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txn: add description about deprecating the tidb_disable_txn_auto_retry system variable #16541

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txn: add description about deprecating the tidb_disable_txn_auto_retry system variable #16541

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Expand Up @@ -65,6 +65,10 @@ However, TiDB transactions also have the following disadvantages:

## Transaction retries

> **Note:**
>
> Starting from v8.0.0, the [`tidb_disable_txn_auto_retry`](/system-variables.md#tidb_disable_txn_auto_retry) system variable is deprecated, and TiDB no longer supports automatic retries of optimistic transactions. It is recommended to use the [Pessimistic transaction mode](/pessimistic-transaction.md). If you encounter optimistic transaction conflicts, you can capture the error and retry transactions in your application.

In the optimistic transaction model, transactions might fail to be committed because of write–write conflict in heavy contention scenarios. TiDB uses optimistic concurrency control by default, whereas MySQL applies pessimistic concurrency control. This means that MySQL adds locks during the execution of write-type SQL statements, and its Repeatable Read isolation level allows for current reads, so commits generally do not encounter exceptions. To lower the difficulty of adapting applications, TiDB provides an internal retry mechanism.

### Automatic retry
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