The first time you reboot a Hortonworks HDP/HDF cluster node, you will notice some services do not auto-start by default. This may include Ambari Server and Agent, depending on how it was initially configured.
These can be managed in a few different ways.
https://community.hortonworks.com/content/supportkb/151076/how-to-enable-ambari-server-auto-start-on-rhelcent.html
Ambari UI makes it easy with the auto-start feature.
https://docs.hortonworks.com/HDPDocuments/Ambari-2.6.0.0/bk_ambari-operations/content/enable_service_auto_start.html
https://cwiki.apache.org/confluence/display/AMBARI/Recovery%3A+auto+start+components
https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=41812517
There are some related JIRAs and documentation around this.
https://issues.apache.org/jira/browse/AMBARI-10029
https://community.hortonworks.com/questions/825/how-to-write-cluster-startup-shutdown-order-and-sc.html
Note that order of startup of services for larger clusters, and some delays starting services may cause issues with this feature. I have seen situations where HDFS needs a little more time to successfully start before other services can properly register themselves.
