Percona XtraDB Cluster strict mode

The Percona XtraDB Cluster (PXC) Strict Mode prevents the use of tech preview features and unsupported operations in Percona XtraDB Cluster. This mode performs several validations during startup and runtime to ensure compliance.

The server’s response to a failed validation depends on the selected mode. The server can halt with an error, stopping startup or denying the operation, or log a warning and continue running. The available modes include:

Mode	Description
DISABLED	Do not perform strict mode validations and run as normal.
PERMISSIVE	If a validation fails, log a warning and continue running as normal.
ENFORCING	If a validation fails during startup, halt the server and throw an error.
	If a validation fails during runtime, deny the operation and throw an error.
MASTER	The same as ENFORCING except that the validation of explicit table locking is not performed. This mode can be used with clusters in which write operations are isolated to a single node.

By default, PXC Strict Mode operates in ENFORCING mode. When a node acts as a standalone server or during bootstrapping, the mode defaults to DISABLED.

Selecting ENFORCING as the default mode is strongly recommended. Using this mode ensures that unsupported operations and tech preview features are denied, thereby protecting data consistency and prompting a review of cluster configuration.

Choosing any mode other than ENFORCING requires a clear understanding of the potential risks to data integrity. Refer to the Validations section for detailed information.

To specify the mode, define the pxc_strict_mode variable in the configuration file or use the --pxc-strict-mode option during mysqld startup.

Note

It is better to start the server with the necessary mode (the default ENFORCING is highly recommended). However, you can dynamically change the mode during runtime. For example, to set PXC Strict Mode to PERMISSIVE, run the following command:

mysql> SET GLOBAL pxc_strict_mode=PERMISSIVE;

To ensure data consistency, all nodes in the cluster should use the same configuration, including the value of the pxc_strict_mode variable.

Validations

PXC Strict Mode validations are implemented to ensure the smooth and reliable operation of cluster environments. These validations are particularly effective for standard cluster setups that do not require tech preview features or operations that are not supported by Percona XtraDB Cluster.

If pxc_strict_mode is set to DISABLED or PERMISSIVE on the originating node, unsupported operations can bypass strict mode validations. When these operations are replicated, they are not validated on destination nodes, even if those nodes have pxc_strict_mode set to ENFORCING. This behavior underscores the importance of carefully managing cluster configurations to avoid inconsistencies.

By adhering to strict mode recommendations, users can ensure smoother cluster operations while minimizing risks associated with unsupported actions.

Auto-increment lock mode

The lock mode for generating auto-increment values must be interleaved to ensure that each node generates a unique (but non-sequential) identifier.

This validation checks the value of the innodb_autoinc_lock_mode variable. By default, the variable is set to 1 (consecutive lock mode), but it should be set to 2 (interleaved lock mode). This validation is not performed during runtime, because the innodb_autoinc_lock_mode variable cannot be set dynamically.

Depending on the strict mode selected, the following happens:

Mode	Description
DISABLED	At startup, no validation is performed.
PERMISSIVE	At startup, if innodb_autoinc_lock_mode is not set to 2, a warning is logged and startup continues.
ENFORCING or MASTER	At startup, if innodb_autoinc_lock_mode is not set to 2, an error is logged and startup is aborted.

Binary log format

Percona XtraDB Cluster supports only the default row-based binary logging format.

In 8.0, setting the binlog_format variable to anything but ROW at startup or runtime is not allowed regardless of the value of the pxc_strict_mode variable.

Combine schema and data changes in a single statement

With the strict mode set to ENFORCING, Percona XtraDB Cluster does not support statements, because they combine both schema and data changes. Note that tables in the SELECT clause should be present on all replication nodes.

With the strict mode set to PERMISSIVE or DISABLED, CREATE TABLE … AS SELECT (CTAS) statements are replicated using the method to ensure consistency.

MyISAM tables are created and loaded even if wsrep_replicate_myisam equals 1. Percona XtraDB Cluster does not recommend using the MyISAM storage engine. The support for MyISAM may be removed in a future release.

Depending on the strict mode selected, the following happens:

Mode	Behavior
DISABLED	At startup, no validation is performed. At runtime, all operations are permitted.
PERMISSIVE	At startup, no validation is performed. At runtime, all operations are permitted, but a warning is logged when a CREATE TABLE … AS SELECT (CTAS) operation is performed.
ENFORCING	At startup, no validation is performed. At runtime, any CTAS operation is denied and an error is logged.

Although CREATE TABLE ... AS SELECT (CTAS) operations for temporary tables are permitted even in STRICT mode, temporary tables should not be used as source tables in CREATE TABLE ... AS SELECT (CTAS) operations due to the fact that temporary tables are not present on all nodes.

If node-1 has a temporary and a non-temporary table with the same name, CREATE TABLE … AS SELECT (CTAS) on node-1 will use temporary and CREATE TABLE … AS SELECT (CTAS) on node-2 will use the non-temporary table resulting in a data level inconsistency.

