Percona XtraDB Cluster strict mode¶
The Percona XtraDB Cluster (PXC) Strict Mode is designed to avoid the use of tech preview features and unsupported features in PXC. This mode performs a number of validations at startup and during runtime.
Depending on the actual mode you select, upon encountering a failed validation, the server will either throw an error (halting startup or denying the operation), or log a warning and continue running as normal. The following modes are available:
-
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 asENFORCING
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 is set to ENFORCING
,
except if the node is acting as a standalone server
or the node is bootstrapping, then PXC Strict Mode defaults to DISABLED
.
It is recommended to keep PXC Strict Mode set to ENFORCING
,
because in this case whenever Percona XtraDB Cluster encounters a tech preview feature
or an unsupported operation, the server will deny it.
This will force you to re-evaluate your Percona XtraDB Cluster configuration
without risking the consistency of your data.
If you are planning to set PXC Strict Mode to anything else than ENFORCING
,
you should be aware of the limitations and effects
that this may have on data integrity.
For more information, see Validations.
To set the mode, use the pxc_strict_mode
variable in the configuration file
or 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 it during runtime.
For example, to set PXC Strict Mode to PERMISSIVE
,
run the following command:
mysql> SET GLOBAL pxc_strict_mode=PERMISSIVE;
Note
To further ensure data consistency,
it is important to have all nodes in the cluster
running with the same configuration,
including the value of pxc_strict_mode
variable.
Validations¶
PXC Strict Mode validations are designed to ensure optimal operation for common cluster setups that do not require tech preview features and do not rely on operations not supported by Percona XtraDB Cluster.
Warning
If an unsupported operation is performed on a node with pxc_strict_mode
set to DISABLED
or PERMISSIVE
, it will not be validated on nodes where it is replicated to, even if the destination node has pxc_strict_mode
set to ENFORCING
.
This section describes the purpose and consequences of each validation.
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
will result 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.
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
, andANALYZE
-
TRUNCATE TABLE
andALTER TABLE
Depending on the selected mode, the following happens:
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 an undesirable operation is performed on an unsupported table.
ENFORCING
or MASTER
At startup, no validation is performed.
At runtime, any undesirable operation performed on an unsupported table is denied and an error is logged.
Note
Unsupported tables can be converted to use a supported storage engine.
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:
DISABLED
At startup, no validation is performed.
At runtime, you can set wsrep_replicate_myisam
to any value.
PERMISSIVE
At startup, if wsrep_replicate_myisam
is set to ON
, a warning is logged and startup continues.
At runtime, it is permitted to change wsrep_replicate_myisam
to any value, but if you set it to ON
, a warning is logged.
ENFORCING
or MASTER
At startup, if wsrep_replicate_myisam
is set to ON
, an error is logged and startup is aborted.
At runtime, any attempt to change wsrep_replicate_myisam
to ON
fails and an error is logged.
Note
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.
Binary log format¶
Percona XtraDB Cluster supports only the default row-based binary logging format. In
8.4, 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.
Tables without primary keys¶
Percona XtraDB Cluster cannot properly propagate certain write operations
to tables that do not have primary keys defined.
Undesirable operations include data manipulation statements
that perform writing to table (especially DELETE
).
Depending on the selected mode, the following happens:
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 an undesirable operation is performed on a table without an explicit primary key defined.
ENFORCING
or MASTER
At startup, no validation is performed.
At runtime, any undesirable operation performed on a table without an explicit primary key is denied and an error is logged.
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:
DISABLED
At startup, no validation is performed.
At runtime, you can set log_output
to any value.
PERMISSIVE
At startup, if log_output
is set only to TABLE
,
a warning is logged and startup continues.
At runtime, it is permitted to change log_output
to any value, but if you set it only to TABLE
,
a warning is logged.
ENFORCING
or MASTER
At startup, if log_output
is set only to TABLE
,
an error is logged and startup is aborted.
At runtime, any attempt to change log_output
only to TABLE
fails
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()
andRELEASE_LOCK()
-
FLUSH TABLES <tables> WITH READ LOCK
-
Setting the
SERIALIZABLE
transaction level
Depending on the selected mode, the following happens:
DISABLED
or MASTER
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 an undesirable operation is performed.
ENFORCING
At startup, no validation is performed.
At runtime, any undesirable operation is denied and an error is logged.
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).
Depending on the strict mode selected, the following happens:
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.
Note
This validation is not performed during runtime, because the innodb_autoinc_lock_mode
variable cannot be set dynamically.
Combine schema and data changes in a single statement¶
With 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 strict mode set to PERMISSIVE
or DISABLED
, CREATE TABLE … AS SELECT (CTAS) statements are
replicated using the method to ensure
consistency.
In Percona XtraDB Cluster 5.7, CREATE TABLE … AS SELECT (CTAS) statements were replicated using DML
write-sets when strict mode was set to PERMISSIVE
or DISABLED
.
Important
MyISAM tables are created and loaded even if wsrep_replicate_myisam
equals to 1. Percona XtraDB Cluster does not recommend using the MyISAM storage engine. The support for MyISAM may be removed in a future release.
See also
MySQL Bug System: XID inconsistency on master-slave with CTAS https://bugs.mysql.com/bug.php?id=93948
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. |
Important
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.
Depending on the strict mode selected, the following happens:
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 you discard or import a tablespace.
ENFORCING
At startup, no validation is performed.
At runtime, discarding or importing a tablespace is denied and an error is logged.
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