> ## Documentation Index
> Fetch the complete documentation index at: https://filament.getgalaxy.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Integrity & Checkpoints

> How Filament verifies every batch and resumes interrupted runs.

Two guarantees run through every Filament pipeline: what gets written is
verified, and an interrupted run doesn't have to start over.

## Integrity checking

Every batch is integrity checked on both the read and write side. The exact
verification (e.g. row-count and checksum accounting) lives in the pipeline
layer between a source's `Extract` and a sink's `Apply`/`Commit`, independent
of which connector is on either end.

## Checkpointed resume

A source that implements the optional
[`Resumable`](/pages/connectors/building-a-connector/optional-interfaces) contract
extracts against a per-resource checkpoint plan instead of reading blind from
the start:

* **`ResumePlanner`** builds the initial plan for a run — the shard layout
  (a primary-key split) plus any cursor carried over from a previous
  attempt. The engine persists this plan *before* extraction starts, so a
  later resume sees a stable, already-decided shard layout rather than
  re-deriving it.
* **`Resumable.ExtractFrom`** reads using that plan, and stamps each record
  with its shard (`Record.Part`) and keyset position (`Record.Key`) so the
  pipeline can advance and persist the cursor as it goes.
* A resource with no primary key can't be resumed — it maps to `nil` in the
  plan and is simply re-read whole on resume.

CDC sources use the same idea with per-resource checkpoints instead of a
shard plan — see [`ChangeSource`](/pages/connectors/building-a-connector/optional-interfaces).

<Note>
  This is why the chosen read strategy for a resumable source is frozen into
  the checkpoint at plan time: the shard layout can't change mid-flight, or
  resume would be reading against a plan that no longer matches reality. See
  [Postgres source](/pages/connectors/sources/postgres) for a concrete example of
  this in practice.
</Note>