CatalogWatcher

The change detection of the catalog: every interval, it lists each database's tables and views (their uuid, metadata modification time and a fingerprint of their columns, system.tables and system.columns: the time is in seconds, so an ALTER in the second of the last look shows only in the columns) and when each last got a wow_catalog row, and sends SyncSource for each table whose listing changed since it last looked, or that is gone: the source's aggregate reads the table again and applies the trust rules. Its first look of a database, at startup, syncs every table, which changes nothing where nothing changed, and every table the catalog holds a source of there (sources) that the warehouse no longer lists: one dropped while the catalog was down is marked missing too.

It sends through the node's CommandClient as CALLER, a catalog administrator (SyncSource reads only what the warehouse says, so whoever sends it changes nothing the warehouse does not). A failed look is logged and tried again at the next one.