Wrapping Datasets

Wrapping a new dataset is challenging because you have to understand all of the quirks of the new dataset and deal with the quirks of PODPAC as well. This reference is meant to record a few rules of thumb when wrapping new datasets to help you deal with the latter.

Rules

  1. When evaluating a node with a set of coordinates:

    1. The evaluation coordinates must include ALL of the dimensions present in the source dataset

    2. The evaluation coordinates MAY contain additional dimensions NOT present in the source dataset, and the source may ignore these

  2. When returning data from a data source node:

    1. The ORDER of the evaluation coordinates MUST be preserved (see UnitsDataArray.part_transpose)

    2. Any multi-channel data must be returned using the output dimension which is ALWAYS the LAST dimension

  3. Nodes should be lightweight to instantiate and users should expect fail on eval. Easy checks should be performed on initialization, but anything expensive should be delayed.

Guide

In theory, to wrap a new DataSource:

  1. Create a new class that inherits from podpac.core.data.DataSource or a derived class (see the podpac.core.data module for generic data readers).

  2. Implement a method for opening/accessing the data, or use an existing generic data node and hard-code certain attributes

  3. Implement the get_coordinates(self) method

  4. Implement the get_data(self, coordinates, coordinates_index) method

    1. coordinates is a podpac.Coordinates object and it’s in the same coordinate system as the data source (i.e. a subset of what comes out of get_coordinates)

    2. coordinates_index is a list (or tuple?) of slices or boolean arrays or index arrays to indexes into the output of get_coordinates() to produce coordinates that come into this function.

In practice, the real trick is implementing a compositor to put multiple tiles together to look like a single DataSource. We tend to use the podpac.compositor.OrderedCompositor node for this task, but it does not handle interpolation between tiles. Instead, see the podpac.core.compositor.tile_compositor module.

When using compositors, it is prefered the that sources attribute is populated at instantiation, but on-the-fly (i.e. at eval) population of sources is also acceptible and sometimes necessary for certain datasources.

For examples, check the podpac.datalib module.

Happy wrapping!