API version: dev

---

# Elements

Ember Nexus is built on top of the property graph database [Neo4j](https://neo4j.com/), which means many of the
API's own concepts are graph-like too. Understanding a few core graph database concepts makes the rest of the API
much easier to reason about.

## Nodes and relationships

The core building block of a graph database is the [node](https://neo4j.com/developer/cypher/intro-cypher/#cypher-nodes):
a record that can carry zero or more types (comparable to labels) and any number of properties. A node can be
compared to a row in a relational database table.

A [relationship](https://neo4j.com/developer/cypher/intro-cypher/#cypher-relationships) connects exactly two nodes,
has exactly one type, and can carry properties of its own, just like a node. A relationship can be compared to a
foreign key in a relational database, except that it's a first-class object with its own identity and data.

```json graph-example
[
  {"id":"start","type":"Data","data":{"name":"Node"}},
  {"id":"end","type":"Data","data":{"name":"Node"}},
  {"id":"related","type":"RELATED","start":"start","end":"end","data":{}}
]
```

## Elements

Ember Nexus combines nodes and relationships into a single concept: the **element**. Every element is either a node
or a relation, and both are addressed, created and modified through the same set of
[element endpoints](/reference/endpoints.md#element).

Every element:

- has at least one `type`, which is a label in the case of a node, and the relation type in the case of a relation;
- has an `id` property, a random [UUIDv4](https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_\(random\))
  assigned automatically on creation and never changed afterward;
- can hold arbitrary additional properties, including nested JSON data — not just flat key/value pairs; and
- may additionally have up to one file attached, see [working with files](/guide/working-with-files.md).

Relations additionally reference a `start` and an `end` element, both of which must be nodes.

## Where elements live in the graph

Every element except the root has at least one parent, forming a tree-like ownership structure on top of the graph.
This structure is what [access control](/reference/access-model.md) and the
[`GET /<uuid>/parents`](/api/get-parents.md) / [`GET /<uuid>/children`](/api/get-children.md) endpoints are built around,
while [`GET /<uuid>/related`](/api/get-related.md) and [search](/reference/search.md) let you traverse relationships
directly, independent of that ownership tree.

---

Home: [Ember Nexus API docs](/index.md)

Full site map: [llms.txt](/llms.txt)

Previous: [Authentication](/guide/authentication.md)

Next: [Search](/guide/search.md)