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

# One-time migration

> Copy a dataset once — bootstrap at bulk speed, confirm parity, stop.

Sometimes you don't want a permanently-synced projection — you want the
data moved once: seed a new search index, populate a staging environment,
or lift a dataset into a serving store and be done.

<Note>
  A dedicated one-shot mode (`bootstrap` **and exit**) is a planned engine
  capability and is not in the released binary yet. The pattern below
  achieves a one-time copy with the shipped engine today.
</Note>

## The pattern today

A one-time copy is the [zero-downtime migration](/docs/guides/zero-downtime-migration)
flow, stopped early:

1. Run the pipeline with `bootstrap: mode: snapshot`. The engine captures
   a cursor, streams every existing row through your projection into the
   target at bulk speed, then switches to tailing.
2. Wait for the tail to catch up (cursor age near zero) — this guarantees
   rows written *during* the copy were also delivered, which a plain dump
   script cannot promise.
3. Stop the engine. The target now holds a complete, consistent copy as
   of the moment you stopped.

Because delivery is idempotent and the cursor is durable, an interrupted
run resumes exactly where it left off — restart the engine and the copy
completes rather than starting over.

## Verifying the copy

Before declaring the migration done:

* Compare source row count with target document count (deterministic ids
  mean one row → one document; no duplicates to distort the count).
* Spot-check recently-written rows — anything committed before you
  stopped the engine must be present.
* The engine's delivery counters (`events_delivered_total`, dlq `0`)
  confirm nothing was routed to the dead-letter queue.

## What the planned one-shot mode adds

The dedicated mode will remove step 3's judgement call: the engine will
bootstrap, drain the change stream to the captured high-water mark,
report parity, and **exit on its own** — making it scriptable in CI and
batch contexts. Watch the
[release notes](https://github.com/ventstream/ventstream/releases) for
availability.