(Subtask to achieve SERVER-84243 - Dedicate a catalog cache and loader to the shard role. For more information refer to this document)

Today, FilteringMetadataCache has a shared_ptr<CatalogCacheLoader> as member of the class, representing the ShardServerCatalogCacheLoader implementation, but accepting the injection of the CatalogCacheLoaderMock (testing purposes) or the ReadOnlyCatalogCacheLoader (queryableBackupMode).

This implies having some dynamic cast - from the loader pointer to ShardServerCatalogCacheLoader - to use some of the methods only implemented by the last.

The goal of this ticket is to create a better architecture, by creating a new interface that implements all the necessary methods that the FilteringMetadataCache’s loader needs, consequently removing all the dynamic casts, and breaking the dependency of the pareten interface CatalogCacheLoader.

Therefore, today:

• CatalogCacheLoader (Interface)
  • ConfigServerCatalogCacheLoader
  • ShardServerCatalogCacheLoader
  • ReadOnlyCatalogCacheLoader
  • CatalogCacheLoaderMock

The proposed architecture is:

• ConfigServerCatalogCacheLoader (Interface)
  • ConfigServerCatalogCacheLoaderImpl
  • ConfigServerCatalogCacheLoaderMock
• ShardServerCatalogCacheLoader (Interface) -> deprecated once shards are authoritative
  • ShardServerCatalogCacheLoaderImpl
  • ReadOnlyCatalogCacheLoader
  • ShardServerCatalogCacheLoaderMock

Open question: Do we want to rename the ShardServerCatalogCacheLoader and ConfigServerCatalogCacheLoader to RouterRoleCatalogCacheLoader and ShardRoleCatalogCacheLoader?