What one scan DID — the poll and the drain it triggered, as a single value.

A scan is two phases that always happen together and were reported separately. The poll says what it fetched, what it could not reach and what it cost; the drain returns a ReactiveDag.Drain.Report saying what recomputed downstream. A host wanting "what happened when this ran" had to collect both from one telemetry payload and know which half answered which question.

This is the pair, named. ReactiveDag.ScanWorker builds one per scan and puts it on [:reactive_dag, :scan, :stop], so a broadcast, a durable row or a log line takes one value instead of reassembling it. ReactiveDag.Insights.record/1 retains it whole for the same reason — a host unwrapping to run.report throws away the poll, which is most of what the scan did and all of what it could not reach.

It does not merge the phases

Deliberately. A poll is fallible network I/O that must not abort a sweep when one upstream is down; a drain is pure recomputation that must RAISE rather than mark keys clean over work that did not happen. Those two rules cannot live in one loop, and nothing here tries to make them.

What was genuinely duplicated was the reporting: two vocabularies for "what did this cost", answered by ReactiveDag.Rollup for both. This is the same move one level up — one value for "what did this run do".

The drain may be absent

report is nil for a scan that never drained: an unscannable source (no credential, integration not enabled) is a completed scan that found nothing, and a host recording scan results still wants the row. drained?/1 says which, rather than making every caller test for nil.