Wikidata/Notes/Entities and Snaks

Loose collection of notes about different kinds of entities and their snaks. This page serves as a place to collect brainstorming and discussion notes that can be used later to amend and extend the Wikibase data model specification.

Note: Terminology used in this notes page, may be different from the one used on the actual data model spec. Needs to be synced.

property refers to the property definition. E.g. the property population is an entity defined on a wiki page. It exists on one, but can be used (instantiated) on multiple entities.
a simple snak refers to the use of a property to assign a plain value to an entity or other structure. Snaks are not given with references, that is, the provenance for a snak is the Wikidata community itself. E.g. the page number in a source reference is an snak; it assigns a plain value (a number) to a property instance ("page"). Note that snaks may be multi-valued. Sometimes other Snaks but PropertyValueSnaks are allowed.
special or system snaks are well-known, pre-defined snaks with specific properties that may be represented and handled differently from ordinary snaks. Examples for such properties are: label, description, alias, data-type, categories, etc.
a Claim is a sets of snaks; Claims have at least one snak; additional snaks are considered qualifiers.
Statements consist of a claim and a (special, multi-value) source-reference snak, and a rank.
Qualifiers are additional snaks of claims, see above.
Source-Reference is a (special) snak of statements, with a reference record as a data type.
Each Source-Reference is a ordered list of snaks, e.g. book (a reference to an item) and page (a number)

All special snaks shall be at least readable like regular snaks.
All (special and regular) snaks shall be at least readable as statements. Such statements will have a special source reference that refers to the local wiki's community itself.
Should statements also be available as snaks, for simplicity? Probably not, that would be misleading.

Properties are defined as entities, each property on its own wiki page in the property namespace.
Properties have the special snaks label, description and data-type.
Properties may have any number of additional snaks, such as help-page or muti-value. Some of these may be known to the system and have a special meaning.
All properties defined this way can be used to create statements about items, or qualifiers in claims, or snaks in source references, or snaks of properties, queries, and so on.

Queries are defined as entities in their own namespace, using snaks, just like properties.
Which snaks should be present in a Query, and what they mean, remains to be determined.

Other entities that may at some point also be defined like this include:

Item templates (schemas)
Reference templates (schemas)
Data types