Backfill while following live

Leani does not make a large historical scan block head following. The live lane starts independently, commits canonical head work, and can recover a short live gap. Historical jobs fill missing committed intervals for each processor.

processor start                                             observed head
      │                                                           │
      ├──── archive / retained / P2P history ────────┐            │
      │                                              ├─ hash join ┤
      └──────────────── live P2P + finality ─────────┴────────────┘

1. Run doctor and inspect which historical sources satisfy the processor’s declared material shape.
2. Start serve with live execution and finality configured. Live readiness is independent of older processor coverage.
3. For history_control = "node_owned", let the node schedule missing ranges. For application_subscriptions, create bounded backfill subscriptions from the application and consume their independent history lanes.
4. Observe exact coverage intervals, not only processedThrough.
5. Accept the handoff only when parent hash, canonical ancestry, and processor coverage meet at the boundary.

The default gap policy

If sources.live.history_fallback_blocks is omitted, P2P fallback is eligible to fill every history gap at or after the processor start block. Retained or archive sources are selected first when capable. Configure a numeric limit only when you intentionally want to restrict P2P recovery to a finalized suffix.

A failed job is terminal until acted upon

Terminal failed backfill jobs are not retried forever. Inspect the recorded error and either retry, cancel, or replace the job after correcting its cause. Completion is published only after all overlapping ownership and coverage invariants are satisfied.

For application-owned work, each history lane has its own lease and acknowledgement. A live acknowledgement never advances a history lane, and one history lane reconnects without resetting its siblings.