effort to publish it in RDF according to the Linked Data principles.

Assuming that your spreadsheet doesn’t contain formulas or merged cells that would make the data irregularly shaped, or that you can create a nice database view that denormalises your relational database tables into one, then the cost of publishing data in CSV basically amounts to running the appropriate export of the data and hosting the static file somewhere on the Web. Dead cheap, right?

Oh wait, you’ll need to write some documentation explaining what each of the columns in the CSV file mean, and what types of data people should expect to find in each of these. You’ll also need to create and maintain some kind of directory so people can discover your data in the crazy haystack that is the Web. Not quite so cheap after all.

So what are the comparable processes and costs in the RDF and Linked Data scenario? One option is to use a tool like D2R Server to expose data from your relational database to the Web as RDF, but let’s stick with the CSV example to demonstrate the lo-fi approach.

This is not the place to reproduce an entire guide to publishing Linked Data, but in a nutshell, you’ll need to decide on the format of the URIs you’ll assign to the things described in your data set, select one or more RDF schemata with which to describe your data (analogous to defining what the columns in your CSV file mean and how their contents relate to each other), and then write some code to convert the data in your CSV file to RDF, according to your URI format and the chosen schemata. Last of all, for it to be proper Linked Data, you’ll need to find a related Linked Data set on the Web and create some RDF that links (some of) the things in your data set to things in the other. Just as with conventional Web sites, if people find your data useful or interesting they’ll create some RDF that links the things in their data to the things in yours, gradually creating an unbounded Web of data.

Clearly these extra steps come at a cost compared to publishing raw CSV files. So why bear these costs?

There are two main reasons: discoverability and reusability.

Anyone (deliberately) publishing data on the Web presumably does so because they want other people to be able to find and reuse that data. The beauty of Linked Data is that discoverability is baked in to the combination of RDF and the Linked Data principles. Incoming links to an RDF data set put that data set “into the Web” and outgoing links increase the interconnectivity further.

Yes, you can create an HTML link to a CSV file, but you can’t link to specific things described in the data or say how they relate to each other. Linked Data enables this. Yes, you can publish some documentation alongside a CSV file explaining what each of the columns mean, but that description can’t be interlinked with the data itself, making it self-describing. Linked Data does this. Yes, you can include URIs in the data itself, but CSV provides no mechanism that for indicating that the content of a particular cell is a link to be followed. Linked Data does this. Yes, you can create directories or catalogues that describe the data sets available from a particular publisher, but this doesn’t scale to the Web. Remember what the arrival of Google did to the Yahoo! directory? What we need is a mechanism that supports arbitrary discovery of data sets by bots roaming the Web and building searchable indices of the data they find. Linked Data enables this.

Assuming that a particular data set has been discovered, what is the cost of any one party using that data in a new application? Perhaps this application only needs one data set, in which case all the developer must do is read the documentation to understand the structure of the data and get on with writing code. A much more likely scenario is that the application requires integration of two or more data sets. If each of these data sets is just a CSV file then every application developer must incur the cost of integrating them, i.e. linking together the elements common to both data sets, and must do this for every new data set they want to use in their application. In this scenario the integration cost of using these data sets is proportional to their use. There are no economies of scale. It always costs the same amount, to every consumer.

Not so with Linked Data, which enables the data publisher to identify links between their data and third party data sets, and make these links available to every consumer of that data set by publishing them as RDF along with the data itself. Yes, there is a one-off cost to the publisher in creating the links that are most likely to be useful to data consumers, but that’s a one-off. It doesn’t increase every time a developer uses the data set, and each developer doesn’t have to pay that cost for each data set they use.

If data publishers are seriously interested in promoting the use of their data then this is a cost worth bearing. Why constantly reinvent the wheel by creating new sets of links for every application that uses a certain combination of data sets? Certainly as a UK taxpayer, I would rather the UK Government made this one-off investment in publishing and linking RDF data, thereby lowering the cost for everyone that wanted to use them. This is the way to build a vibrant economy around open data.