69.3 SpaceDirectory

Logically, a SpaceDirectory maps 32-bit SpaceId values to Spaces and their RPM roots. Each Partition contains a SpaceDirectory for its Spaces.

The SpaceDirectory supports MVCC snapshots associated with transactions on its containing Partition. A published directory snapshot identifies the RPM roots for its Spaces at that partition transaction boundary.

An MVCC snapshot of a SpaceDirectory entails an MVCC snapshot of every RPM referenced by that directory snapshot. The directory snapshot therefore represents one consistent mapping across all Spaces in the containing Partition.

The SpaceDirectory is implemented using the same variable-height RPM as a Space. An empty directory has height zero; a nonempty directory has between one and four levels.