As of 6.4.1 MCS uses systemd to handle processing start and stop routine. It also handles the processes hierarchy restarts, e.g. if mcs-primproc(contains both EM and PP) crashes systemd restarts mcs-writeengineserver unit. If WE is in the middle of a write operation this potentially left the system in an unusable state. If mcs-writeengineserver is restarted systemd restarts both mcs-dmlproc and mcs-ddlproc so if DMLProc was doing some changes it left failed transactions behind. It is tedious to clean these stuck txns from the cluster.
The suggested approach is to decouple pairs: mcs-primproc, mcs-writeengineserver and mcs-writeengineserver and mcs-dmlproc, mcs-writeengineserver and mcs-ddlproc systemd-wise.