Generally, resources can have multiple presentations, mostly because there may be multiple different clients expecting different representations. Asking for a suitable presentation by a client, is referred as content negotiation.
Server-driven Vs Agent-driven Content Negotiation
If the selection of the best representation for a response is made by an algorithm located at the server, it is called server-driven negotiation. If that selection is made at agent or client-side, its called agent-driven content negotiation.
Practically, you will NOT find much usage of server-side negotiations because, in that way, you have to make lots of assumptions about client expectations. Few things like client context or how the client will use the resource representation are almost impossible to determine. Apart from that this approach makes the server-side code complex, unnecessarily.
So, most REST API implementations rely on agent-driven content negotiations. Agent driven content negotiation rely on usage of HTTP request headers or resource URI patterns.
Content negotiation using HTTP headers
At server side, an incoming request may have an entity attached to it. To determine it’s type, server uses the HTTP request header
Content-Type. Some common examples of content types are “text/plain”, “application/xml”, “text/html”, “application/json”, “image/gif”, and “image/jpeg”.Content-Type: application/json
Similarly, to determine what type of representation is desired at client side, HTTP header
ACCEPTis used. It will have one of the values as mentioned forContent-Typeabove.Accept: application/json
Generally, if no
Acceptheader is present in the request, the server can send pre-configured default representation type.ImplementingAcceptheader based content negotiation is most used and recommened way.Content negotiation using URL patterns
Another way to pass content type information to the server, the client may use the specific extension in resource URIs. For example, a client can ask for details using:
http://rest.api.com/v1/employees/20423.xml http://rest.api.com/v1/employees/20423.json
In above case, first request URI will return a XML response whether second request URI will return a JSON response.
Defining preferences
It is possible to have multiple values in Accept header. The client may want to give multiple values in the accept header when the client is not sure about if its desired representation is present or supported by the server at that time. [RFC 2296]
For example,
Accept: application/json,application/xml;q=0.9,*/*;q=0.8
Above Accept header allows you to ask the server a JSON format. If it can’t, perhaps it could return XML format (the second level). If it’s still not possible, let it return what it can.
The preference order is defined through the q parameter with values from 0 to 1. When nothing is specified, the implicit value is 1.
Mike says
Just curious, and I may need to post this under a different topic, but I was looking to determine the correlation between Accept and Content-Type headers versus swagger consumes and produces keywords. I would expect a 1-1 relation, but simply adding a produces in a swagger does not require a user to send in an accept header. Can someone explain the correlation or point me to a site that has this info? Swagger.io discusses the headers and keywords, but not their correlation, which I am looking for.
Thank you in advance for any help, and sorry if this should be under a different topic.
Oswald says
I think there is some confusion in the “Content negotiation using URL patterns” because it shows an example using an extension to locate a URI. While in the “REST Resource Naming Guide” section “Do not use file extenstions” you recommend not to use file extensions.
Admin says
I tried to show the possible ways to achieve it.
KSN says
I have different GET request with the different content-type returned.
@GET
@Produces(“application/text+xml;qs=0.75;charset=’utf-8′”)
public Source getText( ) {
}
@GET
@Produces(“application/xml;qs=0.5;charset=’utf-8′”)
public Source getXml( ){
}
When I request from FireFox which goes into the first method due to FireFox set the Accept-header: “text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8”.
Why it is going into first even the request header has “application/xml”?
Any help will be appreciated!!
Thanks in advance.
James Passmore says
They are too generic, Roy Fielding in comment 31 of his blog tells us:
…
In terms of testing a specification, the hardest part is identifying when a RESTful protocol is actually dependent on out-of-band information or if the authors are just overspecifying things for the purpose of documentation. What I look for are requirements on processing behavior that are defined outside of the media type specification. One of the easiest ways to see that is when a protocol calls for the use of a generic media type (like application/xml or application/json) and then requires that it be processed in a way that is special to the protocol/API.
…
Xavier says
Isn’t “application/xml” or “application/json” a bit too generic?
Wouldn’t it be better to use vendor specific media types like “application/vnd.mycompany.customers.v1+json” or “application/vnd.mycompany.customers+json;version=1”?
You can put a lot of stuff in the ACCEPT header like the version or the charset which helps keeping your URIs clean (no version or file extension… after all, the U in URI stands for Unique, it shouldn’t change when you request another representation for the same resource).
Admin says
Yes, vendor specific media types are good idea and I see them in use in most projects. They work.
U stands for Uniform (NOT Unique). Both have absolutely different meanings.
pramod sahni says
how to produce different response for different client in rest api?
Admin says
Why you want to do that? Please consider it twice before implementing.
Anyway, If you still has a good reason to do this then I will suggest to use “optionality index” concept. In this approach, client will pass an extra request header with predetermined value.
For each different optioanlity index, return a different response. e.g.
steven c says
are you sure about this?
Oscar says
Negotiation via URI patterns is against REST resource naming best practices, isn’t it? I refer to “Consistency is the key” , where it’s advisable “Do not use file extentions”
Admin says
It’s true that this pattern does not add any value, and not advisable. Still it’s mentioned here because it is a possible way of content negotiation and many people may find it useful for their API consumers.
Oscar says
Thanks for your reply!!
Great tutorial 😉
steven c says
true that
Eugen says
If format is present via URI and Accept header what precedence should be in this case?
My personal opinion is: use URI than fall back to Accept header.
But I am not sure this is right. Is there some RFC or spec which define this priority?
Admin says
I second your opinion. I have not heard of any RFC which talk about having format information in URI.