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The Group Replication Resource Manager component monitors secondary server lag time and memory usage, and can expel servers which lag excessively or use too many resources from the group. Allowable lag time and resource usage are configurable for both applier channels and recovery channels, as explained in this section. This component is available as part of MySQL Enterprise Edition.

Prior to installing the Group Replication Resource Manager component, the Group Replication plugin should be installed usingINSTALL PLUGIN or--plugin-load-add (seeSection 20.2.1.2, “Configuring an Instance for Group Replication”). It is possible to install the component without the Group Replication plugin being available, but in this case, the component is useful only for monitoring of memory usage and is not capable of taking any action.

The Group Replication Resource Manager component can be installed and uninstalled usingINSTALL COMPONENT andUNINSTALL COMPONENT, respectively. See the descriptions of these statements, as well asSection 7.5.1, “Installing and Uninstalling Components”, for more information.

The Group Replication Resource Manager component provides a configurable automatic expulsion mechanism which detects when the applier or recovery channel on a group replication secondary is lagging, or when the secondary is swapping excessively, and expels the problematic server from the group, thus helping to maintain high availability. Due to the high availability requirement,in order to use the auto-expulsion functionality with an active replication group, the group must initially consist of no fewer than three members, including the group replication primary; this guarantees that there are at least two members (one primary and one secondary) in the event that one member has been expelled.

Using the system and status variables provided by this component, the operator can separately monitor each of the three areas of concern—applier lag, recovery lag, and system resource exhaustion—and separate thresholds for expulsion set for each of them, as listed here:

The Resource Manager component checks lag and usage on group replication secondaries every 5 seconds. This period is not configurable by the operator.

A server which has been expelled from the group may subsequently try to rejoin it without manual intervention, provided thatgroup_replication_autorejoin_tries is enabled (otherwise the server proceeds as specified bygroup_replication_exit_state_action). The auto-rejoin mechanism and behavior are the same as those described inSection 20.7.7.3, “Auto-Rejoin”.

For a replication group member attempting to join or rejoin a group after encountering issues and being expelled, a quarantine period prevents immediate re-expulsion. This period is tracked individually for each member, so that, during the quarantine period started after group member A has been expelled and subsequently allowed to re-join the group, member B can be expelled safely if the need arises. The duration of the quarantine period determined by the value of thegroup_replication_resource_manager.quarantine_time server system variable. The default length of the quarantine period is 3600 seconds (1 hour).

The Resource Management component provides a number of server status variables which can be used for monitoring the status of Group Replication and the Resource Manager component. In addition to the three such variables discussed previously, these include the following:

In addition, it is possible to determine if and when errors have occurred when attempting to get lag or memory usage information by checking the status variables listed here:

For general information about MySQL Group Replication, seeChapter 20,Group Replication.