Question: I can't figure out how to auto-restart mongodb on Ubuntu 16.04 in the event that it crashes due to memory-pressure. 

I'm using supervisord for other scripts etc, but mongod is through systemctl and I'm not sure how that ties into it.

Answer: 
Figured it out:

1. Edit your mongod service: `sudo vim /lib/systemd/system/mongod.service`
2. Add `Restart=always` under `service`
3. Reload systemctl daemon: `sudo systemctl daemon-reload`

Now whenever mongod gets killed. It'll get respawned by systemctl.
When you install `mongoDB` just run `systemctl enable mongod.service` on terminal.

This will make your mongoDB service auto-start on every restart.