This section introduces the different types of elements of a GeoGig repository so as to give you a better understanding of how the repository is structured.
A GeoGig repository can contain the following elements:
- Features. A feature represents a geographical element and contains a given set of attributes and a geometry. A feature is the equivalent to a shape in a shapefile or a single record in a database.
- Trees. A tree represents a subdivision used to group features with a certain criteria. It is the equivalent of a shapefile or a table in a database. Trees can be nested so they might also serve as folders.
- Commits. A commit is generated each time some data is written (“committed”) from the staging area into the repository database. A commit points to the data after that commit (in other words, it points to a tree under which the data is found), so it represents a given snapshot of the data. You can return to how your data was at a certain point in time by viewing the data at its corresponding commit.
All these objects are stored in GeoGig and referenced by an ID. You can use that ID to refer to the object anytime and in any of the commands available in GeoGig when required. The ID is a 40 character string. For example,
509a481257c5791f50f5a35087e432247f9dc8b7 is a valid ID. All IDs are unique.
In GeoGig, even in the working tree, all elements are given an ID.
Object IDs will be used when describing the different commands used to work on a GeoGig repository.
Another element found in a GeoGig repository is a ref. A ref, or reference, is a short string that references a given element in the GeoGig repository, which can be an object of any of the types described above. Think about it as the GeoGig equivalent of a symbolic link or a shortcut.
There are three main refs that are used:
WORK_HEAD: References the working tree
STAGE_HEAD: References the staging area
HEAD: References the current state of the GeoGig repository.
As you see, they correspond to the three areas in GeoGig.
Whenever you make a commit and add new data to the GeoGig repository, the
HEAD reference is changed to the latest commit.
HEAD reference is an indirect reference, so it is actually slightly more involved, but for now it can be thought of as referring to the latest commit in the repository.
Some GeoGig commands will create other refs as part of their operations.
Trees, features, and feature types¶
A feature contains a set of attributes and these attributes must correspond to a given feature type, also stored in the GeoGig repository. For this reason features are stored along with the ID pointing to their feature type.
A tree in a GeoGig repository can only contain features or other trees. A feature in a GeoGig repository can be seen as a rough equivalent of a file in a file system.
All trees have an associated feature type, and in most cases, all features under a tree share the same feature type. The tree’s feature type is considered to be the default type of the features under the tree, but features are not restricted to it. It is possible to add features with a different feature type to a tree.
Although it is the most common case, a tree is not the same as a feature type. Both trees and features have an associated feature type and feature types are stored independently.