69.4 Space

A Space is a lightweight logical namespace containing serial elements indexed by 32-bit Seids. Each Space belongs to a Partition and is indexed within that partition by a 32-bit SpaceId.

The LSS API represents a Space in an immutable partition snapshot with an ISpaceView. It is owned by its IPartitionView and directly refers to the Space state in that snapshot. A client can therefore call FindSpace() once and use the returned interface for repeated reads without repeatedly looking up the SpaceId in the SpaceDirectory's radix map.

A partition transaction similarly owns an IMutableSpace for each Space it accesses. It initially refers to the state selected from the transaction's base snapshot. On first mutation, copy-on-write may create mutable Space state and make the same interface refer to that copy. The interface is local to the transaction and becomes invalid when the transaction is closed.

The next Seid used for sequential allocation belongs to the Space, while the RPM owns the radix mapping from Seids to log-record positions:

struct Space
{
    SpaceId id;
    Seid nextSeid;
    RPM rpm;
};

A checkpointed or published Space state contains both the allocation state and the corresponding RPM root. Readers use the RPM root; the writer uses nextSeid for allocation.

The allocation state and RPM are separate but coupled. Every explicitly reserved or inserted Seid must be less than nextSeid. Inserting a caller-supplied Seid therefore goes through IMutableSpace, which advances nextSeid when required as well as updating the RPM.