Elasticsearch plugin
Found.no (an Elasticsearch hosting service) has a very good writeup on the Elasticsearch plugin development process. It's as detailed as I've seen out there and is fairly recent (Sept 2013) so should be reasonably up to date. If I was going to build a plugin from scratch that's where I would start:
https://www.found.no/foundation/writing-a-plugin/
The other is to dig around in other plugins on Github:
https://github.com/mobz/elasticsearch-headhttps://github.com/elasticsearch/elasticsearch-river-twitter
Lists of other plugins:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/modules-plugins.html
Between the tutorial and looking at all the source code out there you should have a solid foundation.
It depends what you want to do with your plugin!
You can write a plugin in java to send data in elastic for example. You can do what you want for that.
I think a plugin is freedom! You can use elastic APIs or not, or imagine new function when you connect your plugin (java for example) on elastic.