Regarding understanding the necessity of Graph Creation in ArangoDB - graph

Question

I don't understand the necessity of creating graphs in ArangoDB.
For example, refer the below AQLs,
// Paths between 2 vertices
FOR p IN TRAVERSAL(person, knows, "person/person3", "outbound", {
paths: true, filterVertices: [{_id: "person/person2"}],
vertexFilterMethod: ["exclude"]}) RETURN p.path.vertices[*].name
//All connected Vertices for a given Vertex..**
FOR p IN PATHS(person, knows, "outbound")
FILTER p.source._id == "person/person5"
RETURN p.vertices[*].name
The above two queries are clearly related to Graphs...but you no need to create a graph to make them work.
Why and when should I create a graph?
What advantages will I get if I create a graph?

Answer 1

Creating or registering a 'graph' in ArangoDB is optional. Its purpose is to maintain graph persistency during modifications.
You can use Document features and combinations of graph traversals on the collections without referencing the graph.
However, one main purpose of the graph definition above is to use it during modifying edges or vertices. A Vertex document in a vertex collection may be referenced from several edge documents in several edge collections, with these edge collections in term belonging to several graphs.
When you now remove a vertex via a graph API, all these graph definitions are queried whether they permit edges pointing into this very special vertex collection. Subsequentially all edges in all possible edge collections are searched and removed. All this is done with transactional security.
By doing this, the mentioned graph persistency can be maintained. Graph persistency means that you don't have dangling edges that are pointing to a previously removed vertex.
Plesae note that you should rather use pattern matching traversals; One could rephrase
FOR p IN TRAVERSAL(person, knows, "person/person3", "outbound", {
paths: true, filterVertices: [{_id: "person/person2"}],
vertexFilterMethod: ["exclude"]}) RETURN p.path.vertices[*].name
like this using the more modern pattern matching:
FOR v, e, p IN 1..20 OUTBOUND "person/person3"
FILTER v._ID != "person/person2"
RETURN p.vertices[*].name