The single-writer engine.

The floor begins with a volume, and a volume has exactly one writer. EchoStore.Graft.VolumeServer is one process per volume, and its mailbox is Graft’s global write lock: every commit to that volume passes through one GenServer, serialized by the BEAM’s mailbox, so there is no lock to acquire and no contention to lose. A write begins, stages its pages, and commits; the pages of one business write land at low indices, a page per table row counting up from zero.

The store beneath the writer is CubDB, an append-only immutable B-tree. Entries are never changed in place; a write appends, and a read takes a zero-cost immutable snapshot, so concurrent reads neither block writes nor are blocked by them. A Graft snapshot is a CubDB snapshot — the same structure gives the engine its consistency and the reader its isolation, and it is pure Elixir, with no C extension to build or trust.

Identity runs through the engine as it runs through the rest of BCS. A volume, a segment, and a commit each carry a branded GID — VOL, SEG, CMT — minted from the same Snowflake the cache and the bus use, so a commit sorts by when it happened and a segment names itself on the wire. The engine is single-writer where it must be and immutable where it can be; the rest of the floor is what that one commit becomes once it leaves the node.