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It is best practice to keep a broker in the durable domain configuration while its host is being upgraded. However, in a two-broker domain, in the unusual event that a broker does not restart, broker quorum is lost. Without broker quorum, operations such as the addition of a new host are not allowed. See About Broker Quorum.
In this situation, use the NuoDB Agent Tool to remove the broker from domain membership and regain broker quorum.