Negotiator

This is a library that handles content negotiation in HTTP server applications written in Go.

Usage

Simple

To return JSON/XML out of the box simple put this in your route handler:

import "github.com/rickb777/negotiator"
...
func getUser(w http.ResponseWriter, req *http.Request) {
    user := &User{"Joe","Bloggs"}
    n := negotiator.NewWithJSONAndXML()
    n.MustNegotiate(w, req, negotiator.Offer{Data: user})
}

Custom

To add your own negotiator, for example you want to write a PDF with your model, do the following:

1. Create a type that conforms to the ResponseProcessor interface

2. Where you call negotiator.New(responseProcessors ...ResponseProcessor), pass in a your custom processor. When your request handler calls negotiator.Negotiate(w, req, offers...) it will render a PDF if your Accept header defined it wanted a PDF response.

When a request is Not Acceptable

Having created a Negotiator with one or more response processors, if a request is handled that is not claimed by and processor, a Not Acceptable (406) response is returned.

By default, this uses the standard http.Error function (from net/http) to render the response, If needed, a custom error handler can be plugged in using Negotiator.WithErrorHandler(myHandler).

Accept Handling

The Accept header is parsed using header.ParseMediaRanges(), which returns the slice of media ranges, e.g.

    // handle Accept-Language
    mediaRanges := header.ParseMediaRanges("application/json;q=0.8, application/xml, application/*;q=0.1")

The resulting slice is sorted according to precedence and quality rules, so in this example the order is {"application/xml", "application/json", "application/*"} because the middle item has an implied quality of 1, whereas the first item has a lower quality.

The other content-negotiation headers, Accept-Language, Accept-Charset, Accept-Encoding, are handled by the header.Parse method, e.g.

    // handle Accept-Language
    acceptLanguages := header.Parse("en-GB,en;q=0.5")

This will contain {"en-GB", "en"} in a header.PrecedenceValues slice, sorted according to precedence rules.

This can be used for Accept-Language, Accept-Charset and Accept-Encoding, as well as Accept. The negotiator in this API uses the Accept header only, though.

Acknowledgement

Many thanks to Jonathan Channon (https://github.com/jchannon) for the original concepts and work on which this was based.