https://stackoverflow.com/questions/19959284
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RussianYour question has a lot of answers on Stackoverlow, but most of them skirt the reason you are asking, and I suspect you always find them partially disatisfying.
If we take Roy Fielding at his word, it is impossible to write most commercial interactive Client-apps as SOA RESTful/HATEOAS using HTTP/HTML. It might be possible in other mediums, I can't say.
So the practical answer is "look it up in the documentation" and "write your Client with that app-knowledge in it" with a side-helping of "ignore the fact that we're breaking Fielding's rules by doing that".
I tend to design JSON responses that provide this approach:
GET /account/12345 HTTP/1.1
{
"account": {
"number": "12345",
"currency": "usd",
"balance": "100.00",
"deposit": {
"href": "/account/12345/deposit",
"action": "POST"
},
"withdraw": {
"href": "/account/12345/withdraw",
"action": "POST"
},
"transfer": {
"href": "/account/12345/transfer",
"action": "POST"
},
"close": {
"href": "/account/12345/close",
"action": "DELETE"
}
}
}
... adding additional properties to the design as needed, but these are the basics.
I believe this allows the consuming Client(s) to be written in a RESTful way, but in so doing I am using the Response Body, which Fielding says is not what he intended.
I'd offer this explanation seperate to the answer though:
Fielding says "I am getting frustrated by the number of people calling any HTTP-based interface a REST API." (http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven).
Note how he so stridently says "any" HTTP-based interface.
The most relevant segment in his 'lecture' is this:
"A REST API should be entered with no prior knowledge beyond the initial URI (bookmark) and set of standardized media types that are appropriate for the intended audience (i.e., expected to be understood by any client that might use the API). From that point on, all application state transitions must be driven by client selection of server-provided choices that are present in the received representations or implied by the user’s manipulation of those representations. The transitions may be determined (or limited by) the client’s knowledge of media types and resource communication mechanisms, both of which may be improved on-the-fly (e.g., code-on-demand). [Failure here implies that out-of-band information is driving interaction instead of hypertext.]"
He says this because an HTTP/HTML app URI media-type is just "text/html", and where is the Verb in that? There isn't one. A URI cannot tell you what Verb something requires for use/navigation, ergo you cannot use just the in-band data alone to construct the 'next' navigation dynamically in your Client.
He explained that he believes we render our URI's as part of CDATA that includes the "method" or that the context of the URI would self-evidently provide it, like the FORM element does. He explicitly rails against the OpenSocialst REST API stating that it is not RESTful.
Here: “anchor elements with an href attribute create a hypertext link that, when selected, invokes a retrieval request (GET) on the URI corresponding to the CDATA-encoded href attribute.” Identifiers, methods, and media types are orthogonal concerns — methods are not given meaning by the media type. Instead, the media type tells the client either what method to use (e.g., anchor implies GET) or how to determine the method to use (e.g., form element says to look in method attribute). The client should already know what the methods mean (they are universal) and how to dereference a URI.
Note that he says the Client should already know what the methods mean, he does not say that the Client should already know what they are - which is why you asked your question. A lot of people struggle with this, because we don't in fact build our apps like this in most SOA environments.
Like a lot of engineers, I just wish Fielding would come out with a clarification or re-statement, but not only has he not done so, he has published two further admonishments to us as engineers, doubling-down on his statement, saying that we should stop calling our API's RESTful and accept that we're building RPC's.
I think the JSON elements-like approach is a reasoble bridge, but I have no answer to the fact that we're using the Request Body to do it, not relying on the Media Type to imply it.
Finally, there is a newer Verb in HTTP called OPTIONS which, for a given URI, would return the allowed Verb actions list. I think Fielding had a hand in writing this HTTP revision. That would allow a Client to generically construct URI navigations without the forbidden internal app knowledge. But that has three issues I can think of in the practical world:
- You would have to code a mechanism into your Service Aggregation to make that call for every URI you try to return, and since much data contains many URI's (_links in HAL) that adds a lot of extra 'hops' into your Service Response construction. We'd probably all complain about that.
- Virtually no SOA site claiming to be RESTful actually implements an OPTIONS verb-method-call for you to do this enquiry with anyway.
- We would all complain about the 'unecessary' extra calls it adds (especially in the eCommerce world) to a Client's processing and its tendency to push us beyond SLA requirements.
