Roll child windows up to parent state.

Problem

Parent state is often implied by child state.

A zone may be offline because any device in it is offline. A region may be impacted because any zone is impacted. If every parent state is recalculated later in a separate reporting query, the parent timeline can drift away from the child evidence that created it.

Roll-ups record the parent state at the same temporal level as the child state. That means parent windows can be queried, compared, exported, and explained with the same APIs.

Solution

Declare parent activity from child contribution windows.

A roll-up watches the child windows produced by the pipeline and opens a parent window when the child aggregate satisfies the rule. The rule can be simple, such as active count greater than zero, or it can represent quorum, threshold, or health logic.

var pipeline = Spanfold.Spanfold
    .For<DeviceStateChanged>()
    .RecordWindows()
    .Window("DeviceOffline", window => window
        .Key(update => update.DeviceId)
        .ActiveWhen(update => update.IsOffline)
        .Segment("lifecycle", segment => segment.Value(update => update.Lifecycle)))
    .RollUp(
        "ZoneOffline",
        update => update.ZoneId,
        children => children.ActiveCount > 0,
        segments => segments.Preserve("lifecycle"))
    .RollUp(
        "RegionOffline",
        update => update.RegionId,
        children => children.ActiveCount > 0)
    .Build();