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> (E.g. I can't see a client going "oh!, there's a new business function I haven't seen yet, let me invoke that!".) Of course not, nobody thinks that. That not

by bct 15y ago

> (E.g. I can't see a client going "oh!, there's a new business function I haven't seen yet, let me invoke that!".) Of course not, nobody thinks that. That notion does not exist. > With the rels/links, you're just moving the coupling away explicit URLs to the names/identities of rels/links in the response. I suppose, but that's a much looser coupling than the alternative (i.e. writing in the documentation "the comments URL is http://example.com/comments http://example.com/comments - you can't rearrange your URL structure, you can't start using a different domain (e.g. a CDN) for comments, existing clients can't use other sites that implement the same API, etc). HATEOAS is about building general protocols rather than site-specific APIs. That it makes it easier to change your own URLs is just a bonus.

stephen 15y ago
> Of course not, nobody thinks that. I'm pretty sure Fielding does, see "improved on-the-fly": "The transitions may be determined (or limited by) the client’s knowledge of media types and resource communication mechanisms, both of which may be improved on-the-fly (e.g., code-on-demand)." http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hyperte... Hypermedia is great for intelligent clients (e.g. humans) who can adapt to, say, a webpage changing and new fields suddenly showing up in the hypermedia (HTML) that are now required. However, for an application, it's going to be hard-coded to either do: 1) POST /employee with name=foo&age=1, GET /employee?id=1 Or 2) GET /hateoas-entry-point, select "new employee" link, fill out the 2 fields (and only 2 fields) it knew about when the client was programmed (name, age), post it to the "save employee link", go back to "/hateoas-entry-point", select "get employee" link, fill in the "id=1". (...or something like that). In either scenario, the non-human client is just as hard-coded as the other--it's either jumping to external URLs or jumping to internal links. Either way those URLs/links (or link ids) can't change and the functionality is just as frozen. Perhaps the benefits of hypermedia would be more obvious if Fielding built a toy example or two that we could all touch and feel instead of just dream up. But so far there seem to be a lot of non-HATEOAS REST APIs that are doing just fine sans hypermedia.
- contextfree 15y ago
  I think it's amazing that you're the first person I've ever seen make this very obvious point, which was the first thing that popped into my head when I first started reading about REST and HATEOAS APIs (or as I would call them, navigational APIs*). It's always seemed to me that REST tutorials and evangelism ought to address this basic objection upfront, if they do have good counterarguments, but I've never seen them do so. (A good HATEOS client would take the form of a graph navigator - this style of programming is one I associate more with "AI" than with typical web programming patterns. Which doesn't make it bad necessarily, but the REST material I've seen doesn't actually get into the navigational client programming side of things, which is the actual interesting part.)
- sopooneo 15y ago
  This has been my feeling for a long time, and I have never seen a HATEOAS proponent address it to my satisfaction. Frankly I think it is a pretty important point. Are we expecting automated consumers of a REST API to be curious and spontaneous the way human users of the web are?
  - extension 15y ago
    With the exception of spiders and other AI-like things, no, we are not expecting clients to spontaneously consume RESTful services in meaningful ways. REST clients are generic. They don't know anthing about specific services, thus allowing those services to evolve independently. A client that is coupled to a specific service is not RESTful, nor is any API that can only be consumed by such a client.
    - glassx 15y ago
      You comment is correct, despite the downvotes. The hypermedia isn't there so AI or spiders can navigate, it is there to reduce coupling. -- A Rant follows. Disclaimer: I'm not an expert on the subject so feel free to correct me or anything. The WWW itself is "RESTful". Even Hacker News is RESTful. You don't care about the URLs that show in your address bar. You don't construct them based on an ID-number on the side of each post. You just click around and submit forms. With hypermedia, your consumer doesn't need to know about those specifics of URL construction. URLs are transparent. You just query for Resources and follow links. YES, absolutely ZERO coupling is MUCH easier with boring CRUD-like stuff, such as Microsoft's OData, GData or AtomPub. Maybe when you're writing a good consumer tuned for usability (like a Twitter client) you, app builder, will need to know some details on the service beforehand, but even then, having hypermedia would be cool. With a nice and pretty RESTful API, Twitter could just roll out new features, such as 'people who recently retweeted you' and you'd have a new section show up on your app instantly, since it discovers resources. Maybe you could even, like, load icons for new sections via links on the API itself... Another example: I used to work on an App that used hypermedia: an Enterprise Content Manager with an OData API. Enterprise people used an decoupled enterprise client called Excel to connect to our Enterprise server and build random reports themselves.

