Different RESTful representations of the same reso

问题:

My application has a resource at /foo. Normally, it is represented by an HTTP response payload like this:

{"a": "some text", "b": "some text", "c": "some text", "d": "some text"}

The client doesn't always need all four members of this object. What is the RESTfully semantic way for the client to tell the server what it needs in the representation? e.g. if it wants:

{"a": "some text", "b": "some text", "d": "some text"}

How should it GET it? Some possibilities (I'm looking for correction if I misunderstand REST):

• GET /foo?sections=a,b,d.
  • The query string (called a query string after all) seems to mean "find resources matching this condition and tell me about them", not "represent this resource to me according to this customization".
• GET /foo/a+b+d My favorite if REST semantics doesn't cover this issue, because of its simplicity.
  • Breaks URI opacity, violating HATEOAS.
  • Seems to break the distinction between resource (the sole meaning of a URI is to identify one resource) and representation. But that's debatable because it's consistent with /widgets representing a presentable list of /widget/<id> resources, which I've never had a problem with.
• Loosen my constraints, respond to GET /foo/a, etc, and have the client make a request per component of /foo it wants.
  • Multiplies overhead, which can become a nightmare if /foo has hundreds of components and the client needs 100 of those.
  • If I want to support an HTML representation of /foo, I have to use Ajax, which is problematic if I just want a single HTML page that can be crawled, rendered by minimalist browsers, etc.
  • To maintain HATEOAS, it also requires links to those "sub-resources" to exist within other representations, probably in /foo: {"a": {"url": "/foo/a", "content": "some text"}, ...}
• GET /foo, Content-Type: application/json and {"sections": ["a","b","d"]} in the request body.
  • Unbookmarkable and uncacheable.
  • HTTP does not define body semantics for GET. It's legal HTTP but how can I guarantee some user's proxy doesn't strip the body from a GET request?
  • My REST client won't let me put a body on a GET request so I can't use that for testing.
• A custom HTTP header: Sections-Needed: a,b,d
  • I'd rather avoid custom headers if possible.
  • Unbookmarkable and uncacheable.
• POST /foo/requests, Content-Type: application/json and {"sections": ["a","b","d"]} in the request body. Receive a 201 with Location: /foo/requests/1. Then GET /foo/requests/1 to receive the desired representation of /foo
  • Clunky; requires back-and-forth and some weird-looking code.
  • Unbookmarkable and uncacheable since /foo/requests/1 is just an alias that would only be used once and only kept until it is requested.

回答1:

I would suggest the querystring solution (your first). Your arguments against the other alternatives are good arguments (and ones that I've run into in practise when trying to solve the same problem). In particular, the "loosen the constraints/respond to foo/a" solution can work in limited cases, but introduces a lot of complexity into an API from both implementation and consumption and hasn't, in my experience, been worth the effort.

I'll weakly counter your "seems to mean" argument with a common example: consider the resource that is a large list of objects (GET /Customers). It's perfectly reasonable to page these objects, and it's commonplace to use the querystring to do that: GET /Customers?offset=100&take=50 as an example. In this case, the querystring isn't filtering on any property of the listed object, it's providing parameters for a sub-view of the object.

More concretely, I'd say that you can maintain consistency and HATEOAS through these criteria for use of the querystring:

• the object returned should be the same entity as that returned from the Url without the querystring.
• the Uri without the querystring should return the complete object - a superset of any view available with a querystring at the same Uri. So, if you cache the result of the undecorated Uri, you know you have the full entity.
• the result returned for a given querystring should be deterministic, so that Uris with querystrings are easily cacheable

However, what to return for these Uris can sometimes pose more complex questions:

• returning a different entity type for Uris differing only by querystring could be undesirable (/foo is an entity but foo/a is a string); the alternative is to return a partially-populated entity
• if you do use different entity types for sub-queries then, if your /foo doesn't have an a, a 404 status is misleading (/foo does exist!), but an empty response may be equally confusing
• returning a partially-populated entity may be undesirable, but returning part of an entity may not be possible, or may be more confusing
• returning a partially populated entity may not be possible if you have a strong schema (if a is mandatory but the client requests only b, you are forced to return either a junk value for a, or an invalid object)

In the past, I have tried to resolve this by defining specific named "views" of required entities, and allowing a querystring like ?view=summary or ?view=totalsOnly - limiting the number of permutations. This also allows for definition of a subset of the entity that "makes sense" to the consumer of the service, and can be documented.

