# Consistency and visibility

> What a statement and a transaction guarantee, what a read sees after a write, and what AS OF COMMIT covers.

NYXDB commits every statement atomically, and groups several statements into
one atomic unit with a transaction. This page states the guarantees an
application builds on.

## One statement, one atomic write

A SQL write statement changes one row of one table: `INSERT`, `UPSERT` and
`REPLACE` carry one row tuple, and `UPDATE` and `DELETE` name one complete
primary key. With a trailing `ATTRIBUTE GROUP`, it changes one
[attribute group](/docs/concepts/attribute-groups) of that row. The write is
validated completely before it commits, and it commits as a whole. An error
names its reason, for example "insert target already exists" for an `INSERT`
of an existing key.

Each tablet orders its own commits with a commit sequence and acknowledges a
write once its record is durable in the write-ahead log. See
[Durability](/docs/operations/durability).

## Transactions

`BEGIN` … `COMMIT` groups writes across attribute groups, rows and tables into
one atomic unit; `ROLLBACK` discards them. See
[Transactions](/docs/sql/transactions).

## Reads after a write

When a write returns its receipt, the next read sees it, with no retry. A read
that runs while a write is in flight returns either the state before that write
or the state after it, never a partly applied row.

A [transformation](/docs/concepts/transformations) or materialized view applies
each source commit as PSI publishes it, so its target follows its sources
continuously and always equals its definition over them.

## Snapshots: AS OF COMMIT

`AS OF COMMIT <n>` reads a `VERSIONED` table as it stood at commit `n`:

```sql
SELECT address, token, amount
FROM balances AS OF COMMIT 42
WHERE chain_id = 1;
```

See [Versioned tables](/docs/concepts/versioned-tables).

## Guarantees

| Contract | What it means |
| --- | --- |
| Atomic statements | A statement is applied completely or not at all. |
| Transactions | `BEGIN` … `COMMIT` applies several statements atomically. |
| Read-after-write | The next read after a receipt sees the write. |
| `AS OF COMMIT` | A read of a VERSIONED table at an earlier commit. |
| Exact derived state | Transformations and materialized views equal their definitions. |

## Streaming reads

A `STREAM SELECT` delivers one snapshot of its result, then the changes that
later commits make to it. See
[Streaming reads](/docs/concepts/streaming-reads) for the delivery contract.

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Source: https://nyxdb.io/docs/concepts/consistency/