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Reducer channel that stores only a sentinel in checkpoint blobs and reconstructs state by replaying ancestor writes through the reducer.
DeltaChannel avoids re-serializing the full accumulated value at every
step. Instead of writing the value into channel_values, the channel is
omitted entirely and its state is reconstructed on read by walking the
ancestor chain and replaying the per-step writes through the reducer (see
BaseCheckpointSaver.getDeltaChannelHistory).
Snapshot cadence is driven by two counters: a per-channel update count and
the total supersteps since the last snapshot. A full DeltaSnapshot
blob is written when EITHER the update count reaches snapshotFrequency 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.
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 (getDeltaChannelHistory, the
DeltaSnapshot blob shape, the counters_since_delta_snapshot metadata
field) is not yet stable.
The name of the channel.
Return a string representation of the channel's current state.
Mark the current value of the channel as consumed. By default, no-op. A channel can use this method to modify its state, preventing the value from being consumed again.
Returns True if the channel was updated, False otherwise.
Compare this channel with another channel for equality. Used to determine if two channels with the same key are semantically equivalent. Subclasses should override this method to provide a meaningful comparison.
Notify the channel that the Pregel run is finishing. By default, no-op. A channel can use this method to modify its state, preventing finish.
Returns True if the channel was updated, False otherwise.
Return a new identical channel, optionally initialized from a checkpoint. Can be thought of as a "restoration" from a checkpoint which is a "snapshot" of the channel's state.
Return the current value of the channel.
Return True if the channel is available (not empty), False otherwise. Subclasses should override this method to provide a more efficient implementation than calling get() and catching EmptyChannelError.
Apply ancestor writes oldest-to-newest via a single reducer call.
If any write is an Overwrite, the last one in the sequence acts as the reset point: its value becomes the new base and only writes after it are passed to the reducer.
Update the channel's value with the given sequence of updates. The order of the updates in the sequence is arbitrary. This method is called by Pregel for all channels at the end of each step. If there are no updates, it is called with an empty sequence.
Raises InvalidUpdateError if the sequence of updates is invalid. Returns True if the channel was updated, False otherwise.
import { Annotation } from "@langchain/langgraph";
import { DeltaChannel, messagesDeltaReducer } from "@langchain/langgraph";
const State = Annotation.Root({
messages: Annotation<BaseMessage[]>({
reducer: () => [], // ignored; DeltaChannel is supplied below
}),
});