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## Summary
Adds cooperative drain support for Pregel runs so a graph can be asked
to stop at the next superstep boundary, persist its checkpoint, and
surface a resumable terminal exception.
- New `RunControl` (in `langgraph.runtime`) — a thread-safe handle whose
`request_drain(reason="shutdown")` sets a single flag.
- New `GraphDrained(GraphBubbleUp)` exception (in `langgraph.errors`)
raised when a run exits early due to drain. Carries the `reason` string.
- New `control: RunControl | None` kwarg on `invoke` / `ainvoke` /
`stream` / `astream` / `stream_v2` / `astream_v2`. Wired through to
`Runtime.control`, so nodes can read `runtime.control.drain_requested` /
`drain_reason` and even call `request_drain()` from inside a node.
- Stream transformers learn `"drained"` as a terminal `SubgraphStatus`.
The intended use is hooking SIGTERM (or any external supervisor signal)
to `control.request_drain("sigterm")` so an in-flight graph run can stop
cleanly and be resumed later from the saved checkpoint.
## Semantics: cooperative, between-superstep
`request_drain()` flips a flag. The Pregel loop checks it at the top of
each `tick()`, **after** the previous superstep's writes have been
applied and checkpointed. It never preempts work that is already
running.
| Scenario | Behavior |
|---|---|
| Node mid-execution (blocking I/O, sleeps, etc.) | Runs to completion.
Drain takes effect on the next superstep. |
| Node with a retry policy currently retrying | Retry loop runs to
exhaustion or success (drain is not checked between retries). Drain
takes effect on the next superstep. |
| Functional API: `@entrypoint` with pending `@task` futures |
Entrypoint and all dispatched tasks complete; drain takes effect after
the entrypoint returns. |
| Graph naturally finishes on the same tick where drain was requested
(no more tasks) | Treated as `done`; returns normally. **No
`GraphDrained` is raised.** The caller can inspect
`control.drain_requested` afterwards to distinguish a
drained-but-completed run from a normal one. |
| More tasks remain | Raises `GraphDrained(reason)`. The checkpoint of
the last completed superstep is saved (also under `durability="exit"`).
Resume with `invoke(None, config)` / `ainvoke(None, config)`. |
| Subgraph requests drain | `GraphDrained` bubbles up through the parent
loop and stops it at its own next superstep boundary; the parent's
checkpoint is saved and resumable. |
Drain does **not** cancel asyncio tasks or kill threads. Pair it with a
graceful timeout + `task.cancel()` (or process exit) if you need a hard
upper bound — see `test_drain_then_cancel_after_graceful_timeout` for
the recommended pattern.
## Usage
```python
from langgraph.runtime import RunControl
from langgraph.errors import GraphDrained
control = RunControl()
# In a signal handler, supervisor, etc.:
# control.request_drain("sigterm")
try:
result = graph.invoke(input, config, control=control)
if control.drain_requested:
# finished naturally on the same tick where drain was requested
...
except GraphDrained as e:
# checkpoint saved; resume later with the same config
log.info("graph drained: %s", e.reason)
```
## Test plan
- [x] Sync + async drain stops the next superstep
(`test_run_control_request_drain_stops_future_steps[_async]`)
- [x] Drain on the terminal step finishes normally
(`test_drain_requested_in_terminal_step_finishes_normally[_async]`)
- [x] `durability=\"exit\"` persists a resumable checkpoint on drain
(`test_drain_with_exit_durability_persists_resume_checkpoint`)
- [x] Subgraph drain bubbles up and parent resumes correctly
(`test_drain_from_subgraph_can_resume_parent`)
- [x] External thread / task triggering drain mid-run
(`test_external_drain_concurrent_sync` / `_async`)
- [x] Drain + hard cancel after graceful timeout
(`test_drain_then_cancel_after_graceful_timeout`)
- [x] Functional API: in-flight `@task` futures still resolve after
`request_drain()`
(`test_request_drain_allows_inflight_[a]call_scheduling`)
- [x] `control` kwarg wired through `stream_v2`
(`test_stream_v2_accepts_control_for_drain`)
- [x] `Runtime.merge` preserves `control`
(`test_merge_runtime_preserves_run_control`)
---------
Co-authored-by: Quanzheng Long <long@langchain.dev>
Co-authored-by: Will Fu-Hinthorn <will@langchain.dev>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Low-level orchestration framework for building stateful agents.
Trusted by companies shaping the future of agents – including Klarna, Replit, Elastic, and more – LangGraph is a low-level orchestration framework for building, managing, and deploying long-running, stateful agents.
pip install -U langgraph
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Note
Looking for the JS/TS library? Check out LangGraph.js and the JS docs.
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Tip
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Acknowledgements
LangGraph is inspired by Pregel and Apache Beam. The public interface draws inspiration from NetworkX. LangGraph is built by LangChain Inc, the creators of LangChain, but can be used without LangChain.