modifier: adding modifier tag concept

[jonmorehouse]

When digging into fabio, I found the need to be able to route to upstreams based upon a "prefix" in a URI.

For instance, let's say I have a service I want to call test at localhost:8000 and I want it accessible at localhost:9999/test/ via fabio. I would like a request to localhost:9999/test to point to localhost:8000/, not localhost:8000/test.

I added the concept of a "modifier" tag, which allows you to trigger whether or not we strip the prefix when routing to a specific target. I'd love to hear any thoughts, its my first steps in this code base and I'm sure there are plenty of things to do better!

[CLAassistant]

All committers have signed the CLA.

[jonmorehouse]

I was digging through this, and it seems like the Target object could have a Modify method which accepts an http.Request object and modifies the request to its liking. In the case that we had more modifier tags, this could be quite useful

[magiconair]

Hi @jonmorehouse, this has been requested multiple times in #2 and #44 which is still open. Personally, I think stripping the URL prefix is the wrong approach since it moves essential configuration from your application into the load balancer and it makes your application behave differently with and without load balancer. IMO, in a lot of cases adding an additional handler isn't that much work but it makes the routing explicit.

However, I get that some people would like to this support for this and there are some edge cases where adding handlers is not easy. I've outlined a different syntax for the urlprefix tag in #42 which would allow this option more organically. Feel free to work on that and build the strip prefix option on top of that.

[aaronhurt]

This is absolutely needed in our environment. We're currently using a very complex consul-template design that is based on tags including "proxy-standard" and "proxy-nostrip". Our standard tag does a regrep on the request and strips the route prefix before passing it on the the backend whereas the nostrip does the opposite.

[magiconair]

@jonmorehouse I'm going to close this PR as the strip prefix feature has been implemented for #44 and shipped in v1.3.7. In any case I want to thank you for your submission to show interest in the project and this feature.