# Transaction and persistence settings

Choose isolation and persistence settings for application correctness rather than benchmark speed.

Source: https://particledb.ai/operations/transactions-and-durability/

Transactions and persistence answer different questions. Isolation controls concurrent visibility and conflicts. Durability controls what remains after a process or machine failure.

## Application transactions

Use one connection and commit only after all required database changes succeed. Read [transactions](/sql/transactions/) for examples and retry handling.

## Server modes

In the standard `1.1.19-rc.12` download, `--txn-mode` selects an engine, not a session isolation level:

| Engine | Behavior |
|---|---|
| `occ` (default) | Optimistic engine with commit-time validation |
| `serializable` | Engine supporting strict serializable transactions when SERIALIZABLE is requested |
| `fast` | No isolation guarantee; concurrent writers can lose updates. Use only for disposable or controlled single-writer work |

Sessions default to READ COMMITTED under `occ` and `serializable`. Request REPEATABLE READ for snapshot isolation or SERIALIZABLE for strict serializable transactions. An explicit level has the same meaning under either engine. Selecting the `serializable` engine alone does not change the session default.

Use `--default-isolation` to set the server-wide session default, or select a level through the client or [SQL transaction commands](/sql/transactions/). Check the installed version's help before changing either setting.

### Pre-release compatibility

Historical pre-release builds accepted additional engine names. The standard `1.1.19-rc.12` binary rejects `occ-v3`, `zero-copy-occ`, `table-2pl`, `serializable-ssi`, `strict-serializable` and `row-2pl`. These are removed configuration values, not current modes. Removing a name does not invalidate findings about the versions and settings that accepted it.

### Persistence is separate

`--wal-sync-mode` accepts `sync`, `groupsync` and `nosync`. Durability flags do not select the engine or the session's isolation level. `nosync` disables the write-ahead log and cannot preserve acknowledged writes across restart. Review [durability](/operations/durability/) before using any benchmark configuration for application data.

## Verify your required behavior

Test concurrent updates and application invariants with separate connections. Test restart and recovery using the deployment's actual storage. A setting name or an acknowledged response is not a substitute for a documented guarantee.

[Durability](/operations/durability/) · [Concurrency](/sql/locking/) · [Benchmarks](/benchmarks/overview/)

## Related

[Documentation home](/docs/) · [Troubleshooting](/guides/troubleshooting/)