By default, TiDB implements an optimistic transaction mode, where the transaction commit might fail because of transaction conflicts. To make sure that the commit succeeds, you need to modify the application and add an automatic retry mechanism. You can avoid this issue by using the pessimistic transaction mode of TiDB.
The pessimistic transaction mode of TiDB is an experimental feature. It is not recommended to apply it in the production environment.
Pessimistic transactions in TiDB behave similarly to those in MySQL. See the minor differences in Known restrictions.
When you perform the
SELECT FOR UPDATE statement, transactions read the last committed data and apply a pessimistic lock on the data being read.
When you perform the
UPDATE/DELETE/INSERT statement, transactions read the last committed data to execute on them and apply a pessimistic lock on the modified data.
When a pessimistic lock is applied on a row of data, other write transactions attempting to modify the data are blocked and have to wait for the lock to be released.
When a pessimistic lock is applied on a row of data, other transactions attempting to read the data are not blocked and can read the committed data.
All the locks are released when the transaction is committed or rolled back.
Deadlocks in concurrent transactions can be detected by the deadlock detector. A DEADLOCK error which is the same as that in MySQL is returned.
TiDB supports both the optimistic transaction mode and pessimistic transaction mode in the same cluster. You can specify either mode for transaction execution.
The pessimistic transaction mode is disabled by default because it is currently an experimental feature. Before enabling it, you need to add the following setting in the configuration file:
[pessimistic-txn] enable = true
enable is set to
true, the default transaction mode in TiDB is still optimistic. To enable the pessimistic transaction mode, choose any of the following methods that suits your needs:
BEGIN PESSIMISTIC; statement to start the transaction in the pessimistic transaction mode. You can write it in comment style as
BEGIN /*!90000 PESSIMISTIC */; to make it compatible with the MySQL syntax.
set @@tidb_txn_mode = 'pessimistic'; statement to allow all the transactions processed in this session to be in the pessimistic transaction mode.
Enable the pessimistic transaction mode in the configuration file. This allows all transactions (except auto-committed single-statement ones) to adopt the pessimistic transaction mode.
[pessimistic-txn] enable = true default = true
If the pessimistic transaction mode is enabled in the configuration file by default, use one of the following methods to adopt the optimistic transaction mode for the transaction:
BEGIN OPTIMISTIC; statement to start the transaction in the optimistic transaction mode. You can write it in comment style as
BEGIN /*!90000 OPTIMISTIC */; to make it compatible with the MySQL syntax.
set @@tidb_txn_mode = 'optimistic'; statement to allow all the transactions processed in this session to be in the optimistic transaction mode.
The three methods to enable the transaction mode are ordered from highest priority to lowest as follows:
BEGIN PESSIMISTIC; or
The session variable
default configuration item in the configuration file. If you use a regular
BEGIN statement and set the value of
tidb_txn_mode to an empty string, then you can use
default to determine whether to enable the pessimistic or optimistic transaction mode.
The related configuration parameters are under the
[pessimistic-txn] category. Besides
default, you can also configure the following parameters:
ttl = "30s"
ttl is the timeout for pessimistic transaction locking. Its default value is “30s” (30 seconds). You must set it to a value between 15~60 seconds, and a value out of this range can result in an error.
A transaction fails when its execution time exceeds
ttl. If the value of
ttl is set too high, the remaining pessimistic lock might block the write transaction for a long time when
tidb-server is down. If set too low, the transaction might be rolled back by other transactions before it can finish execution.
max-retry-count = 256
A pessimistic transaction can automatically retry a single statement. You can specify the maximum retrying times by setting the
max-retry-count parameter to avoid retrying a statement endlessly in some extreme cases. Normally, you do not need to modify this configuration.
TiDB does not support GAP Lock or Next Key Lock. When multiple rows of data are updated through range conditions in a pessimistic transaction, other transactions can insert data without being blocked in this range.
TiDB does not support
SELECT LOCK IN SHARE MODE.