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Change group_replication_exit_state_action default option to READ_ONLY

In MySQL 8.0.16, the Group Replication plugin variable group_replication_exit_state_action default value has been changed to READ_ONLY.

The group_replication_exit_state_action plugin variable was introduced in MySQL 8.0.12 to allow the user to configure how Group Replication behaves when a server instance leaves the group unintentionally.
The group_replication_exit_state_action has following two values:
ABORT_SERVER: the server shuts itself down;

READ_ONLY:          the server switches itself to super read-only mode.

Earlier group_replication_exit_state_action default value was set to ABORT_SERVER, so when member leave group unintentionally the server shuts itself down. The rationale behind the default ABORT_SERVER was to fence the failed server against reads and writes since all connections will be dropped. But after gathering community feedback, we revisited this decision and decided to go with the READ_ONLY action as default from MySQL 8.0.16.

We hope the change of default for group_replication_exit_state_action will help users. Please let us know what you think. We are looking forward to your valuable feedback.