Saturday, March 18, 2017


Inspired by Why should I prefer HTTP REST over HTTP RPC JSON-Messaging style in conjunction with CQRS?

As with most REST questions, the easy answers can be found by looking at the web through a browser.

In RPC, the client creates an application/x-www-url-encoded document and sends it to the server.
In REST, the client completes a form and submits it.

In both cases, the client-stateless-server architectural constraint still applies; the server has no way of knowing anything about the nature of the client.  The message looks the same regardless of whether the client walks through the guided path or skips to the end.

In 2008, Roy T. Fielding made the following observation of a solution he was outlining.
I should also note that the above is not yet fully RESTful, at least how I use the term. All I have done is described the service interfaces, which is no more than any RPC. In order to make it RESTful, I would need to add hypertext to introduce and define the service, describe how to perform the mapping using forms and/or link templates, and provide code to combine the visualizations in useful ways.
My translation: having identified the endpoint of the protocol, you work toward REST by relaxing the understanding of the endpoint required by the client.

The RPC approach to a form submission requires that the client know in advance
  1. The location of the resource that accepts the request
  2. The appropriate HTTP method to use
  3. The correct spellings of the keys in the key/value pairs
  4. The semantic meaning of the keys/value pairs
In the REST approach, this information is delivered on demand, in the hypermedia.  So the location, the method, the keys are all covered.  The client just loads the form, uses semantic cues to recognize which fields to change, and the submits the form.

Of course, this also means that the client needs to understand forms and how to interpret them.  That's not free, but it shifts the problem from knowing in advance about the specific service to knowing in advance about a generic media type, and the conventions for semantic cues.

The riddle of finding the form gets pushed up to the bookmarked representation.  The client still needs a starting point, which is the bookmark.  It loads a representation of the bookmark provided by the server, and uses the available semantic cues to find a link to the required form.

The server can direct the client to a new representation of the form by changing the representation of the bookmark.  The new form representation can include additional fields with new semantic cues; the client, knowing only about the original semantics, simply ignores these fields when filling out the form; which in turn means that the values submitted will be those provided by the server in the form representation itself.

This isn't free, by any stretch -- we're buying the decoupling of the client and server by doing more upfront api design.
REST is intended for long-lived network-based applications that span multiple organizations. If you don’t see a need for the constraints, then don’t use them.
Stable APIs require a design investment.  The stability constraint, however, is optional; you should reject that constraint when the benefits are insufficient to offset the costs.

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