The best way to understand REST is by example. First check out the blog post by Roy Fielding mentioned in this thread which summarizes the constraints [1]. Then let's examine how several popular restful services work versus those constraints.

Here are some popular well-known services to use as examples: Google.com Amazon.com Facebook.com News.ycombinator.com.

- You enter these services through the site root with no prior knowledge about the services aside from standard media types (HTML/CSS/JS)

- Hypermedia is the engine of application state – meaning that you load a webpage a.k.a. resource that contains hypermedia links to other resources, and you navigate between states in the application by following those hypermedia links. For example, the hacker news homepage contains links to news articles and comment threads about them, which contains links to reply to those comments, and so on.

- The application describes how to compose requests using standard media types such as web forms and JavaScript. Because the pages instruct the client about what requests to compose and to which URL, the services have control over their own name spaces which have no fixed resource names or hierarchy.

I consider REST to be most easily understood as a departure to application protocol design prior to the Web, where each application and service had a unique and custom binary protocol, and a client for these services had to be reprogrammed to interact with it. REST is a set of constraints for designing services that run at Internet scale and that highly decouples clients and servers. Fielding's thesis on REST has more detail on the principles by which the REST architectural style was derived.

[1] http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hyperte...

The big problem with using the hypertext Web as an example of REST is that there is a human operator literally driving that "engine of application state". Most API clients cannot afford to compute their state transitions on a network of 80 billion neurons.

REST is a human driven protocol. That's why resources have a link to a description of what the resource is.

Are you suggesting that Fielding did not intend REST to be used in automated systems?

By "distributed hypermedia systems", did he really mean "alternative browsers for alternative WWWs"?

I can't adequately convey what I think is Fielding's position on how machines should interact with RESTful systems, but I will convey my understanding of the purpose and intent of his thesis:

REST is a characterization of the Web architecture itself. It's not an alternative WWW, it is an abstract description of the principles behind the WWW. You can see this theme in Chapter 4 of the thesis in which Fielding characterizes REST [1].

> This chapter presents the requirements of the World Wide Web architecture and the problems faced in designing and evaluating proposed improvements to its key communication protocols. I use the insights garnered from the survey and classification of architectural styles for network-based hypermedia systems to hypothesize methods for developing an architectural style that would be used to guide the design of improvements for the modern Web architecture. [...]

> The early Web architecture was based on solid principles--separation of concerns, simplicity, and generality--but lacked an architectural description and rationale. The design was based on a set of informal hypertext notes, two early papers oriented towards the user community, and archived discussions on the Web developer community mailing list. In reality, however, the only true description of the early Web architecture was found within the implementations of libwww (the CERN protocol library for clients and servers), Mosaic (the NCSA browser client), and an assortment of other implementations that interoperated with them.

> An architectural style can be used to define the principles behind the Web architecture such that they are visible to future architects. As discussed in Chapter 1, a style is a named set of constraints on architectural elements that induces the set of properties desired of the architecture. The first step in my approach, therefore, is to identify the constraints placed within the early Web architecture that are responsible for its desirable properties. (internal citations elided)

[1] https://www.ics.uci.edu/~fielding/pubs/dissertation/web_arch...

Yes, they can be used by automated system but the automated system was developed by humans who explored the RESTful service.

No there isn't, a great deal of the web is loaded without direct human requests. For example:

* Web crawlers

* Archival services

* Embedded resources (stylesheets, JavaScript, images)

* Newsfeeds

All those examples are hard coded algorithms in the bulk copy category. They pretty much just use the GET method to download all or a specific subset of a site.

To truly drive a REST API requires a human or an AI. The AI doesn't need to be human equivalent, just smart enough to inteligently interpret the hypermedia. If the API changes the AI would be able to adapt.

I'm talking about the complexity of the computer on the Enterprise in ST:TNG. You give it a command and it figures out how to go about solving it.

You don't need an AI. A client for automated interaction will hard-code assumptions about content rather than URIs. If the client cannot find a link inside a document, it should just throw an error. A good service would keep links as long as possible, even when deprecated or provide redirects to new resources.

A client for human interaction though (like a native application or a SPA), can just implement the visitor pattern to display the UI.

Instead of moaning about the architecture principles, we must implement solutions / examples of how to write good clients against REST-based services!

I don't know where you got the idea that REST needs futuristic AI, and you haven't really explained why you think that way.

Why does a REST client have to intelligently interpret the hypermedia? Why are hard-coded algorithms a problem? Why does it need to adapt to changes in the API? None of those things are requirements for REST, you've inserted them for seemingly no reason.

