REST - What exactly is meant by Uniform Interface?

问题:

Wikipedia has:

Uniform interface

The uniform interface constraint is fundamental to the design of any REST service.[14] The uniform interface simplifies and decouples the architecture, which enables each part to evolve independently. The four guiding principles of this interface are:

Identification of resources

Individual resources are identified in requests, for example using URIs in web-based REST systems. The resources themselves are conceptually separate from the representations that are returned to the client. For example, the server may send data from its database as HTML, XML or JSON, none of which are the server's internal representation, and it is the same one resource regardless.

Manipulation of resources through these representations

When a client holds a representation of a resource, including any metadata attached, it has enough information to modify or delete the resource.

Self-descriptive messages

Each message includes enough information to describe how to process the message. For example, which parser to invoke may be specified by an Internet media type (previously known as a MIME type). Responses also explicitly indicate their cacheability.

Hypermedia as the engine of application state (A.K.A. HATEOAS)

Clients make state transitions only through actions that are dynamically identified within hypermedia by the server (e.g., by hyperlinks within hypertext). Except for simple fixed entry points to the application, a client does not assume that any particular action is available for any particular resources beyond those described in representations previously received from the server.

I'm listening to a lecture on the subject and the lecturer has said:

"When someone comes up to our API, if you are able to get a customer object and you know there are order objects, you should be able to get the order objects with the same pattern that you got the customer objects from. These URI's are going to look like each other."

This strikes me as wrong. It's not so much about what the URI's look like or that there is consistency as it is the way in which the URI's are used (identify resources, manipulate the resources through representations, self-descriptive messages, and hateoas).

I don't think that's what Uniform Interface means at all. What exactly does it mean?

回答1:

Using interfaces to decouple classes from the implementation of their dependencies is a pretty old concept. I am surprised that you haven't heard of it...

By REST you use the same concept to decouple the client from the implementation of the REST service. In order to do define such an interface (contract between the client and the service) you have to use standards. This is because if you want an internet size network of REST services, you have to enforce global concepts, like standards to make them understand each other.

• Identification of resources - You use the URI (IRI) standard to identify a resource. In this case a resource is a web document.

• Manipulation of resources through these representations - You use the HTTP standard to describe communication. So for example GET means that you want to retrieve data about the URI identified resource. You can describe an operation with a HTTP method and an URI.

• Self-descriptive messages - You use standard MIME types and (standard) RDF vocabs to make messages self-descriptive. So the client can find the data by checking the semantics, and it don't have to know the application specific data structure the service uses.

• Hypermedia as the engine of application state (A.K.A. HATEOAS) - You use hyperlinks and possibly URI templates to decouple the client from the application specific URI structure. You can annotate these hyperlinks with semantics e.g. IANA link relations, so the client will understand what they mean.

回答2:

Ok I think I understand what it means.

From Fieldings dissertation:

The central feature that distinguishes the REST architectural style from other network-based styles is its emphasis on a uniform interface between components (Figure 5-6). By applying the software engineering principle of generality to the component interface, the overall system architecture is simplified and the visibility of interactions is improved.

He's saying that the interface between components must be the same. Ie. between client and server and any intermediaries, all of which are components.

回答3:

Your question is somewhat broad, you seem to be asking for a restatement of the definitions you have. Are you looking for examples or do you not understand somethings specifically stated.

I agree that the line:

These URI's are going to look like each other

is fundamentally wrong. URIs needn't look anything like each other for the Uniform interface constraint to be met. What needs to be present is a uniform way to discover the URIs that identify the resources. This uniform way is unique to each message type, and there must be some agreed upon format. For example in HTML one document resource links to another via a simple tag:

<a href="URI of related resource" rel="defined relationship">fallback relationship</a>

HTTP servers return html as a text/html resource type which browsers have an agreed upon way of parsing. The anchor tag is the hypermedia control (HATEOAS) that has the unique identifier for the related resource.

The only point that wasn't covered was manipulation. HTML has another awesome example of this, the form tag:

<form action="URI" method="verb">
  <input name=""></input>
</form>

again, browser know how to interpret this meta information to define a representation of the resource acted upon at the URI. Unfortunately HTML only lets you GET and POST for verbs...

more commonly in a JOSN based service, when you retrieve a Person resource, it's easy to manipulate that representation and then PUT or PATCH it right back to it's canonical URL. No pre-existing knowledge of the resource is needed to modify it. Now when we write client code we get all wrapped up with the idea that we do in fact need to know the shape before we consume it...but that really is just to make our parsers efficient and easy. We could make parsers that analyze the semantic meaning of each part of a resource and modify it by interpreting the intent of the modification. IE: a command of make the person 10 years older would parse the resource looking for the age, identify the age, and then add 10 years to that value, then send that resource back to the server. Is it easier to have code that expects the age to be at a JSON path of $.age? absolutely...but it's not specifically necessary.

回答4:

"Uniform interface" actually means that server responses should announce available actions and resources. In other words, querying resources should allow the client to request other actions and resources without knowing them in advance.

JSON-API spec offers a good example:

{
  "links": {
    "self": "http://example.com/articles",
    "next": "http://example.com/articles?page[offset]=2",
    "last": "http://example.com/articles?page[offset]=10"
  },
  "data": [{
    "type": "articles",
    "id": "1",
    "attributes": {
      "title": "JSON API paints my bikeshed!"
    },
    "relationships": {
      "author": {
        "links": {
          "self": "http://example.com/articles/1/relationships/author",
          "related": "http://example.com/articles/1/author"
        },
      },
      "comments": {
        "links": {
          "self": "http://example.com/articles/1/relationships/comments",
          "related": "http://example.com/articles/1/comments"
        }
      }
    },
    "links": {
      "self": "http://example.com/articles/1"
    }
  }]
}

Just by analysing this single response, a client knows:

1. what he was querying (articles)
2. how are structured articles (id, title, author, comments)
3. how to retrieve related objects (i.e. author, comments list)
4. that there are more articles (10, based on current response length and pagination links)

I Hope this helps.
For those passionate about the topic, I strongly recommend reading Thomas Fielding's dissertation!