Ultimately, I think that this comes down to an issue of consistency more than anything: you can meet HATEOAS guidance using the querystring relatively easily, but the choices you make need to be consistent across your API and, I'd say, well documented.

回答2:

I've decided on the following:

Supporting few member combinations: I'll come up with a name for each combination. e.g. if an article has members for author, date, and body, /article/some-slug will return all of it and /article/some-slug/meta will just return the author and date.

Supporting many combinations: I'll separate member names by hyphens: /foo/a-b-c.

Either way, I'll return a 404 if the combination is unsupported.

Architectural constraint

REST

Identifying resources

From the definition of REST:

a resource R is a temporally varying membership function M_R(t), which for time t maps to a set of entities, or values, which are equivalent. The values in the set may be resource representations and/or resource identifiers.

A representation being an HTTP body and an identifier being a URL.

This is crucial. An identifier is just a value associated with other identifiers and representations. That's distinct from the identifier→representation mapping. The server can map whatever identifier it wants to any representation, as long as both are associated by the same resource.

It's up to the developer to come up with resource definitions that reasonably describe the business by thinking of categories of things like "users" and "posts".

HATEOAS

If I really care about perfect HATEOAS, I could put a hyperlink somewhere in the /foo representation to /foo/members, and that representation would just contain a hyperlink to every supported combination of members.

HTTP

From the definition of a URL:

The query component contains non-hierarchical data that, along with data in the path component, serves to identify a resource within the scope of the URI's scheme and naming authority (if any).

So /foo?sections=a,b,d and /foo?sections=b are distinct identifiers. But they can be associated within the same resource while being mapped to different representations.

HTTP's 404 code means that the server couldn't find anything to map the URL to, not that the URL is not associated with any resource.

Functionality

No browser or cache will ever have trouble with slashes or hyphens.

回答3:

Actually it depends on the functionality of the resource. If for example the resource represents an entity:

/customers/5

Here the '5' represents an id of the customer

Response:

{
   "id": 5,
   "name": "John",
   "surename": "Doe",
   "marital_status": "single",
   "sex": "male",
   ...
}

So if we will examine it closely, each json property actually represents a field of the record on customer resource instance. Let's assume consumer would like to get partial response, meaning, part of the fields. We can look at it as the consumer wants to have the ability to select the various fields via the request, which are interesting to him, but not more (in order to save traffic or performance, if part of the fields are hard to compute).

I think in this situation, the most readable and correct API would be (for example, get only name and surename)

/customers/5?fields=name,surename

Response:

{
   "name": "John",
   "surename": "Doe"
}

HTTP/1.1

• if illegal field name is requested - 404 (Not Found) is returned
• if different field names are requested - different responses will be generated, which also aligns with the caching.
• Cons: if the same fields are requested, but the order is different between the fields (say: fields=id,name or fields=name,id), although the response is the same, those responses will be cached separately.

HATEOAS

• In my opinion pure HATEOAS is not suitable for solving this particular problem. Because in order to achieve that, you need a separate resource for every permutation of field combinations, which is overkill, as it is bloating the API extensively (say you have 8 fields in a resource, you will need permutations!).
• if you model resources only for the fields but not all the permutations, it has performance implications, e.g. you want to bring the number of round trips to minimum.

回答4:

If a,b,c are property of a resource like admin for role property the right way is to use is the first way that you've suggested GET /foo?sections=a,b,d because in this case you would apply a filter to the foo collection. Otherwise if a,b and c are a singole resource of foo collection the the way that would follow is to do a series of GET requests /foo/a /foo/b /foo/c. This approach, as you said, has a high payload for request but it is the correct way to follow the approach Restfull. I would not use the second proposal made ​​by you because plus char in a url has a special meaning.

Another proposal is to abandon use GET and POST and create an action for the foo collection like so: /foo/filter or /foo/selection or any verb that represent an action on the collection. In this way, having a post request body, you can pass a json list of the resource you would.

回答5:

you could use a second vendor media-type in the request header application/vnd.com.mycompany.resource.rep2, you can't bookmark this however, query-parameters are not cacheable (/foo?sections=a,b,c) you could take a look at matrix-parameters however regarding this question they should be cacheable URL matrix parameters vs. request parameters