# Thinking about graph data

## Introduction

xGT is a tool for reading in massive amounts of data into RAM for performing fast pattern search operations. The best data for this analytic approach is where there are relationships between data objects described in the data (i.e. linked data). The classic example of this is a social network graph where people have relationships with each other represented in the data such as "friend-of", "knows", or "family".

We start with the assumption you have data and want to analyze it with xGT.
Let's walk through how to approach this activity, how to build a mental model of your data, how to describe that model using the Trovares `xgt`

Python library, and how to get xGT to help you understand what it is in your data.

We will begin with the data and follow a simple example to illustrate this process. Consider that you have data in two separate comma-separated-values (CSV) files:

person_id | name |
---|---|

123456789 | Manny |

123454321 | Bob |

987654321 | Frank |

987656789 | Alice |

person_id | person_id | start_date | end_date |
---|---|---|---|

123456789 | 987654321 | 20150103 | 20170414 |

123454321 | 987654321 | 20160402 | 20170414 |

987656789 | 987654321 | 20160707 | 20170414 |

123456789 | 987656789 | 20170415 | - |

123454321 | 987656789 | 20170415 | - |

987654321 | 987656789 | 20170415 | - |

## Graph model

These two data sources correspond nicely to components of a graph.
The first is a collection of information about an object (in this case, a person), which can be represented as a *vertex* in the graph.
A vertex is a mathematical name for the "bubble" in a graph drawing.

The second is a collection of information about relationships between objects, which can be represented as an *edge* in the graph.
An edge is a mathematical name for the line that connects two vertices.
We usually consider the edge to have a direction, meaning it goes from one specific vertex to another specific vertex.
We can call the two vertices in this relationship the *source* and *target* of the edge, respectively.

### Graph image

## Setting up the graph model in xGT

We can use the `xgt`

library to build a graph model of the CSV data above.
First, we need to describe the kind of data that our graph components will hold.
We create a `VertexFrame`

object which will hold the employee data and an `EdgeFrame`

object which will hold the data describing who reports to whom.

We link vertices and edges by specifying *key* properties.
These relationships give xGT the information required to link this data into a single, connected graph and hop along edges from vertex to vertex quickly and efficiently during queries.
(Details for the commands below are described in our xGT reference manual.)

```
employees = xgt.create_vertex_frame(name = 'Employees',
schema = [['person_id', xgt.INT],
['name', xgt.TEXT]],
key = 'person_id')
reports = xgt.create_edge_frame(name = 'ReportsTo',
schema = [['employee_id', xgt.INT],
['boss_id', xgt.INT],
['start_date', xgt.DATE],
['end_date', xgt.DATE]],
source = employees,
target = employees,
source_key = 'employee_id',
target_key = 'boss_id')
```

### Understanding the vertices

As mentioned earlier, the employee data is represented in our graph model as vertices, and each vertex is uniquely identified by a single columns from the vertex schema.
All of the columns from the vertex schema that are not used as key columns are called *properties* of the object.
In our case, the vertex frame has a `name`

property for each employee.

### Understanding the edges

To connect two vertices (employees) with a `ReportsTo`

relationship we have an *edge*.
The data associated with each edge comes from the edge's *schema*.
Note that there are no columns of the `ReportsTo`

schema that are directly construed as vertices.

To a human, the `employee_id`

and `boss_id`

look like identifiers for vertices, but the schema itself doesn't require that.
Instead, we establish this explicitly using the `source`

, `source_key`

, `target`

, and `target_key`

parameters of the `create_edge_frame`

method.

The `ReportsTo`

edge connects two `Employees`

vertices therefore the `employees`

vertex frame is used in both `source`

and `target`

parameters of the `create_edge_frame`

method.

The direction of our `ReportsTo`

relationship is from employee to boss, so the `employee_id`

column is used as the `source_key`

parameter of the edge, and the `boss_id`

column is used as the `target_key`

parameter of the edge.

### Data loading

Normally, having described the schema of the graph components, our next step would be to actually fill those components with data from our tables. For brevity, we'll skip over this step, but you can learn about the various mechanisms xGT provides in our data management documentation. Let's pretend we've accomplished this and skip straight to searching for patterns in our graph.

