Dgraph Glossary

Alpha

Dgraph consists of Zero and Alpha nodes. Alpha nodes host predicates and indexes. Dgraph scales horizontally by adding more Alphas.

Badger

Badger is a fast, open-source key-value database written in pure Go that provides the disk layer for Dgraph. More at Badger documentation

DQL

Dgraph Query Language is Dgraph’s proprietary language to insert, update, delete and query data.

Edge

In the mental picture of a graph: bubbles connected by lines ; the bubbles are nodes, the lines are edges. In Dgraph terminology edges are relationships i.e an information about the relation between two nodes.

Facet

A facet represent a property of a relationship.

Graph

A graphs is a simple structure that maps relations between objects. In Dgraph terminology, the objects are nodes and the connections between them are relationships.

GraphQL

GraphQL is a declarative language for querying data used by application developers to get the data they need using GraphQL APIs. Dgraph supports the deployment of GraphQL data model (GraphQL schema) and automatically exposes a GraphQL API endpoint accepting GraphQL queries.

gRPC

gRPC is a high performance Remote Procedure Call (RPC) framework used by Dgraph to interface with clients. Dgraph has official gRPC clients for go, C#, Java, JavaScript and Python. Applications written in those language can perform mutations and queries inside transactions using Dgraph clients.

Lambda

Dgraph takes care of all the logic to execute GraphQL queries when a GraphQL data model is deployed. Lambda provides a way to extend this capability and to write your custom logic integrated with Dgraph execution model.

Mutation

A mutation is a request to modify server-side data. It is covering insert, update, or delete operations.

Node

An node is “a thing” or an object of the business domain. For every node, Dgraph stores and maintains a universal identifier UID, a list of properties and the relationships this node has with other nodes.

Node is also used in software architecture to reference a physical computer or a virtual machine running a module of Dgraph in a cluster. See for example Aplha node.

Predicate

In RDF terminology, a predicate is the smallest piece of information about an object. A predicate can hold a literal value or can describe a relation to another entity :

• when we store that an entity name is “Alice”. The predicate is name and predicate value is the string “Alice”. It becomes a node property.
• when we store that Alice knows Bob, we may use a predicate knows with the node representing Alice. The value of this predicate would be the uid of the node representing Bob. In that case, knows is a relationship.

RATEL

Ratel is an open source tool for data visualization and cluster management that’s designed to work with Dgraph and DQL. More at Ratel User’s guide.

RDF

RDF 1.1 is a Semantic Web Standards for data interchange. It allows us to make statements about resources. The format of these statements is simple and in the form of <subject>> <predicate> <object>. Dgraph supports RDF format to create, import and export data. Note that Dgraph also supports JSON format.

Relationship

A relationship is simply an information that a node is related to another node. A relationship is directed from one node to another and has a name. It may also have properties usually used to store quantitative information about the relation, such as weigh or cost. In Dgraph the properties of a relationship are facets.

Sharding

Sharding is a database architecture pattern to achieve horizontal scaling allowing near-limitless scalability. This allows for data to be split and stored in multiple data nodes or servers and for nodes to be added when required to share the load. Dgraph colocates data per predicate, an approach referred to as ‘predicate sharding’.

Triple

Because RDF statements consist of three elements <subject> <predicate> <object> they are called triples.

UID

UID is the Universal Identifier of a node. uid is a reserved property holding the UID value. UIDs are generated by Dgraph when creating nodes.

Upsert

An upsert operation is an operation where an entity is searched for, and then depending on if it is found or not, a new entity is created with associated predicates or the entity predicates are updated. Upsert operations are important to implement uniqueness of predicates.

Zero

Dgraph consists of Zero and Alpha nodes. Zero nodes control the Dgraph database cluster. It assigns Alpha nodes to groups, re-balances data between groups, handles transaction timestamp and UID assignment.