GetData

Reads one or more values from data sources and emits them downstream, on the IO dispatcher. Each readings entry (built with reading) pairs a data source with an access mode; readings are processed in order into a single accumulator, so a later reading overwrites an earlier one on key collision. Full access mode, and Batch without unwrapValuesToList, produce a map keyed by data id; Batch with unwrapValuesToList produces a list of values; with only Single readings, one reading emits the bare value and several emit a list — a map result wins over a list result when the two are mixed. Does not consume incomingData. Dispatches onStart before any reading runs, onSuccess (carrying the assembled result) once every reading resolves, and onFailure on the first problem — a Single id resolving to null, a missing Batch id with allowMissingData false, or a database read failure.

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