In chapter 4.4.3. Data Replication - in https://access.stage.redhat.com/documentation/en/red-hat-jboss-enterprise-application-platform/version-7.0/configuring-messaging-applications/#data_replication

there is note:

Synchronization occurs in parallel with current network traffic so this won't cause any blocking on current clients. 

This is not fully correct. Client are blocker for initial-replication-sync-timeout which is 30 seconds by default. Then they're unblocked.