Reducer channel that stores only a sentinel in checkpoint blobs and reconstructs state by replaying ancestor writes through the reducer.
The reducer receives the current accumulated value and a batch of writes
in one call: reducer(state, [write1, write2, ...]) -> new_state.
Reducers must be deterministic and batching-invariant (associative across folds): applying two consecutive write batches separately must produce the same state as applying their concatenation once:
reducer(reducer(state, xs), ys) == reducer(state, xs + ys)
This lets LangGraph replay checkpointed writes in larger batches than they were originally produced without changing reconstructed state.
snapshot_frequency=None (default): pure delta; stores only
DELTA_SENTINEL in checkpoint blobs; reads replay all ancestor writes.
snapshot_frequency=N: create_checkpoint writes a full _DeltaSnapshot
blob every N steps, bounding replay depth to N.
The value type (e.g. list, dict). Inferred automatically
from the outer type when used inside Annotated[T, DeltaChannel(...)].
Every Nth pregel step writes a snapshot blob.
None (default) = pure delta, never snapshot.