CA PPM: Process engine is not working and bg-ca log shows the following error heart beat is old

Document ID : KB000106773
Last Modified Date : 10/08/2018
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Question:
Process engine is not working and bg-ca log shows the following error 'heart beat is old': 

ERROR 2018-07-07 11:30:02,101 [Process Monitor (tenant=clarity)] bpm.engine (clarity:process_admin:26463084__8398ADB6-0DAA-4A14-A005-F7C0D814B7B3:none) ----> Engine: bg-sc9app999.casaasops.com heart beat is old. Heartbeat: Sat Jul 07 06:42:43 PDT 2018 Limit: Sat Jul 07 09:30:01 PDT 2018. Engine will be shutdown.. If this is not normal, please fix any problems and restart the process engine. Trying to shudown process enigne. 
java.lang.Exception 
at com.niku.bpm.engine.ProcessMonitor.execute(ProcessMonitor.java:188) 
at com.niku.bpm.engine.ProcessMonitor.run(ProcessMonitor.java:118) 

what does the error mean and how can we resolve the issue?
Answer:
Heart Beat is Old. It means that one of the process pipelines is not responding to the process monitor thread. When this happens the heart beat is old error, it is sent when the thread has not responded for two hours and shuts the whole process engine down.
Restarting BG services resolve the issue.