lancedb/docs/web/basics/guarantees.mdx

---
title: "Guarantees"
sidebarTitle: "Guarantees"
description: "What a write promises when it returns, what a reader sees, and what survives a failure."
icon: shield-check
keywords: ["guarantees", "consistency", "atomicity", "durability", "concurrency", "acid"]
---

What the system promises, and — just as usefully — what it does not. Most
surprises in practice come from assuming one of the guarantees below is
stronger or weaker than it is.

## Writes {#writes}

**A write is atomic.** Every write commits a new table version. Either the
version exists and holds all of the change, or it does not exist and holds none
of it. There is no state in which a reader sees half an `add()`.

**A write returns only after the commit.** When the call returns, the version
is durable in object storage. Nothing is buffered in your process waiting to be
flushed.

**Every write advances the version**, including one that affects zero rows. A
delete whose predicate matches nothing still commits. Version numbers count
commits, not changes.

**Concurrent writes are resolved optimistically.** A transaction records the
change and the version it was based on. At commit time, a conflicting
concurrent operation is detected rather than silently overwritten, and the
commit is retried against the newer version. Operations that do not collide —
appending while another process appends — both succeed.

<Note>
`merge_insert` reports its retry attempts in the result. A high count means
contention on the same keys, not an error.
</Note>

## Reads {#reads}

**A read sees a consistent snapshot.** A query reads one table version
throughout; it never sees a mixture of two.

**Which version, by default, is the one open at the time the table was opened.**
This is the guarantee most often assumed to be stronger than it is:

<Warning>
A long-lived connection **does not observe other processes' writes** until it
reopens the table or its `read_consistency_interval` elapses. The default is
right for a batch job reading a stable snapshot and wrong for a service
expected to see fresh data.
</Warning>

Set `read_consistency_interval` when several processes write the same table and
readers must observe those writes. The cost is a metadata check on that
interval. See [Consistency](/tables/consistency).

**Your own writes are always visible to you.** The interval governs other
writers, not yourself.

## Durability and history {#durability}

**Old versions are retained, not overwritten.** A new version does not destroy
the one before it, which is what makes [time travel](/management/table#versioning) work
and what lets a reader finish a scan that started on an older version.

**Deletion is a mark, not an erase.** A deleted row is recorded in a deletion
file. The data is recoverable for as long as the version that contains it
exists.

<Warning>
Retention is finite. [Compaction](/maintenance/compaction) prunes versions
older than the retention window — **7 days by default** — and a tag or branch
pointing at a pruned version no longer resolves. If you rely on reading old
versions, widen the window before running it, not after.
</Warning>

**Dropping a table ends its history.** Versioning does not survive
`drop_table`; there is no undrop.

## Jobs {#jobs}

**A refresh never revisits a filled row.** It gives values to rows that lack
them and leaves the rest alone. Two consequences, both relied on elsewhere:

- **Retrying is safe and cheap.** A failed backfill is resumed, not restarted —
  re-running costs only the rows that did not complete.
- **A filled value does not track its input.** If a column derives from `price`
  and a row's `price` later changes, the derived value stays as computed.
  Recomputing means dropping the column and declaring it again.

**A failure is not a rollback.** Rows filled before a failure keep their values,
because each commits as it completes.

**Cancellation is a request.** `cancel()` asks a job to stop; it does not
guarantee it stops before doing more work, and cancelling a finished job does
nothing.

**A job id is opaque.** It correlates with server logs and the jobs API.
Parsing it, or storing it to resume the job later, is not supported.

See [Jobs](/basics/jobs).

## Schema {#schema}

**Schema changes are versioned like data changes.** Adding, renaming, retyping
or dropping a column commits a version, so a schema change can be travelled back
through.

**A computed column's inputs are protected.** While a declaration references a
column, that column cannot be renamed, retyped, or dropped — the declaration is
part of the schema and will not be invalidated out from under itself.

**Most schema changes do not rewrite data.** Adding a derived column writes only
the new column. Type changes are the exception and can be substantial.

## What is not guaranteed {#not-guaranteed}

Worth stating plainly, because each is occasionally assumed:

| | |
| --- | --- |
| Multi-table transactions | A commit covers one table. There is no cross-table atomic write. |
| Read-your-writes across processes | Governed by `read_consistency_interval`, not automatic. |
| Unenforced primary keys | `SET UNENFORCED PRIMARY KEY` records intent; it does not enforce uniqueness. |
| Ordering of concurrent appends | Two appends both succeed; their relative row order is not defined. |
| Index freshness | An index does not cover rows added after it was built until [optimized](/maintenance/compaction). |
| Automatic compaction | Nothing runs in the background. `optimize()` runs when you call it. |

The last two are the most common source of "why is this query slow" — see
[Limits](/basics/limits) for the bounds that pair with them.