This section covers the commands used in the MySQL Cluster Manager for getting and setting values of various types used in MySQL Cluster configuration. We begin with a discussion of what we mean by the term “configuration attribute”, and how this relates to the manual configuration of a MySQL Cluster using MySQL Cluster configuration parameters and MySQL Server options and variables that you may already be familiar with.

Configuration attributes.  Traditionally, when administering MySQL Cluster, it has been necessary to distinguish between 3 types of configuration data:

MySQL Cluster Manager simplifies this configuration scheme by treating all 3 types of configuration data as attributes, where the term “attribute” refers to a MySQL Cluster configuration parameter, a MySQL Server variable, or a command-line option used with one or more MySQL Cluster binary programs. It does this transparently, handling all necessary changes in a unified interface.

For example, suppose that you wish to know how much data memory is allocated to the data nodes in a given MySQL Cluster. Rather than having to determine that this is controlled using the DataMemory configuration parameter that is written in the config.ini file and then reading that file to find the value, you merely invoke the MySQL Cluster Manager get command, and MySQL Cluster Manager handles reading from the file for you, and displays the value without the necessity of opening the file in a separate application such as more or less. If you wish to change the amount of data memory allocated to the data nodes, you can issue a MySQL Cluster Manager set (or reset) command; MySQL Cluster Manager then writes the desired value to config.ini. If—as is the case with DataMemory—updating a configuration value in a running MySQL Cluster requires a rolling restart to be performed, MySQL Cluster Manager can perform this operation automatically so that the configuration change takes effect without further intervention required on the part of the operator.

Configuration attribute levels.  A configuration attribute value applies at one of the three levels, described here:

Default values are hard-coded into MySQL Cluster; you can override a default value for a given configuration attribute (using the set command) or reset a given attribute value to its default (using the reset command), but you cannot change a default value itself. You can set or reset an configuration attribute's value on either the process level or the instance level using a single set or reset command. Once you have set or reset the value of a configuration attribute, this value persists until it is changed by executing another set or reset command.

MySQL Cluster Manager determines what value to use for a configuration attribute relating to a given process by following these steps for each MySQL Cluster process:

(For each configuration attribute:)

(In the steps just shown, “exit” can be taken to mean “If there are more configuration attributes applicable to this process that have not yet been set, proceed to the next attribute until there are no more attributes to be set for this process”.)

Mandatory attributes.  Some attributes must be defined in the MySQL Cluster Manager at the process type or instance level for all processes of the applicable type or types for the cluster configuration to be valid. Such mandatory attributes may be changed, but not reset; in other words, the definition can be changed, but the definition itself cannot be removed entirely. Another way of stating this is that a mandatory attribute has no default value.

An example of a mandatory attribute is NodeId. If you try to reset a mandatory attribute, the attempt fails with an error, as shown here:

mcm> reset NodeId:ndb_mgmd:1 mycluster;
ERROR 6007 (00MGR): Config attribute NodeId is mandatory and cannot be reset
mcm> reset NodeId:ndbd:2 mycluster;
ERROR 6007 (00MGR): Config attribute NodeId is mandatory and cannot be reset
mcm> reset NodeId:mysqld:4 mycluster;
ERROR 6007 (00MGR): Config attribute NodeId is mandatory and cannot be reset

Read-only attributes.  A read-only attribute is an attribute that must be defined by the MySQL Cluster Manager when a cluster is created. A read-only attribute can be neither changed nor reset by the user. This means that a read-only attribute is always a mandatory attribute.

One such attribute is HostName, which is read only for any type of MySQL Cluster process. Any attempt to change or reset a read-only attribute fails, as shown here:

mcm> reset HostName:ndb_mgmd mycluster;
ERROR 6008 (00MGR): Config attribute HostName is readonly and cannot be changed
mcm> reset HostName:ndbd mycluster;
ERROR 6008 (00MGR): Config attribute HostName is readonly and cannot be changed
mcm> reset HostName:mysqld mycluster;
ERROR 6008 (00MGR): Config attribute HostName is readonly and cannot be changed

mcm> set HostName:ndb_mgmd mycluster;
ERROR 6008 (00MGR): Config attribute HostName is readonly and cannot be changed
mcm> set HostName:ndbd mycluster;
ERROR 6008 (00MGR): Config attribute HostName is readonly and cannot be changed
mcm> set HostName:mysqld mycluster;
ERROR 6008 (00MGR): Config attribute HostName is readonly and cannot be changed

An attribute that is mandatory or read only is set when a cluster is created. Neither a mandatory attribute nor a read-only attribute can be reset. (Neither type of attribute has a default value other than what is set for it when the cluster is created.) A mandatory attribute can be changed at any time by the user; a read-only attribute cannot be changed once the cluster has been created. You can obtain a listing of mandatory and read-only attributes using the get command (see Section 3.5.1, “The get Command”).

A listing of attribute properties also can be found in the output of ndb_config --configinfo --xml (see ndb_config — Extract MySQL Cluster Configuration Information); for more more complete information, see MySQL Cluster Configuration.

MySQL Cluster Manager determines internally which attributes are considered read-only for reasons of cluster stability and performance. You can use the get command to see which attributes are read only.