This section discusses how to perform a rolling restart of a MySQL Cluster installation, so called because it involves stopping and starting (or restarting) each node in turn, so that the cluster itself remains operational. This is often done as part of a rolling upgrade or rolling downgrade, where high availability of the cluster is mandatory and no downtime of the cluster as a whole is permissible. Where we refer to upgrades, the information provided here also generally applies to downgrades as well.

There are a number of reasons why a rolling restart might be desirable. These are described in the next few paragraphs.

Configuration change.  To make a change in the cluster's configuration, such as adding an SQL node to the cluster, or setting a configuration parameter to a new value.

MySQL Cluster software upgrade or downgrade.  To upgrade the cluster to a newer version of the MySQL Cluster software (or to downgrade it to an older version). This is usually referred to as a “rolling upgrade” (or “rolling downgrade”, when reverting to an older version of MySQL Cluster).

Change on node host.  To make changes in the hardware or operating system on which one or more MySQL Cluster node processes are running.

System reset (cluster reset).  To reset the cluster because it has reached an undesirable state. In such cases it is often desirable to reload the data and metadata of one or more data nodes. This can be done any of three ways:

Resource Recovery.  To free memory previously allocated to a table by successive INSERT and DELETE operations, for re-use by other MySQL Cluster tables.

The process for performing a rolling restart may be generalized as follows:

The specifics for implementing a given rolling upgrade depend upon the changes being made. A more detailed view of the process is presented here:

In the previous diagram, the Stop and Start steps indicate that the process must be stopped completely using a shell command (such as kill on most Unix systems) or the management client STOP command, then started again from a system shell by invoking the ndbd or ndb_mgmd executable as appropriate.

Restart indicates that the process may be restarted using the ndb_mgm management client RESTART command (see Section 17.5.2, “Commands in the MySQL Cluster Management Client”).