Node Identifiers: From Structured Data to Linked Data | Sitebulb

Node Identifiers: From Structured Data to Linked Data

Patrick Hathaway

Published October 27, 2020

Most people working in digital marketing are familiar with the concept of structured data by now. It is reasonably straightforward to understand the basic concepts, and there are numerous structured data generators that can provide a head-start when actually writing the code.

This article is about taking the next step, beyond disparate structured data entities and towards one of the true goals of The Semantic Web: linked data.

"The Semantic Web isn't just about putting data on the web. It is about making links, so that a person or machine can explore the web of data.  With linked data, when you have some of it, you can find other, related, data." Tim Berners-Lee, Linked Data - Design Issues

As we will see, node identifiers are a fundamental part of making data interconnected.

What is a node?

We can't throw terms like 'node identifier' around without explaining what the language means, so we'll take a brief detour and return to first principles: graph theory.

We are already familiar with graphs as part of common digital marketing parlance - Google's Knowledge Graph, or Facebook's Open Graph, for example.

Graph theory is a mathematical structure used to describe the relationship between objects. In graph theory, graphs are made up of nodes (also known as 'vertices') which are connected by edges (also known as 'links').

One of the simplest graphs we can create shows two nodes connected by an edge:

In this example, the arrow indicates that this is a directed graph, rather than an undirected graph, which you get from a simple line with no arrowhead.

The directed edge means that the edge has an orientation associated with it. When it comes to structured data, we use directed graphs to describe relationships. For example:

So the 'Patrick Hathaway' (moi!) node is linked to the Sitebulb node via the 'founder' relationship (co-founder really, don't tell Gareth I said that!).

Let's look at a simple example of structured data that we might find on a web page like this:

Here, we have named the entities and relationships in a way that has a consistent meaning through a shared vocabulary (Schema.org);

So the nodes represent entities, and the directed edges describe the relationship between the entities.

While this graphical representation is helpful for us mere humans to understand what's going on, the important thing is that this data can be mapped to a matrix in a machine-readable format.

And, by using node identifiers, we can interconnect this data with other structured data on the web.

Now that we understand what a node is in terms of structured data, we can move onto node identifiers themselves.

What is a node identifier?

Simply put, a node identifier is a unique 'name' for an entity, which is publicly accessible and can be looked up or linked to.

"Entities need to be uniquely identifiable. There must be a one to-one correspondence between each entity identifier (ID) and the (real-world or fictional) object it represents (i.e., within a given entity catalog; the same entity may exist under different identifiers in other catalogs)." Krisztian Balog, Entity-Oriented Search

By using node identifiers, we can reference multiple, otherwise disparate, blocks of structured markup, so that they are linked together such that a machine can understand the context of the relationship.

There are 3 major formats for Schema.org: JSON-LD, Microdata and RDFa, and the node identifier attribute is different for each;

Google has put it's weight behind JSON-LD as the recommended notation, and it is fast becoming the 'de facto', so the examples we explore below will focus on JSON-LD - however it is important to note that the same functionality can be achieved with the other formats.

The 'value' of the @id property is typically a URI (Uniform Resource Identifier)  a URL with a fragment identifier - for example https://example.com#website. The URL identifies the page, and the fragment identifier (hash mark #) points to the subordinate resource.

Node identifier property in JSON-LD

JSON-LD is a JavaScript notation embedded in a