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Front-State Checkpoint

Artifact — instantiates Mobile-Defect Reconfiguration

Snapshots the substrate state at the advancing front into a resumable, restorable record, so the transition can pause, resume, or roll back from a known point.

Front-State Checkpoint is the saved snapshot of the substrate at the moving front. As the transition advances, it captures the local state at and around the front — enough to reconstruct where things stood — and appends it to a running lineage of such snapshots. Its defining trait is that it is an artifact, not an action: it neither moves the defect nor verifies it, it records a trustworthy point-in-time state so that other mechanisms — pause, resume, rollback — have a known-good position to act from. A transition without checkpoints can only go forward or start over; with them, it gains memory.

Example

A data team runs a months-long backfill, rewriting billions of rows in a huge table one batch at a time — a front sweeping through the keyspace. A backfill that can't be interrupted is a liability: a deploy, an outage, or a bad batch would mean restarting from zero. So each batch writes a Front-State Checkpoint — the highest key processed, the batch's parameters, a checksum of what it wrote — appended to a durable lineage of checkpoints. When the job is paused for a release and resumed a day later, it reads the last checkpoint and continues from exactly there. When a bad batch is found, the lineage says precisely which checkpoint preceded it, so recovery targets that point instead of the whole table. The snapshots turn an all-or-nothing migration into a resumable, restorable one.

How it works

  • Snapshot at the front. Capture the local substrate state at and around the advancing front — position plus enough surrounding state to reconstruct it.
  • Make it durable and verifiable. Persist each snapshot with a checksum or equivalent, so a restore can trust what it reads.
  • Append to a lineage. Each checkpoint joins an ordered history, so the sequence of front positions is a queryable record, not just the latest cursor.
  • Serve, don't act. Expose the checkpoints for other mechanisms to resume from, hold at, or roll back to; the checkpoint itself changes nothing.

Tuning parameters

  • Checkpoint frequency — every step versus every N steps. Frequent checkpoints lose less on restart but cost storage and write overhead.
  • State depth — a bare position cursor versus a full local state capture. Deeper snapshots enable true restore and verification; shallow ones only mark progress.
  • Durability and retention — how safely and how long snapshots are kept, and how far back the lineage reaches.
  • Verification hooks — whether each checkpoint stores a checksum or hash so restores and audits can confirm integrity.

When it helps, and when it misleads

Its strength is that checkpoints give a transition memory and reversibility: they are the restore points a rollback returns to, the resting states an arrest holds at, and the resume points after any interruption — converting all-or-nothing runs into pausable, recoverable ones. This is checkpoint-restart applied to a moving front.[1]

The dangerous checkpoint is the one that records a position but not enough state to actually resume or verify from — a cursor that looks like safety but can't reconstruct the front, so a restore silently lands in an inconsistent state. Checkpoints that are never test-restored are in the same class: assumed good, never proven. And over-frequent deep snapshots can cost more than the safety is worth. The discipline is to capture verifiable state (not just a marker), to periodically exercise a real restore, and to match frequency to how expensive re-work actually is.

How it implements the components

  • substrate_state_map — captures the local substrate state at the front as a durable snapshot: the point-in-time map the run can be reconstructed from.
  • history_and_lineage_record — appends each snapshot to an ordered lineage, so the sequence of front positions is a queryable history.

It records state but does not verify it (that is Behind-Front Stabilization Pass) and does not itself reverse to a saved point — restoring is done by Counter-Defect Rollback Runbook, which consumes these checkpoints.

Notes

A checkpoint's worth is set entirely by what it captures. A position marker resumes work but cannot verify or restore correctness; a full verifiable snapshot can do both but costs more to write and store. Decide which you need per transition, and test a restore before trusting the lineage.

References

[1] Checkpoint-restart is the long-standing practice of periodically saving enough state to resume from the save point after an interruption rather than from the beginning — here applied to the state at a moving reconfiguration front. ↩