The _configOpTime used in Database, Collection and Chunks metadata to ensure that DBConfig <= CollectionInfo <= ChunkManager configOpTime,
Sometimes the check for those invariants is too strong: e.g. https://github.com/mongodb/mongo/blob/13271aa2d75ff3c15444aad8b4e0d67837e113bc/src/mongo/s/config.cpp#L411-L421

configOpTime setting/getting and comparison responsibilities should be encapsulated in a new class for example that can be used as a base for DBConfig, CollectionInfo, ChunkManager.