Single user mode enables the database administrator to restrict access to the database system to a single API node, such as a MySQL server (SQL node) or an instance of ndb_restore. When entering single user mode, connections to all other API nodes are closed gracefully and all running transactions are aborted. No new transactions are permitted to start.
Once the cluster has entered single user mode, only the designated API node is granted access to the database.
You can use the ALL STATUS command to see when the cluster has entered single user mode.
ENTER SINGLE USER MODE 5
After this command has executed and the cluster has entered single
user mode, the API node whose node ID is
becomes the cluster's only permitted user.
The node specified in the preceding command must be an API node; attempting to specify any other type of node will be rejected.
When the preceding command is invoked, all transactions running on the designated node are aborted, the connection is closed, and the server must be restarted.
The command EXIT SINGLE USER MODE changes the state of the cluster's data nodes from single user mode to normal mode. API nodes—such as MySQL Servers—waiting for a connection (that is, waiting for the cluster to become ready and available), are again permitted to connect. The API node denoted as the single-user node continues to run (if still connected) during and after the state change.
EXIT SINGLE USER MODE
There are two recommended ways to handle a node failure when running in single user mode:
Finish all single user mode transactions
Issue the EXIT SINGLE USER MODE command
Restart the cluster's data nodes
Restart database nodes prior to entering single user mode.