For instance, consider an unattended webcam you want to use to show off, e.g. the growth of a flower bed remotely. It has the URI for a web service that returns a resource describing actions it can take:

    {
        "actions": [
            {
                "rel": "report-problem",
                "href": "https://example.com/report-problem",
                "method": "POST",
                "accept": "application/problem+json"
            },
            {
                "rel": "update-frame",
                "href": "https://example.com/update-frame",
                "method": "POST",
                "accept": "image/*"
            }
        ]
    }

The definition of the report-problem relationship is "This is the action to take when an error occurs."

The definition of the update-frame relationship is "This is the action to take when a new frame is available to upload."

This API defines one media type and two relationships. It requires no futuristic artificial intelligence or human intervention, it just needs to speak HTTP, parse simple JSON, and trigger the actions based on simple conditions.

It also doesn't provide much benefit over a plain RPC-like interface, because, for practical reasons, the webcam will be coded to send requests directly to https://example.com/report-problem and https://example.com/update-frame (the "bookmarks"). If you want to change those URLs, your best hope is that the webcam will understand a 307 or 308 redirect.

RPC vs REST is an entirely different argument. We're talking about REST best practices here, not "let's do RPC instead".

There's no reason for the webcam to hard-code URIs, and that's a violation of the architectural constraints. Just arbitrarily deciding to disregard facets of the architectural pattern without giving a reason does not add to this discussion.

None of this relates to whether or not you need futuristic AI or human intervention. I think from the example, it's pretty clear you need neither for a REST API.

> We're talking about REST best practices here, not "let's do RPC instead".

I don't think so. Avernar above seems to argue against REST as an architectural style (RPC being the default as everybody uses it). I thought you were trying to defend REST with your example, and I wanted to point out a weakness in that example.

> There's no reason for the webcam to hard-code URIs, and that's a violation of the architectural constraints.

The webcam has to hardcode some URIs (entry points). Consider that it could be using other services: perhaps a social network to post updates on. Then it must hardcode another entry point URI. But at that point -- and since every resource is supposedly independent and only explicit relations matter -- why can't the webcam treat the "problem report service" and the "update frame service" as two entirely separate services to hardcode? Which constraint of REST does this violate?

In your example, insisting that the webcam go through the entry point doesn't even buy the server a lot of flexibility. You reduce the client's knowledge from 2 URLs to 1 URL -- a URL that you may still want to change etc.

An extra HTTP roundtrip every time can well be an unpleasant overhead. HTTP client caches are sparsely supported and notoriously complex (RFC 7234 is 41 pages long). Rolling your own cache is, well, rolling your own.

And then there is the human problem. I was blown away by a discussion [1] where people reported using cryptography to prevent in-house clients from hardcoding URLs. I was also saddened when I read the JSON API spec [2] -- a great example of REST, I think -- only to discover that most existing implementations [3] disregard the linking aspect and hardcode the URL structure that the spec uses for examples. But this is not a complaint against your webcam example so much as it is a general hurdle with REST.

[1] http://blog.ploeh.dk/2013/05/01/rest-lesson-learned-avoid-ha... [2] http://jsonapi.org/ [3] http://jsonapi.org/implementations/

> None of this relates to whether or not you need futuristic AI or human intervention. I think from the example, it's pretty clear you need neither for a REST API.

Absolutely.

True, and some of these use cases are very interesting.

jcrites does not mention them, though. I realize now that his comment does not preclude them, either, but still this example would be more convincing if automated clients were emphasized, because that is what most people are actually building and care about.

I suggest you go to the original source, ie, Roy Fielding:

https://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arc...

http://roy.gbiv.com/untangled/tag/rest

https://www.infoq.com/articles/roy-fielding-on-versioning

I was going to post the same. I was lucky enough to get a few answers from Fielding himself when I first started learning this.

This is the video that made it finally "click" for me. https://vimeo.com/20781278

We don't yet have the abstractions, libraries, conventions or standards for this to work fully yet, but we're close.

> REST seems like an elephant in a room with some blind folks. Everybody who touches it thinks it's something different than the next guy, and they're all describing only one element of the thing.

I don't see it that way at all. Sure, there are minor quibbles about various approaches, but you can easily place everybody into one of two camps: the ones who don't know/care about REST's technical definition, and the ones that do. The two groups largely agree about what REST is amongst members of their own group, but disagree with the other group.

I think the above commentor got it right. You map the CRUD actions to HTTP verbs and statuses, as many of them make sense for your API, with links to further resources along the way.