introduction.qmd

---
title: Introduction to Labeled Property Graphs
subtitle: Introduction
author: Jakob Voß
date: 2024-11-22
---

# Data Modelling with Graphs (and Star Wars)

## "Little boxes with arrows and stuff"

![](Netwerkje.jpg)

:::{.notes}
Ken Thompson (inventor of Unix, and UTF-8) about how he begins a program with scribbling data structures
:::

## Example

```{dot}
graph {
  rankdir=LR;
  C3PO [fontcolor=white];
  R2D2 [fontcolor=white];
  Luke [fontcolor=white];
  R2D2 -- C3PO [label="friends", fontcolor=white];
  R2D2 -- Luke [label="⮜ owns", fontcolor=white];
  Luke -- C3PO [label="owns ➤", fontcolor=white];
}
```

## Example

```{dot}
graph {
  rankdir=LR;
  R2D2 -- C3PO [label="friends"];
  R2D2 -- Luke [label="⮜ owns", fontcolor=black];
  Luke -- C3PO [label="owns ➤", fontcolor=black];
}
```

## Example

```{dot}
graph {
  rankdir=LR;
  C3PO [label=Robot];
  R2D2 [label=Robot];
  Luke [label=Person];
  R2D2 -- C3PO [label="friends", fontcolor=white];
  R2D2 -- Luke [label="⮜ owns", fontcolor=white];
  Luke -- C3PO [label="owns ➤", fontcolor=white];
}
```

## Basic graph elements for data modeling

- **nodes** (aka vertices) representing entities
- **edges** (aka connections, relations...)
- node labels as
  - **node identifiers** and/or
  - **node types** (aka **node labels**, classes...)
- edge labels as **edge types** (aka **edge labels**...)

## Data modeling

![](levels-of-data-modeling.svg)

. . .

- Arbitrary graphs used for models
- Models expressed in data formats

# Some Graph Data Formats

## RDF/Turtle

~~~ttl
# directed edges
<Luke> <owns> <R2D2> , 
              <C3PO> .

# node types (additional edges)
<R2D2> a <robot> .
<C3PO> a <robot> .
<Luke> a <person> .

# no undirected edges!
<R2D2> <friend> <C3PO> .
~~~

. . .

- Requires IRIs
- More limitations later

## CSV

robot ownership

~~~csv
owner,robot
Luke,C3PO
Luke,R2D2
~~~

robot friendship

~~~csv
friend1,friend2
R2D2,C3PO
~~~

. . .

- Requires contextual information
- Least common denominator: resistance is futile!

## SQL

~~~sql
# nodes
INSERT INTO robots VALUES ("R2D2");
INSERT INTO robots VALUES ("C3PO");
INSERT INTO people VALUES ("Luke");

# edges
INSERT INTO robot_ownership VALUES ("Luke", "C3PO");
INSERT INTO robot_ownership VALUES ("Luke", "R2D2");
INSERT INTO robot_friends VALUES ("R2D2", "C3PO");   # directed!
~~~

. . .

- Requires a database schema. Pros and cons?

## GraphML

~~~xml
<?xml version="1.0" encoding="UTF-8"?>
<graphml xmlns="http://graphml.graphdrawing.org/xmlns">
  <graph edgedefault="undirected">
    <node id="C3PO"/>
    <node id="Luke"/>
    <node id="R2D2"/>
    <edge source="Luke" target="C3PO" directed="true"/>
    <edge source="Luke" target="R2D2" directed="true"/>
    <edge source="C3PO" target="R2D2"/>
  </graph>
</graphml>
~~~

. . .

- Many more graph data formats exist
- Mostly for applications other than metadata\
  (e.g. network analysis)

## Cypher

~~~cypher
CREATE (C3PO:robot)
CREATE (Luke:person)
CREATE (R2D2:robot)
CREATE (Luke)-[:owns]->(C3PO)
CREATE (Luke)-[:owns]->(R2D2)
CREATE (Luke)-[:friend]->(C3PO)  # directed!
~~~

. . .

- Used in property graph databases
- Established standard (more or less)

## Property Graph Exchange Format (PG)

~~~pg
# nodes
R2D2 :robot
C3PO :robot
Luke :person

# edges
Luke -> C3PO :owns      # directed
Luke -> R2D2 :owns      # undirected
C3PO -- R2D2 :friends   # undirected
~~~

. . .

- Specified, documented, implemeted, but less known
- <https://pg-format.github.io/>
- Command line tool `pgraph` to convert formats

## Try out PG in your browser!

- <https://pg-format.github.io/pg-formatter/>

- Create a graph of some robots in Star Wars!

# Additional features

## Additional graph features

```{dot}
digraph {
  rankdir=LR;
  X -> X [label="loops"];
  subgraph cluster {
    style=filled;
	color=lightgrey;
	node [style=filled,color=white];
	a -> b;
	label = "subgraph";
  }
  X -> a;
  X -> Y [label="multi"];
  X -> Y [label="edges"];
  orphan;
}
```

. . .

- Support depends on the actual format or software!

## Properties / Attributes

~~~pg
# node properties
Padmé  :person  gender: female
Anakin :person  gender: male                 
Luke   :person  gender: male                 
C3PO   :robot   color:  golden, silver   # multi-value!
R2D2   :robot              
   
# edge properties
Padmé  -> R2D2   :owns     episode:1
Anakin -> R2D2   :owns     episode:2    
Anakin -> Luke   :child    episode:3
Padmé  -> Luke   :child    episode:3 
Luke   -> R2D2   :owns     episode:4
Luke   -> C3PO   :owns     episode:4
~~~

## Details depend on format & software

- Special properties (name, id, visual, reserved...)

- Which datatypes are supported (string, number, date...)?

- Can properties have values of mixed type? Empty set? Null?

- What are node/edge ids (internal, numeric, name...)?

- Can nodes/edges have multiple labels/types?

## Wikidata as a property graph

![](marcia-lucas-wikidata-1.png)
![](marcia-lucas-wikidata.png)


## Wikidata as a property graph

```{mermaid}
flowchart LR
  Q28193["<u>Academy Award for Best Film Editing (Q28193)</u><br>alias: Oscar for Best Film Editing"]
  Q463119["<u>Marcia Lucas (Q463119)</u><br><tt>alias:</tt> Marcia Griffin"]
  Q463119 -- "<u>award received (P166)</u><br>for work: Star Wars<br>date: 1978" --> Q28193
```

. . .

- Node identifiers and edge labels (property identifiers)
- Data model and terminology differ from\
  both RDF and common property graph models
  - aliases and descriptions with language
  - properties can link to entities

# First Summary

## (Labeled) Property Graphs

- A class of graph structures where

  - nodes and edges have **labels** (aka types)
  - nodes and edges have **properties** (aka attributes)

. . .

- Specific features differ depending on data format and software

- Useful for data modeling and schema-less data management

## Tow hard things

> There are only two hard things in Computer Science: cache invalidation and naming things. --- Phil Karlton

- "Property"
  - attribute-value pair in a property graph
  - IRI used as middle part in an RDF triple
  - attribute, field, ...
- "Label"
  - type, class, ...
  - name, ...

## Some property Graph data formats

- CSV
- Cypher
- GraphML
- Property Graph Exchange Format (PG)
- ...

Converter NPM package [pgraphs](https://www.npmjs.com/package/pgraphs)