18.4.3.1 Cloning for Distributed Recovery

For full instructions to set up and configure the clone plugin, see Section 5.6.7, “The Clone Plugin” . Detailed prerequisites for a remote cloning operation are covered in Section 5.6.7.3, “Cloning Remote Data” . For Group Replication, note the following key points and differences:

When group members have been set up to support cloning, the group_replication_clone_threshold system variable specifies a threshold, expressed as a number of transactions, for the use of a remote cloning operation in distributed recovery. If the gap between the transactions on the donor and the transactions on the joining member is larger than this number, a remote cloning operation is used for state transfer to the joining member when this is technically possible. Group Replication calculates whether the threshold has been exceeded based on the gtid_executed sets of the existing group members. Using a remote cloning operation in the event of a large transaction gap lets you add new members to the group without transferring the group's data to the server manually beforehand, and also enables a member that is very out of date to catch up more efficiently.

The default setting for the group_replication_clone_threshold Group Replication system variable is extremely high (the maximum permitted sequence number for a transaction in a GTID), so it effectively deactivates cloning wherever state transfer from the binary log is possible. To enable Group Replication to select a remote cloning operation for state transfer where this is more appropriate, set the system variable to specify a number of transactions as the transaction gap above which you want cloning to take place.

Group Replication attempts to execute a remote cloning operation regardless of your threshold when state transfer from a donor's binary log is impossible, for example because the transactions needed by the joining member are not available in the binary log on any existing group member. Group Replication identifies this based on the gtid_purged sets of the existing group members. You cannot use the group_replication_clone_threshold system variable to deactivate cloning when the required transactions are not available in any member's binary log files, because in that situation cloning is the only alternative to transferring data to the joining member manually.

When group members and joining members are set up for cloning, Group Replication manages remote cloning operations for you. A remote cloning operation might take some time to complete, depending on the size of the data. See Section 5.6.7.9, “Monitoring Cloning Operations” for information on monitoring the process.

A remote cloning operation clones settings that are persisted in tables from the donor to the recipient, as well as the data. Group Replication manages the settings that relate specifically to Group Replication channels. The credentials used by the donor for the group_replication_recovery replication channel (the replication user and password) are transferred to and used by the joining member after cloning, and they must be valid there. All group members that received state transfer by a remote cloning operation therefore use the same replication user and password for distributed recovery. However, Group Replication preserves the channel settings that relate to the use of SSL, so these are unique to the individual member. Group Replication member settings that are persisted in configuration files, such as the group replication local address, are not cloned and are not changed on the joining member.