## Summary
Replaces the single-roundtrip `UNION ALL` DeltaChannel read with a
two-stage query that avoids fetching unused snapshot blobs, then removes
the old combined path entirely.
### Problem
`_get_channel_writes_history` used a single `UNION ALL` query that
fetched **all** checkpoint metadata, writes, and blobs for a
`(thread_id, channel)` in one shot. With `snapshot_frequency=N`, this
pulled back O(N/freq) full-size snapshot blobs even though only the
nearest one is needed to seed reconstruction. At 500 turns with
`snapshot_frequency=10`, this meant fetching ~100 complete
message-history snapshots per read.
### Solution
Two-stage read:
- **Stage 1** — lightweight scan of `checkpoints` only (no blob bytes):
walks the parent chain from the target checkpoint and stops at the first
ancestor with a snapshot, returning `chain_cids` and `seed_version`
- **Stage 2** — targeted fetch: only the writes for `chain_cids` and the
single seed blob at `seed_version`
The two-stage path is now unconditional — the old combined query and
`LG_DELTA_TWO_STAGE_QUERY` env-var gate have been removed.
### Sentinel cleanup
`DELTA_SENTINEL` is now a pure in-memory signal and is never written to
storage:
- Postgres `put()` already stripped it from `channel_values` before
writing blobs
- Memory saver `put()` now stores `"empty"` instead of serializing the
sentinel
- `EXT_DELTA_SENTINEL` (msgpack ext code 8) removed from
`JsonPlusSerializer`
- `DELTA_SENTINEL` is kept as an in-memory marker:
`DeltaChannel.checkpoint()` returns it so savers know to skip it, and
`_ChannelWritesHistory.seed` uses it to mean "no snapshot found, start
from empty"
## Performance
Benchmarked at `snapshot_frequency=10` on Postgres (`~100 tok/msg`):
| turns | old combined query | two-stage |
|------:|-------------------:|----------:|
| 50 | 6.0ms | 2.8ms (2.1x faster) |
| 100 | 10.1ms | 5.6ms (1.8x faster) |
| 500 | **216.1ms** | 15.3ms (**14x faster**) |
The old query's read time grew super-linearly with turn count because
each read fetched O(N/freq) full snapshot blobs. Two-stage keeps read
depth bounded by `snapshot_frequency` regardless of thread length.
## Test plan
- `make test` in `libs/checkpoint`, `libs/checkpoint-postgres`,
`libs/langgraph`
- Removed `test_delta_sentinel_serde_round_trip` (sentinel no longer
serializable)
- Updated `test_memory.py` — delta channel blobs stored as `"empty"`,
not serialized sentinel
- Updated `test_channels.py` — `channel_values` no longer contains
sentinel key for DeltaChannels
- Deleted `test_delta_channel_two_stage_benchmark.py` (one-stage vs
two-stage comparison; path no longer exists)
---------
Co-authored-by: Sydney Runkle <54324534+sydney-runkle@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
- Leave it up to each checkpointer implementation to decide whether to merge in configurable/metadata (previously PregelLoop would do some of this always)
- Never copy over internal langgraph keys into checkpoint.metadata (these are redundant/misleading to include)
Prepare langgraph-checkpoint for 0.5
- Given we have no upper bound on langgraph-checkpoint dep need to undo all changes in langgraph-checkpoint that might break previous versions of langgraph
- This has been superseded by saving the individual writes of each task through put_writes()
- Removing this speeds up checkpoint operations as it was duplicating data saved elsewhere already
- Instead store sends in a Topic channel, removing the need to fetch sends as writes against the parent checkpoint
- Remove deprecated/unused functions in langgraph-checkpoint (will require bumping min range for langgraph-checkpoint in langgraph lib)
- Implement migration of old pending sends in langgraph-checkpoint-postgres
- Ensure parent config of `checkpoint_during=False` checkpoints always points to checkpoints that were also saved
- This makes our checkpoint benchmarks more closely resemble the behavior of our prod checkpointers
- Also found and fixed a bug w multiple subgraphs in same node accidentally sharing checkpoints
(Keep MemorySaver around for backwards compatibility)
"MemorySaver" is ambiguous: is it saving memories? Where is it saving
memories to?
InMemorySaver aligns naming InMemoryStore as well as similar LangChain
objects (InMemoryVectorStore, etc.)
