CatalogHolder

The one mutable thing in the type system: which TypeCatalog is the current one for a database.

A catalog is a value and is replaced whole rather than edited, so something has to hold which value is current, and that something cannot be the catalog. This is it, and it is deliberately the whole of it - it does not know that a converter or a codec exists, only how to put one catalog in the place of another, and, where the database is listed as several servers, which cluster those catalogs are read from.

lock guards more than the swap, which is why it is a lock rather than a compare-and-set: GlobalCatalogStore.ensureLoaded holds it across the round trip that reads the catalog out of the database, so twenty connections opening at once read it once between them. A lock that spans that has to live on something outlasting any single catalog, which is the second reason this class exists.