1. Content Negotiation
Generally, the REST resources can have multiple presentations, mostly because there may be different clients expecting different representations. Asking for a suitable presentation by a client is referred to as content negotiation.
HTTP has provisions for several mechanisms for “content negotiation” — the process of selecting the best representation for a given response when there are multiple representations available.
— RFC 2616
2. 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 the agent or client-side, it is called agent-driven negotiation.
Practically, we will NOT find much usage of server-side negotiations because, in that way, we have to make a lot of assumptions about the client’s expectations.
Few things, like the client context or how the client will use the resource representation, are almost impossible to determine. Also, the server-driven negotiation makes the server-side code more complex, unnecessarily.
So, most REST API implementations rely on agent-driven content negotiations. Agent-driven content negotiation depends on the usage of HTTP request headers or resource URI patterns.
2.1. 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/jsonSimilarly, to determine what type of representation is desired on the client-side, an HTTP header ACCEPT is used. It will have one of the values mentioned for Content-Type above.
Accept: application/jsonGenerally, if no Accept header is present in the request, the server can send pre-configured default representation type.
Implementing
Acceptheader based content negotiation is most used and recommened way.
2.2. 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.jsonIn the above case, the first request URI will return an XML response whether the second request URI will return a JSON response.
3. Defining Preferences
It is possible to have multiple values in Accept header using the ‘q’ parameter i.e. relative quality factor.
The client may want to provide multiple values in the accept header when the client is not sure 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=0.8Above Accept header allows you to ask the server a JSON format (first choice). If it can’t, perhaps it could return XML format (the second choice). 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 default value is 1.
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.
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.
I tried to show the possible ways to achieve it.
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.
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.
…
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).
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.
how to produce different response for different client in rest api?
Why do you want to do that? Please consider it twice before implementing.
Anyway, If you still have a good reason to do this then I will suggest using “optionality index” concept. In this approach, the client will pass an extra request header with a predetermined value.
For each different optionality index, return a different response. e.g.
are you sure about this?
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”
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.
Thanks for your reply!!
Great tutorial 😉
true that
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?
I second your opinion. I have not heard of any RFC which talk about having format information in URI.