by bct 15y ago

stephen 15y ago

> Of course not, nobody thinks that. I'm pretty sure Fielding does, see "improved on-the-fly": "The transitions may be determined (or limited by) the client’s knowledge of media types and resource communication mechanisms, both of which may be improved on-the-fly (e.g., code-on-demand)." http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hyperte... Hypermedia is great for intelligent clients (e.g. humans) who can adapt to, say, a webpage changing and new fields suddenly showing up in the hypermedia (HTML) that are now required. However, for an application, it's going to be hard-coded to either do: 1) POST /employee with name=foo&age=1, GET /employee?id=1 Or 2) GET /hateoas-entry-point, select "new employee" link, fill out the 2 fields (and only 2 fields) it knew about when the client was programmed (name, age), post it to the "save employee link", go back to "/hateoas-entry-point", select "get employee" link, fill in the "id=1". (...or something like that). In either scenario, the non-human client is just as hard-coded as the other--it's either jumping to external URLs or jumping to internal links. Either way those URLs/links (or link ids) can't change and the functionality is just as frozen. Perhaps the benefits of hypermedia would be more obvious if Fielding built a toy example or two that we could all touch and feel instead of just dream up. But so far there seem to be a lot of non-HATEOAS REST APIs that are doing just fine sans hypermedia.

contextfree 15y ago
I think it's amazing that you're the first person I've ever seen make this very obvious point, which was the first thing that popped into my head when I first started reading about REST and HATEOAS APIs (or as I would call them, navigational APIs*). It's always seemed to me that REST tutorials and evangelism ought to address this basic objection upfront, if they do have good counterarguments, but I've never seen them do so. (A good HATEOS client would take the form of a graph navigator - this style of programming is one I associate more with "AI" than with typical web programming patterns. Which doesn't make it bad necessarily, but the REST material I've seen doesn't actually get into the navigational client programming side of things, which is the actual interesting part.)
sopooneo 15y ago
This has been my feeling for a long time, and I have never seen a HATEOAS proponent address it to my satisfaction. Frankly I think it is a pretty important point. Are we expecting automated consumers of a REST API to be curious and spontaneous the way human users of the web are?
- extension 15y ago
  With the exception of spiders and other AI-like things, no, we are not expecting clients to spontaneously consume RESTful services in meaningful ways. REST clients are generic. They don't know anthing about specific services, thus allowing those services to evolve independently. A client that is coupled to a specific service is not RESTful, nor is any API that can only be consumed by such a client.
  - glassx 15y ago
    You comment is correct, despite the downvotes. The hypermedia isn't there so AI or spiders can navigate, it is there to reduce coupling. -- A Rant follows. Disclaimer: I'm not an expert on the subject so feel free to correct me or anything. The WWW itself is "RESTful". Even Hacker News is RESTful. You don't care about the URLs that show in your address bar. You don't construct them based on an ID-number on the side of each post. You just click around and submit forms. With hypermedia, your consumer doesn't need to know about those specifics of URL construction. URLs are transparent. You just query for Resources and follow links. YES, absolutely ZERO coupling is MUCH easier with boring CRUD-like stuff, such as Microsoft's OData, GData or AtomPub. Maybe when you're writing a good consumer tuned for usability (like a Twitter client) you, app builder, will need to know some details on the service beforehand, but even then, having hypermedia would be cool. With a nice and pretty RESTful API, Twitter could just roll out new features, such as 'people who recently retweeted you' and you'd have a new section show up on your app instantly, since it discovers resources. Maybe you could even, like, load icons for new sections via links on the API itself... Another example: I used to work on an App that used hypermedia: an Enterprise Content Manager with an OData API. Enterprise people used an decoupled enterprise client called Excel to connect to our Enterprise server and build random reports themselves.