Effective URIs

Effective URIs (also sometimes referred to by the somewhat more specific term URL [1]) are central to the design of a usable HTTP API. Uniform Resource Identifiers are defined by RFC 3986 and have their use in HTTP clarified in RFC 7230. A URI is the identifier of a resource in an API and is also used to locate and address that resource on network.

A URI is divided up into several sections: scheme, authority, path, query, and fragment (please refer to RFC 3986 for more detail). As a developer of API services, path and query are most relevant; fragment is not usually sent to the service and there is little opportunity nor reason to exert control over scheme and authority.

What follows will concern itself solely with the path and query.

Things Worth Knowing

  • The value of path and query are case sensitive. That is if you have two URIs that look similar, http://example.com/foo/BAR?dEtail=1 and http://example.com/foo/bar?detail=1, they are not equivalent. A server application can choose to normalize the URI but this not recommended as it does not correspond with common use nor RFC 7230.
  • While a path often looks like a hierarchical path (/collection/item/sub-resource/detail), akin to the full filename of a file on disk, this is a matter of convenience and an artifact of making the URI consumable by humans. The truth of the matter is that the entirety of the URI identifies the resource, not just the last segment of the path.
  • The entire point of URI design [2] is to make the URIs consumable by humans in a meaningful fashion both for users of the API and future maintainers of the API.

General Advice on URI Design

Note

This is far from an exhaustive list. This is merely a starting point from which we can accumulate reasonable advice on how to form good URIs.

  • Since a single URI identifies a single resource it is also useful for two more things to be kept true when building services:

    • Any resource should only have one URI. Where possible do not provide multiple ways to reference the same thing. Use HTTP redirects to resolve indirect requests to the correct canonical URI and content negotiation to request different representations.

    • Any given resource, since it should only have one URI, should respond to all relevant HTTP methods at just that URI and not have secondary URIs for some methods. For example:

      Use:

      GET /resources/1322b203bdc64c13b6e72b04d43e8690
      DELETE /resources/1322b203bdc64c13b6e72b04d43e8690
      

      Never:

      GET /resources/1322b203bdc64c13b6e72b04d43e8690
      DELETE /resources/1322b203bdc64c13b6e72b04d43e8690/delete
      
  • It is often the case that an API will have URIs that represent collections for resources and individual members of that collection in a hierarchy. For example [3]:

    GET /birds
    
    {"birds": [
        {
            "name": "alpha",
            "type": "crow"
        },
        {
            "name": "beta",
            "type": "jackdaw"
        }
    }
    
    GET /birds/alpha
    
    {
        "name": "alpha",
        "type": "crow"
    }
    

    This is a reasonable thing to do as it goes a long way to making the elements of an API comprehensible.

  • If the hierarchy described above is used it is important that all URIs that take the second form (/birds/alpha) have the same semantics and are always identifying a resource which is a member of this collection (in this case “is a bird”).

    There are multiple examples throughout OpenStack of this concept being violated. For example in the os-cells API in nova it is possible to GET /os-cells to get a list of cells, GET /os-cells/some-name to get information about a single cell named some-name and GET /os-cells/details to get the same information as GET /os-cells but with additional detail.

    For this particular example one way (of several options) to achieve the same result while preserving URI semantics would have been to use a query to augment the existing collection URI to indicate the desire for more detail [3]:

    GET /os-cells?details=true
    

    Todo

    There is an as yet unresolved debate on the best way to indicate boolean query parameters. Any of details=true, details=1 or details could make sense here. The above should not be taken to indicate support for any query format. Rather it is merely to demonstrate cleaner semantics in the path portion of the URI.

Complex Queries

In some cases it may be necessary to return a set of resources that match a set of filter conditions. For those situations, use the GET method, and create a query string that concatenates all the requirements. As an example, if you needed to return all the birds which are blue and are migratory and that swim, the URI would look like:

GET /birds?color=blue&migratory=true&swimming=true

There are restrictions on the length of URIs that vary depending on the server and client in use. The most restrictive are some browsers that have a maximum URI length of about 2K, while web servers such as Apache limit URIs to around 8K. If the length of the URI needed to express the complex requirements of a request may possibly exceed those limits, it is acceptable to use the POST method with the filter conditions passed in the body of the request.

Footnotes

[1]https://en.wikipedia.org/wiki/Uniform_Resource_Locator
[2]There is another school of thought which insists that URIs should be entirely opaque identifiers which computers use to exchange information. There’s a lot of value in this line of thinking as it allows the identifiers to act as references to fungible referents, but it discounts the value and cost of creating a diverse collection of clients for services. If we wish to encourage that diverse collection then having URIs which are consumable by humans is helpful.
[3](1, 2) These are example requests and responses only and should not be taken as explicitly describing correct form.

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