StateKey

A typed slot in a graph's thread state.

A is the stored value and U the update a node emits for it. Every update - including a single writer's - is applied to the committed value with applyUpdate, in deterministic task and emission order; replacement is the special case U = A (see StateKey.replace). The function belongs to the compiled graph, never to a checkpoint: checkpoints hold values encoded with stateCodec and pending updates encoded with updateCodec, each tagged with the codec's SchemaVersion so an older checkpoint is migrated before it is decoded.

Keys are compared by identity. A graph rejects two distinct keys with the same id.