msyncDerived data, kept in sync

Describe the derivation the way you would write a MongoDB aggregation. msync runs it once over the data you have, then again on every change.

One description, two runs

The same stages run as a full pass over the collection and as a before/after delta on each change.

A renamed account updates the orders that point at it. A deleted order subtracts from the total. The parent is not recomputed from scratch.

A view declares the fields a stream may read. Later stages, lookups, and sinks see that slice and nothing else.