Unset sentinel value.
Base class for all channels.
Raised when attempting to update a channel with an invalid set of updates.
Troubleshooting guides:
Reducer channel that stores only a sentinel in checkpoint blobs and reconstructs state by replaying ancestor writes through the reducer.
DeltaChannel is in beta. The API and on-disk representation may
change in future releases. Threads written with DeltaChannel today
are expected to remain readable, but the surrounding contract
(BaseCheckpointSaver.get_delta_channel_history, the
_DeltaSnapshot blob shape, the counters_since_delta_snapshot
metadata field) is not yet stable.
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 cadence is driven by two counters: per-channel update count and
total supersteps since last snapshot. create_checkpoint writes a full
_DeltaSnapshot blob when EITHER the update count reaches
snapshot_frequency OR the supersteps count reaches the system-wide
DELTA_MAX_SUPERSTEPS_SINCE_SNAPSHOT bound (default 5000), bounding
replay depth even for channels that stop receiving writes.