# nodus.checkpoint > The checkpoint handshake and gang checkpoints (resources.md §8, distributed-training.md §10.3). Source: https://www.nodus-compute.ai/docs/reference/python/nodus-checkpoint/ Build revision: 53bfc045e800bf7bb44fa18623013c7aa9c9971e The checkpoint handshake and gang checkpoints (resources.md §8, distributed-training.md §10.3). For file snapshots, Nodus owns cadence and storage; programs write their files into `nodus.state_dir()`. Before a snapshot the node sends `checkpoint.request`: `on_request(callback)` saves, then `ack()` tells the node the files are consistent. Restoring files never restores process memory: a restarted program loads what it saved. The collective DCP helper requests step-scoped storage access and publishes only after every process finishes. For a distributed loop, call `dcp.save_if_requested(state, step)` on every global worker at every optimizer boundary. It saves when any worker has a pending request, and acknowledges captured requests only after publication. Use `force=True` for a scheduled save. Never run distributed collectives in an `on_request` background callback. ## `ack` ```python ack(seq: str | None = None) -> None ``` Tell the node the checkpoint files are complete for request `seq` (the latest one by default). Prefer the ID captured by `pending_request()` before saving: acknowledging it leaves a newer request pending. ## `handle` ```python handle(message: dict[str, Any]) -> None ``` Process one message from the events socket. ## `on_request` ```python on_request(callback: Callable[[], None]) -> None ``` Call `callback` (then `ack()`) whenever the node asks for a checkpoint; a no-op outside Nodus. ## `pending_request` ```python pending_request() -> str | None ``` Capture the pending request ID before saving; pass that ID to `ack` after the save completes. A request arriving during the save remains pending. Distributed loops must coordinate saves across every worker; `dcp.save_if_requested` performs that coordination at a boundary every worker reaches. ## `requested` ```python requested() -> bool ``` True while a checkpoint request is pending; poll it at safe points in a training loop. ## `subscribe` ```python subscribe() -> None ``` Listen for checkpoint requests without a callback: poll `requested()` and call `ack()` after saving.