Elasticsearch plugin troubleshooting

Problem: Elasticsearch won't restart

After trying to start Elasticsearch, using

sudo service elasticsearch start

, it confirmed that it had started elasticsearch, but when I ran

sudo service elasticsearch status

all I got in response was:

* elasticsearch is not running 

Nothing more than that, which was goddammed annoying because I was busy trying to do real work (Reddit).

I found evidence of the cause in /var/log/elasticsearch/elasticsearch.log:

So it was a plugin. But what plugin?

I ran these commands to try and reinstall the licenses:

cd /usr/share/elasticsearch/bin
sudo ./plugin remove license
sudo ./plugin install license

After running that, I got a better error message:

ERROR: Plugin [watcher] is incompatible with Elasticsearch [2.4.6]. Was designed for version [2.4.4]

So it was the watcher plugin causing problems. Fixing it was easy enough though:

sudo ./plugin install watcher
sudo ./plugin install license
 

That's all it took. I restarted elasticsearch and I was on my way.