Discard and import tablespaces

DISCARD TABLESPACE and IMPORT TABLESPACE are not replicated using TOI. This can lead to data inconsistency if executed on only one node.

At startup no validation is performed.

Depending on the strict mode selected, the following happens:

Mode	Runtime Behavior
DISABLED	All operations are permitted.
PERMISSIVE	All operations are permitted, but a warning is logged when you discard or import a tablespace.
ENFORCING	Discarding or importing a tablespace is denied and an error is logged.

Explicit table locking

Percona XtraDB Cluster provides only the tech-preview-level of support for explicit table locking operations, The following undesirable operations lead to explicit table locking and are covered by this validation:

• LOCK TABLES

• GET_LOCK() and RELEASE_LOCK()

• FLUSH TABLES <tables> WITH READ LOCK

• Setting the SERIALIZABLE transaction level

At startup, no validation is performed. Depending on the selected mode, the following happens:

Mode	Runtime Behavior
DISABLED / MASTER	All operations are permitted.
PERMISSIVE	All operations are permitted, but a warning is logged when an undesirable operation is performed.
ENFORCING	Any undesirable operation is denied and an error is logged.

Group replication

Group replication is a feature of MySQL that provides distributed state machine replication with strong coordination between servers. It is implemented as a plugin which, if activated, may conflict with PXC. Group replication cannot be activated to run alongside PXC. However, you can migrate to PXC from the environment that uses group replication.

For the strict mode to work correctly, make sure that the group replication plugin is not active. In fact, if pxc_strict_mode is set to ENFORCING or MASTER, the server will stop with an error:

Error message with pxc_strict_mode set to ENFORCING or MASTER

The error message

Group replication cannot be used with PXC in strict mode.

If pxc_strict_mode is set to DISABLED you can use group replication at your own risk. Setting pxc_strict_mode to PERMISSIVE results in a warning.

Warning message with pxc_strict_mode set to PERMISSIVE

Warning message

Using group replication with PXC is only supported for migration. Please
make sure that group replication is turned off once all data is migrated to PXC.

Log output

Percona XtraDB Cluster does not support tables in the MySQL database as the destination for log output. By default, log entries are written to file. This validation checks the value of the log_output variable.

Depending on the selected mode, the following happens:

Major version check

This validation checks that the protocol version is the same as the server major version. This validation protects the cluster against writes attempted on already upgraded nodes.

Expected output

ERROR 1105 (HY000): Percona-XtraDB-Cluster prohibits use of multiple major versions while accepting write workload with pxc_strict_mode = ENFORCING or MASTER

MyISAM replication

Percona XtraDB Cluster provides support for replication of tables that use the MyISAM storage engine. The use of the MyISAM storage engine in a cluster is not recommended and if you use the storage engine, this is your own risk. Due to the non-transactional nature of MyISAM, the storage engine is not fully-supported in Percona XtraDB Cluster.

MyISAM replication is controlled using the wsrep_replicate_myisam variable, which is set to OFF by default. Due to its unreliability, MyISAM replication should not be enabled if you want to ensure data consistency.

Depending on the selected mode, the following happens:

Mode	Startup Behavior	Runtime Behavior
DISABLED	No validation is performed.	You can set wsrep_replicate_myisam to any value.
PERMISSIVE	If wsrep_replicate_myisam is set to ON, a warning is logged and startup continues.	Changing wsrep_replicate_myisam to any value is permitted, but setting it to ON logs a warning.
ENFORCING / MASTER	If wsrep_replicate_myisam is set to ON, an error is logged and startup is aborted.	Any attempt to change wsrep_replicate_myisam to ON fails and an error is logged.

The wsrep_replicate_myisam variable controls replication for MyISAM tables, and this validation only checks whether it is allowed. Undesirable operations for MyISAM tables are restricted using the Storage engine validation.

Storage engine

Percona XtraDB Cluster currently supports replication only for tables that use a transactional storage engine (XtraDB or InnoDB). To ensure data consistency, the following statements should not be allowed for tables that use a non-transactional storage engine (MyISAM, MEMORY, CSV, and others):

• Data manipulation statements that perform writing to table (for example, INSERT, UPDATE, DELETE, etc.)

• The following administrative statements: CHECK, OPTIMIZE, REPAIR, and ANALYZE

• TRUNCATE TABLE and ALTER TABLE

Depending on the selected mode, the following happens:

Mode	Startup Behavior	Runtime Behavior
DISABLED	No validation is performed.	All operations are permitted.
PERMISSIVE	No validation is performed.	All operations are permitted, but a warning is logged when an undesirable operation is performed on an unsupported table.
ENFORCING / MASTER	No validation is performed.	Any undesirable operation on an unsupported table is denied and an error is logged.

Unsupported tables can be converted to use a supported storage engine.