The Catalog

A catalog is a set of data sources that contained in a single other data source

Design Rationale

Data sources are always "contained" in something else. Be it a simple folder, a ftp location or even the list of available data sources from a service. They belong, often temporarily, in that container. The container is the context in which they operate and are addressed. This container is labeled 'Catalog' in Ilwis.

The catalog concept allows for a coherent view on the hierarchical organization of data. It hides the underlying physical structure of containers and thus simplifies addressing locations in the system.

Description

Basically a catalog is a list of data sources that can be translated into ilwis objects. As a Catalog itself is an ilwis object, it can contain other catalogs. Catalogs do not contain non-ilwis objects even if the container (on which the catalog maps) contains them, they are not part of the set. Catalogs can not be filtered, they always show their full content. CatalogView, a different concept, can be filtered.

Catalogs come in many different guises. some examples

• The simpel folder on disk. It may contain other folders or other Catalogs.
• A zip (or other archive format) file. A zip file can contain zipped folders and other catalogs.
• An Hdf5 file. In a sense similar to a zip file. Hierarchical organization of data sources
• WFS getcapabilities. A list of Featureatypes available on a wfs server.
• GPX file. multiple feature data sets (lines/points) in a file.

At the backend Catalogs are supported by a catalog connector. This catalog connector contains one or more objects that are called catalog-explorers. The explorers are tied to a specific set of data source types it can recognize. For example a gdal-catalog-explorer is able to recognize the formats that the gdal library recognizes but no others. While the WFS catalog explorer is able to query wfs services for their content.