## Looking for interesting patterns

If you have looked over our sample data you may have guessed that one interesting patterns is finding a pattern of an employee (let's call them "X") that reports to a boss ("Y") who later had their roles reversed so "Y" was reporting to "X".

You can imagine that spotting such a pattern is easy in a few instances, but if you had 100,000 employees to look through, it would be very challenging for a person to notice these kinds of patterns. If your data consisted of employee data from many companies, it is easy to imagine getting to hundreds of millions of graph edges.

So let's see how to convert our image of a pattern into the Trovares Query Language (TQL) to have xGT perform an automated search for all patterns.

### Describing one relationship

To describe the first X-Y relationship we formulate a `MATCH`

statement as follows:

```
MATCH (x:Employees)-[edge1:ReportsTo]->(y:Employees)
```

Note that in TQL the vertices must be given a vertex frame (in our case this is `Employees`

for both the employee and for the boss) because the xGT data model supports multiple vertex and edge frames in a graph.
For a similar rationale we must supply an edge frame for the connecting edge (in our case it is `ReportsTo`

).

But this MATCH statement is incomplete. To finish it, we need to tell xGT what to do with the answers it finds:

```
MATCH (x:Employees)-[edge1:ReportsTo]->(y:Employees)
RETURN x.person_id, edge1.start_date, edge1.end_date, y.person_id
```

Be careful: this example produces an exact copy of the `ReportsTo`

table, which may be much larger than you want to deal with.
The original question we asked needs the reverse relationship, too.

### Describing the second relationship

The role-reversing pattern that we want to find can be thought of as a *two-path* (two contiguous edges) through the graph.
At a high level of abstraction, it comes down to: X → Y → X.
We also need to add the constraint that the end date of the first edge comes on or before the start date of the second edge.

We begin by showing how to describe a two-path:

```
MATCH (x:Employees)-[edge1:ReportsTo]->(y:Employees)-[edge2:ReportsTo]->(x)
RETURN x.person_id, y.person_id,
edge1.start_date, edge1.end_date,
edge2.start_date, edge2.end_date,
```

To add the constraint about the second edge coming on or after the first edge, we add a WHERE clause:

```
MATCH (x:Employees)-[edge1:ReportsTo]->(y:Employees)-[edge2:ReportsTo]->(x)
WHERE edge1.end_date <= edge2.start_date
RETURN x.person_id, y.person_id,
edge1.start_date, edge1.end_date,
edge2.start_date, edge2.end_date,
```

It is common that queries include constraints in the form of the WHERE clause.

## Understanding the query result

There are really two graphs involved in a query: the large *data graph* and the smaller *query graph*.
The query graph is the graph structure (vertices and edges) described in the MATCH statement without the constraints of the WHERE clause.

When xGT finds a matching pattern in the large data graph—here "matching" means that the graph structure is aligned and that the attributes attached to the subgraph of the large data graph being matched satisfies the constraints of any WHERE clauses—a row is added to a result table.

The result table will have columns that correspond to the names in the RETURN clause.
If the query has an `AS name`

component in the RETURN clause, `name`

will be used as the name of the column in the result table.
Otherwise, xGT will select a unique name for the result table column.
The data type of each column is automatically determined from the data type of the corresponding return value.
The name of the table frame holding the results can be changed by using the `INTO`

keyword.

```
MATCH (x:Employees)-[edge1:ReportsTo]->(y:Employees)-[edge2:ReportsTo]->(x)
WHERE edge1.end_date <= edge2.start_date
RETURN x.person_id AS employee1_id, y.person_id AS employee2_id,
edge1.start_date AS start1, edge1.end_date AS end1,
edge2.start_date AS start2, edge2.end_date AS end2
INTO Results
```

### Exploring the query result

Once a query finishes, xGT will contain a table frame that is populated with data that you described in the query.

## Concluding remarks

We've presented a mental model of how to work with graphs and graph data and how some snippets of TQL relate to the mental model. To begin working with real data inside xGT, you're ready to download the xGT Python interface and get started.