INSERT into one partition* in one table* of MergeTree* family up to max_insert_block_size rows* is transactional (ACID):
- Atomic: INSERT is succeeded or rejected as a whole: if confirmation is sent to the client, all rows INSERTed; if error is sent to the client, no rows INSERTed.
- Consistent: if there are no table constraints violated, then all rows in an INSERT are inserted and the INSERT succeeds; if constraints are violated, then no rows are inserted.
- Isolated: concurrent clients observe a consistent snapshot of the table–the state of the table either as if before INSERT or after successful INSERT; no partial state is seen;
- Durable: successful INSERT is written to the filesystem before answering to the client, on single replica or multiple replicas (controlled by the `insert_quorum` setting), and ClickHouse can ask the OS to sync the filesystem data on the storage media (controlled by the `fsync_after_insert` setting).
* If table has many partitions and INSERT covers many partitions–then insertion into every partition is transactional on its own;
* INSERT into multiple tables with one statement is possible if materialized views are involved;
* INSERT into Distributed table is not transactional as a whole, while insertion into every shard is transactional;
* another example: insert into Buffer tables is neither atomic nor isolated or consistent or durable;
* atomicity is ensured even if `async_insert` is enabled, but it can be turned off by the wait_for_async_insert setting;
* max_insert_block_size is 1 000 000 by default and can be adjusted as needed;
* if client did not receive the answer from the server, the client does not know if transaction succeeded, and it can repeat the transaction, using exactly-once insertion properties;
* ClickHouse is using MVCC with snapshot isolation internally;
* all ACID properties are valid even in case of server kill / crash;
* either insert_quorum into different AZ or fsync should be enabled to ensure durable inserts in typical setup;
* "consistency" in ACID terms does not cover the semantics of distributed systems, see https://jepsen.io/consistency which is controlled by different settings (select_sequential_consistency)
* this explanation does not cover a new transactions feature that allow to have full-featured transactions over multiple tables, materialized views, for multiple SELECTs, etc.
## Transactions, Commit, and Rollback
In addition to the functionality described at the top of this document, ClickHouse has experimental support for transactions, commits, and rollback functionality.
### Requirements
- Deploy ClickHouse Keeper or ZooKeeper to track transactions
- Atomic DB only (Default)
- Non-Replicated MergeTree table engine only
- Enable experimental transaction support by adding this setting in `config.d/transactions.xml`:
- This is an experimental feature, and changes should be expected.
- If an exception occurs during a transaction, you cannot commit the transaction. This includes all exceptions, including `UNKNOWN_FUNCTION` exceptions caused by typos.
- Nested transactions are not supported; finish the current transaction and start a new one instead
### Configuration
These examples are with a single node ClickHouse server with ClickHouse Keeper enabled.
#### Basic configuration for a single ClickHouse server node with ClickHouse Keeper enabled
:::note
See the [deployment](docs/en/deployment-guides/terminology.md) documentation for details on deploying ClickHouse server and a proper quorum of ClickHouse Keeper nodes. The configuration shown here is for experimental purposes.
Issue a `BEGIN TRANSACTION` followed by a `ROLLBACK` to verify that experimental transactions are enabled, and that ClickHouse Keeper is enabled as it is used to track transactions.
If you see the following error, then check your configuration file to make sure that `allow_experimental_transactions` is set to `1` (or any value other than `0` or `false`).
```
Code: 48. DB::Exception: Received from localhost:9000.
DB::Exception: Transactions are not supported.
(NOT_IMPLEMENTED)
```
You can also check ClickHouse Keeper by issuing
```
echo ruok | nc localhost 9181
```
ClickHouse Keeper should respond with `imok`.
:::
```sql
ROLLBACK
```
```response
Ok.
```
#### Create a table for testing
:::tip
Creation of tables is not transactional. Run this DDL query outside of a transaction.
:::
```sql
CREATE TABLE mergetree_table
(
`n` Int64
)
ENGINE = MergeTree
ORDER BY n
```
```response
Ok.
```
#### Begin a transaction and insert a row
```sql
BEGIN TRANSACTION
```
```response
Ok.
```
```sql
INSERT INTO mergetree_table FORMAT Values (10)
```
```response
Ok.
```
```sql
SELECT *
FROM mergetree_table
```
```response
┌──n─┐
│ 10 │
└────┘
```
:::note
You can query the table from within a transaction and see that the row was inserted even though it has not yet been committed.
:::
#### Rollback the transaction, and query the table again
Verify that the transaction is rolled back:
```sql
ROLLBACK
```
```response
Ok.
```
```sql
SELECT *
FROM mergetree_table
```
```response
Ok.
0 rows in set. Elapsed: 0.002 sec.
```
#### Complete a transaction and query the table again
```sql
BEGIN TRANSACTION
```
```response
Ok.
```
```sql
INSERT INTO mergetree_table FORMAT Values (42)
```
```response
Ok.
```
```sql
COMMIT
```
```response
Ok. Elapsed: 0.002 sec.
```
```sql
SELECT *
FROM mergetree_table
```
```response
┌──n─┐
│ 42 │
└────┘
```
### Transactions introspection
You can inspect transactions by querying the `system.transactions` table, but note that you cannot query that
See this [meta issue](https://github.com/ClickHouse/ClickHouse/issues/48794) to find much more extensive tests and to keep up to date with the progress.
