DataHub uses the Pegasus schema (PDL) language extended with a custom set of annotations to model metadata.
Conceptually, metadata is modeled using the following abstractions
- Entities: An entity is the primary node in the metadata graph. For example, an instance of a Dataset or a CorpUser is an Entity. An entity is made up of a unique identifier (a primary key) and groups of metadata which we call aspects.
- Aspects: An aspect is a collection of attributes that describes a particular facet of an entity. They are the smallest atomic unit of write in DataHub. That is, Multiple aspects associated with the same Entity can be updated independently. For example, DatasetProperties contains a collection of attributes that describes a Dataset. Aspects can be shared across entities, for example the "Ownership" an aspect is re-used across all the Entities that have owners.
- Keys & Urns: A key is a special type of aspect that contains the fields that uniquely identify an individual Entity. Key aspects can be serialized into Urns, which represent a stringified form of the key fields used for primary-key lookup. Moreover, Urns can be converted back into key aspect structs, making key aspects a type of "virtual" aspect. Key aspects provide a mechanism for clients to easily read fields comprising the primary key, which are usually generally useful like Dataset names, platform names etc. Urns provide a friendly handle by which Entities can be queried without requiring a fully materialized struct.
- Relationships: A relationship represents a named edge between 2 entities. They are declared via foreign key attributes within Aspects along with a custom annotation (@Relationship). Relationships permit edges to be traversed bi-directionally. For example, a Chart may refer to a CorpUser as its owner via a relationship named "OwnedBy". This edge would be walkable starting from the Chart or the CorpUser instance.
Here is an example graph consisting of 3 types of entity (CorpUser, Chart, Dashboard), 2 types of relationship (OwnedBy, Contains), and 3 types of metadata aspect (Ownership, ChartInfo, and DashboardInfo).
DataHub’s modeling language allows you to optimize metadata persistence to align with query patterns.
There are three supported ways to query the metadata graph: by primary key lookup, a search query, and via relationship traversal.
Querying an Entity by primary key means using the "entities" endpoint, passing in the urn of the entity to retrieve.
For example, to fetch a Chart entity, we can use the following CURL:
As you'll notice, we perform the lookup using the url-encoded Urn associated with an entity. The response would be an "Entity" record containing the Entity Snapshot (which in turn contains the latest aspects associated with the Entity).
A search query allows you to search for entities matching an arbitrary string.
For example, to search for entities matching the term "customers", we can use the following CURL:
The notable parameters are
input specifies the query we are issuing and
entity specifies the Entity Type we want to search over. This is the common name of the Entity as defined in the @Entity definition. The response contains a list of Urns, that can be used to fetch the full entity.
A relationship query allows you to find Entity connected to a particular source Entity via an edge of a particular type.
For example, to find the owners of a particular Chart, we can use the following CURL:
The notable parameters are
types. The response contains Urns associated with all entities connected
to the primary entity (urn:li:chart:customer) by an relationship named "OwnedBy". That is, it permits fetching the owners of a given
There are 2 "special" aspects worth mentioning:
- Key aspects
- Browse aspect
As introduced above, Key aspects are structs / records that contain the fields that uniquely identify an Entity. There are some constraints about the fields that can be present in Key aspects:
- All fields must be of STRING or ENUM type
- All fields must be REQUIRED
Keys can be created from and turned into Urns, which represent the stringified version of the Key record. The algorithm used to do the conversion is straightforward: the fields of the Key aspect are substituted into a string template based on their index (order of definition) using the following template:
By convention, key aspects are defined under
A CorpUser can be uniquely identified by a "username", which should typically correspond to an LDAP name.
Thus, it's Key Aspect is defined as the following:
and it's Entity Snapshot model is defined as
Using a combination of the information provided by these models, we are able to generate the Urn corresponding to a CorpUser as
Imagine we have a CorpUser Entity with the username "johnsmith". In this world, the JSON version of the Key Aspect associated with the Entity would be
and its corresponding Urn would be
The BrowsePaths aspect allows you to define a custom "browse path" for an Entity. A browse path is a way to hierarchically organize entities. They manifest within the "Explore" features on the UI, allowing users to navigate through trees of related entities of a given type.
To support browsing a particular entity, simply include the "BrowsePaths" aspect in its aspect union:
By declaring this aspect, you can produce custom browse paths as well as query for browse paths manually using a CURL like the following:
Notice that you must provide
a. A "/"-delimited root path for which to fetch results. b. An entity "type" using its common name ("dataset" in the example above).