Content negotiation refers to mechanisms defined as a part of HTTP that make it possible to serve different versions of a document (or more generally, representations of a resource) at the same URI, so that user agents can specify which version fits their capabilities the best. One classical use of this mechanism is to serve an image in GIF or PNG format, so that a browser that cannot display PNG images (e.g. MS Internet Explorer 4) will be served the GIF version.
A resource may be available in several different representations; for example, it might be available in different languages or different media types. One way of selecting the most appropriate choice is to give the user an index page and let them select the most appropriate choice; however it is often possible to automate the choice based on some selection criteria.
HTTP provides for several different content negotiation mechanisms including: server-driven (or proactive), agent-driven (or reactive), transparent, and/or hybrid combinations thereof.
Server-driven or proactive content negotiation is performed by algorithms on the server which choose among the possible variant representations. This is commonly performed based on a user-agent provided acceptance criteria.
To summarize how this works, when an user agent submits a request to a server, the user agent informs the server what media types it understands with ratings of how well it understands them. More precisely, the user agent provides an
Accept HTTP header that lists acceptable media types and associated quality factors. The server is then able to supply the version of the resource that best fits the user agent's needs.
This works because browsers can send information as part of each request about the representations they prefer. For example, a browser could indicate that it would like to see information in German, if possible, else English will do. Browsers indicate their preferences by headers in the request. To request only German representations, the browser would send:
Note that this preference will only be applied when there is a choice of representations and they vary by language.
As an example of a more complex request, this browser has been configured to accept German and English, but prefer German, and to accept various media types, preferring HTML over plain text or other text types, and preferring GIF or JPEG over other media types, but also allowing any other media type as a last resort:
Accept-Language: de; q=1.0, en; q=0.5 Accept: text/html; q=1.0, text/*; q=0.8, image/gif; q=0.6, image/jpeg; q=0.6, image/*; q=0.5, */*; q=0.1
In addition to server-driven content negotiation by content type and by language, there is an extension to use content negotiation to retrieve prior version in time with the
RFC 7231 does not specify how to resolve trade-offs (such as, in the above example, choosing between an HTML page in English and a GIF image in German).
Agent-driven or reactive content negotiation is performed by algorithms in the user-agent which choose among the possible variant representations. This is commonly performed based on a server provided list of representations and metadata about them.
To summarize how this works, when a user agent submits a request to a server, the server informs the user-agent which representations it has available as well as any metadata it has about each representation (e.g., content-type, quality, language, etc.). The user-agent then resubmits the request to a specific URL for the chosen representation. This can be automatically chosen by the user-agent or the user-agent can present the user with the choices and the user can directly choose such. More precisely, the server responds with either 300 Multiple Choices or 406 Not Acceptable (when server-driven, user-agent provided acceptance criteria is provided but the server cannot automatically make a selection). Unfortunately HTTP leaves the format of the list of representations and metadata along with selection mechanisms unspecified.
User-agents can request data in specified formats from web services or web APIs, such as application/json or application/xml.