This will depend on what these other agents are to a large extent, but in the general case there are two options:

1) Run a cron job: one, check to see if the process is already running; two, check to see if HDFS is running; three start the process if neither of the above are true.

2) I believe Ubuntu 16 uses systemd. In systemd start order can be controlled via dependencies in the unit file of the process. You will have to reference it specifically for your version, but I believe there are two relevant settings: wants or requires determines which processes run together; before and after determines the order in which they are run. The most thorough solution is to set these other agents' unit files as requires HDFS and after HDFS.