Dgraph Glossary
Alpha
A Dgraph cluster consists of Zero and Alpha nodes. Alpha nodes host relationships (also known as 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 storage layer for Dgraph. More at Badger documentation
DQL
Dgraph Query Language is Dgraph’s proprietary language to insert, update, delete and query data. It is based on GraphQL, but is more expressive. (See also: GraphQL)
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 represents a property of a relationship.
Graph
A graph 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](https://graphql.org/) is a declarative language for querying data used by application developers to get the data they need using GraphQL APIs. GraphQL is an open standard with a robust ecosystem. Dgraph supports the deployment of a 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
A Lambda Resolver (Lambda for short) is a GraphQL resolver supported within Dgraph. A Lambda is a user-defined JavaScript function that performs custom actions over the GraphQL types, interfaces, queries, and mutations. Dgraph Lambdas are unrelated to AWS Lambdas.
Mutation
A mutation is a request to modify the database. Mutations include insert, update, or delete operations. A Mutation can be combined with a query to form an Upsert.
Node
Conceptually, a 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 the node has with other nodes.
The term “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 Aplha node and Zero 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
nameand predicate value is the string “Alice”. It becomes a node property. - when we store that Alice knows Bob, we may use a predicate
knowswith the node representing Alice. The value of this predicate would be the uid of the node representing Bob. In that case,knowsis a relationship.
RATEL
Ratel is an open source GUI tool for data visualization and cluster management that’s designed to work with Dgraph and DQL. See also: Ratel Overview.
RDF
RDF 1.1 is a Semantic Web Standard 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 the RDF format to create, import and export data. Note that Dgraph also supports the JSON format.
Relationship
A relationship is a named, directed link relating one node to another. It is the Dgraph term similar to edge and predicate. In Dgraph a relationship may itself have properties representing information about the relation, such as weight, cost, timeframe, or type. In Dgraph the properties of a relationship are called facets.
Sharding
Sharding is a database architecture pattern to achieve horizontal scale by distributing data among many servers. Dgraph shards data per relationship, so all data for one relationship form a single shard, and are stored on one (group of) servers, an approach referred to as ‘predicate-based sharding’.
Triple
Because RDF statements consist of three elements:
- when we store that a node name is “Alice”. The predicate is
nameand predicate value is the string “Alice”. The string becomes a node property. - when we store that Alice knows Bob, we may use a predicate
knowswith the node representing Alice. The value of this predicate would be the uid of the node representing Bob. In that case,knowsis a relationship.
UID
A UID is the Universal Identifier of a node. uid is a reserved property holding the UID value for every node. UIDs can either be generated by Dgraph when creating nodes, or can be set explicitly.
Upsert
An upsert operation combines a Query with a Mutation. Typically, a node is searched for, and then depending on if it is found or not, a new node is created with associated predicates or the exixting node relationships 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.