In this 3-part series, IMVU senior engineer Bill Welden describes the means and technology behind IMVU’s web services.
Part 2: Nodes and Edges
The Uniform Contract constraint of the REST discipline means that IMVU web applications are able to provide functionality to many different web applications in a robust and reusable manner.
There are, of course, many different ways of implementing a uniform contract. At IMVU, we have chosen a structured network model – a graph consisting of nodes, which represent the objects of interest to our users, and edges representing the relationships among those objects. The functionality of our web services is then framed in terms of inspecting and manipulating this graph.
For example, here is a network diagram showing some high school enrollment data.
Two students, Jan and Chris are each enrolled in two classes. They are both in the same Algebra class, but in different English classes.
Each of the students, and each of the classes are represented by nodes, shown here as circles. The relationship between the students and the classes are represented by edges, shown here as lines connecting the circles.
Nodes come in different types. In this diagram we have student nodes and class nodes connected by edges representing course selections.
Nodes have properties, and the type of the node determines the properties. Each student, for example, has a name and an age. Each class has a starting time.
All of the nodes of a given type constitute a node group. This example has two node groups so far, one for students and one for classes. The node group determines the properties of the nodes in the group.
Node groups are important — they distinguish a structured network model from an unstructured one. Network models are usually assumed to be unstructured, where any sort of data can be associated with any node. In fact, data and links are often intermixed, as in a hyperlinked network of text documents.
IMVU REST services are built on a network model, but a structured one, where every node belongs to a node group, and it is the node group – not the node – that determines the specific properties found in each node. In this way, a structured network model is like a relational data model.
Let’s add some teachers.
Janos teaches the 10 AM Algebra class and Mariska teaches both of the English classes. It is useful to group the edges, as they attach to the node, into edge groups:
All of the edges in an edge group attach, at the opposite end, to the same type of node. Each class node, for example, has two edge groups: one for students and one for teachers.
Because edges create relationships between nodes, the edge groups are sometimes named according to the role that the links play. Here we call the edge group listing the students in the class the “roster”, and the symmetrical view the student’s “schedule”. More often, though, edge groups are named simply by the type of node they link to, as with “teachers” here.
We will see later that edges are implemented using directional links – but the edges themselves are not directional, and can be viewed from either end. Jan is enrolled in Algebra and Algebra has Jan as a student.
Nodes have properties, but edges can also have properties. The number of times the student has been late to class is a property of the edge between the student and the class. It is not a property of the student, because the student will have many classes and may not be late to all of them, nor is it a property of the class, because many students will be enrolled in the class, and not all of them will be late.
The edge group determines what kind of node is at the other edge, whether the node has properties, and if so what the properties are.
We model node groups, nodes, edge groups and edges using JSON documents served over HTTPS, and link them using URLs. In my next post, I will go into detail on the structure of these URLs and documents, and follow through on the high school enrollment example.
Up next: Part 3 — Documents and Links