Return Partial Data

[Frencil]

If a data layer is requesting data from multiple sources and only one of those sources fails, try to find a way to still display what data was received.

A good example of where this would help: Positions data layer that pulls down positions from one source and LD from another. Since the LD data is only used to color positions, and the coloring function will return a default color if passed an undefined value, ideally it would be nice to still see the default-colored positions data if the LD source is not responding.

In practice this may be very difficult to pull off since the data layer makes one request and LocusZoom.Data does the logic to generate actual requests to each source necessary given the fields requested. Right now it's an all-or-none type thing, where either all data is returned or an error is passed through. To return partial data means potentially returning both data and errors in some cases, which does not have clearly defined behavior.

[abought]

The LD example is a very good one, because it revealed some subtle distinctions between errors and dependencies.

Addressed by https://github.com/abought/locuszoom/blob/619da458d2f4e663d1256515af7ea3b608d442f9/assets/js/app/Data.js#L639

Design solution record as follows:

• If any source completely fails to load, show an error
• If a source successfully returns, but has no data in that region, allow other data layers to render
• If a second source is completely dependent on a previous request (that returned no data of interest), skip the second request. This is because there is nothing to combine with.