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

# Transactions

Transactions in databases refer to logical units of work that consist of one or more database operations. A transaction is a sequence of database operations, such as reads (queries) and writes (updates or inserts), that are treated as a single indivisible and consistent unit. The main purpose of transactions is to ensure data integrity and maintain the ACID (Atomicity, Consistency, Isolation, Durability) properties of the database.

## Read-only transactions

RisingWave supports read-only transactions, where all reads within a transaction are executed against the consistent Hummock snapshot. Hummock is the LSM-Tree-based storage engine in RisingWave that is specifically optimized for streaming workloads.

To initiate a transaction, use either the [`START TRANSACTION READ ONLY`](/sql/commands/sql-start-transaction) or [`BEGIN READ ONLY`](/sql/commands/sql-begin) command. Subsequently, you can execute queries to read data from the consistent snapshot. To finalize the transaction and submit the queries as a single unit, use the [`COMMIT`](/sql/commands/sql-commit) command.

Please note that data modifications are not allowed while a transaction is initiated but not yet committed. The statements listed below are not allowed within a transaction:

* All DDL statements (`CREATE`, `ALTER`, and `DROP`)
* Most of DML statements (`INSERT`, `UPDATE`, and `DELETE`)
* Statements related to `USER`. This category may overlap with DDL statements.
* All privilege-related statements, including `GRANT` and `REVOKE`.

## Transactions within a CDC table

When you create a table to ingest CDC streams, you can enable this feature by setting `transactional` to `true` in the `WITH` clause of the `CREATE TABLE` statement. Note that this feature is only available if you are using the native [MySQL CDC](/ingestion/sources/mysql/mysql-cdc) or [PostgreSQL CDC](/integrations/sources/postgresql-cdc) connectors.

For performance considerations, transactions involving changes to more than 4096 rows cannot be guaranteed.