Module checkpoint 

Chandy-Lamport barrier checkpointing for DAG pipelines.

Note: This module is used by the DAG execution model only. The TPC pipeline path uses CheckpointCoordinator and barrier injection via CheckpointBarrierInjector. Both models share the same barrier types from crate::checkpoint::barrier.

This module implements barrier-based checkpointing:

• CheckpointBarrier — marker injected at source nodes
• BarrierAligner — buffers events at fan-in (MPSC) nodes until all upstream inputs have delivered their barrier
• DagCheckpointCoordinator — Ring 1 orchestrator that triggers checkpoints, tracks progress, and produces snapshots
• DagCheckpointConfig — tuning knobs (interval, timeout, retention)

Barriers do NOT flow through event queues — they are handled by a separate orchestration path, keeping the hot-path Event type unchanged.

Structs

BarrierAligner

Buffers events at fan-in (MPSC) nodes until all upstream inputs have delivered their checkpoint barrier.

CheckpointBarrier

A checkpoint barrier injected at source nodes and propagated through the DAG.

DagCheckpointConfig

Configuration for DAG checkpointing.

DagCheckpointCoordinator

Ring 1 checkpoint coordinator.

Enums

AlignmentResult

Result of presenting a barrier to the aligner.

BarrierType

Barrier type for checkpoint coordination.

Type Aliases

CheckpointId

Checkpoint identifier.