Question: I am using jboss amq7.1/apache amq, When using replication as the HA policy for my cluster, it is documented that all data synchronization is done over the network, All persistent data received by the master broker is synchronized to the slave when the master drops from the network. A slave broker first needs to synchronize all existing data from the master broker before becoming capable of replacing it. 

Per my understanding, if master broker is crashed instead of shutdown by administrator, no persistent data can be synced, therefore messages persisted in journal of master will be lost if the disk used by journal is broken, am I right?

Answer: 
Your understanding is not correct.

All persistent data received by the master broker is replicated to the slave when the master broker receives it *so that* when the master broker drops from the network (e.g. due to a crash) the slave can replace the master.

Replicating the data from the master to the slave *when* the master drops from the network would completely defeat the purpose of high availability.
Actually, if HA is configured as Master/Slave, whether network or journal replicated, the receipt of a message to the broker is FIRST replicated and ONLY if successful, it will be confirmed as received to the client.