Output Modes

Concepts covered: paStreamProcessing

The beginner tier named the output mode as a writeStream setting and moved on. Here is what it actually decides: of the result table's rows, which ones does the sink receive at the end of each micro-batch? For a query that only transforms, the question sounds trivial, but the moment a query aggregates, the result table contains rows that change as new data folds in, and delivering a changing row is a genuinely different act from delivering a finished one. The three modes are three answers, and each is only legal where its answer makes sense. Append delivers only rows that are final: rows that will never change no matter what arrives later. For a pure filter-and-transform query, every output row is final the moment it is computed, so append is natural and cheap. For an aggregation it is a p

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