I am a ruby on rails developer and I have developed several plugins, may be i will be selling them to some web sites and they can use my plugin in there application. But i want to assure that the plugin code once given to them is not used for any other application, if they do so i must know where is it deployed.
I just need a way to track number of deployments, for a given plugin.
You can't. Ruby is interpreted, so they can simply remove the tracking code from it, and use it where they want. You might want to build C extensions for ruby if you really want resellable components.
You cannot assure it. You can make it unprofitable by creating some obfuscated code, which will limit the use. On assumption that analyzing that code will cost a lot more, than just paying you another license. Of course that doesn't guarantee anything at all.