It's impossible for me to see how it would be possible to write a HATEOAS client, and I can't in practice see anyone doing so.
Optimizing for HATEOAS seems to me to be optimizing for entirely the wrong metrics, and a complete waste of development time and effort.
There's nothing complicated about a client that understands Hypermedia links. You start at the root, it'll give you a set of links to follow, and you recurse. Here's a browser that can take a HATEOAS-compatible API and will let you work your way through the API: http://dracoblue.github.io/hateoas-browser/
> Optimizing for HATEOAS seems to me to be optimizing for entirely the wrong metrics, and a complete waste of development time and effort.
Correct. HATEOAS is a neat parlor trick: point a dumb client at your API and watch it enumerate all the possible operations. Wow! The problem is that APIs aren't consumed that way, they're consumed by humans who need to decide which API calls to make based on user actions in the application... so developers will resort to consulting documentation in order to piece together the correct calls in the correct order. That's not something you can encode through Hypermedia. At best you can just model the relations between calls, but not the "how" or "why" of everything.
Fundamentally, your display logic should not be linked to your API schema, but HATEOAS essentially enforces that, because you can't predict what links will be available.
Why not? The whole point of HATEOAS is to design your API schema in function of your application flow. You know, just like we do on websites.
Just for clarity, are you talking about API Developers or API Client Developers? I would see the primary beneficiaries of HATEOAS being developers consuming your API - i.e. client developers.
The HAL hypermedia format is in use at Comcast, powering the Xfinity TV API. Not only does it take advantage of hypermedia and HATEOAS, it also splits requests/responses in order to make the most use of HTTP caches. https://boston2016.apistrat.com/speakers/ben-greenberg
Dunno if you're serious about not seeing how one could write a HATEOAS-driven client library or if I'm being trolled, but here's one that I wrote a couple of years ago: https://github.com/gamache/hyperresource There are many others as well (especially for the HAL format).
I don't write a client by doing a random walk through the API until I get to the information I need, I want to call the API that I know I need to give me the information that I want, and for performance, I want it all to be returned on a single call ideally.
e.g. If I want user B's playlist, I don't want to have to crawl:
/friends/ => ['b': { 'links': {'info': '/friends/619'} }]
/friends/619 => {'links': {'playlist': '/friends/619/playlist' } }
/friends/619/playlist
Sometimes you do a few more web calls than you might otherwise. If you can get away with it then great! Because the benefits are plenty
Hell! Even just having the uri of where the resource came from is useful enough. That's before you've introduced related resources.
The thing is, if you use well-known terms, your client will work against ANY API that uses those terms, not just the one that you coded it to work against.
Not only that, but the APIs will have flexibility to move things around, or delegate functionality to other hosts by linking through.
As for the cost of walking the chain, once a target resource has been found the can be cached, and you only need to rewalk if the resource 404/410s.
you get some html with embedded links and then the browser automatically goes and fetches css, js, images...
the remaining links it just presents to you, the user, to follow or not as you choose
hateoas is not a complicated idea. it's not meant to replace SOAP or gRPC or thrift. it's something else
The HATEOAS idea is that you throw that out, just use some aribitrary XML (sorry, "hypermedia") to return your application state, and that this somehow magically empowers the client to be able to puzzle out all the things that can be done to your state.
Except it can't. It's just arbitrary junk if you don't have a schema and a spec. And it will always be so. Discoverability is (1) not something you can get from a data structure and (2) something best provided by documentation, not runtime behavior.
(One of the other problems with Fielding's work is precisely this word-salad of new jargon and usages, leading to exactly this kind of what-does-it-REALLY-mean-anyway confusion. But that's an argument for a different thread.)
As for Fielding's work having a word-salad of new jargon and uses, I frankly didn't get that by reading his dissertation, which I found quite clear. There are a few concepts (Resources, Representations), but I think they make sense in the context.
I think REST/HATEOAS purists have always overplayed the browser example.
Pure machine to machine interaction is hard to automate.
It depends, it's just a way to reduce coupling between a webservice and its client apps. Instead of hardcoding all the URL endpoints into the client, the client has to follow the links provided by the webservice instead. If this layer of indirection has advantages for you depends on your use case. You could compare it to using DNS instead of static IPs. Static IPs work fine but you end up with more coupling than using DNS.
a) know how to retrieve that link (i.e. what resource do I need to retrieve in order to get that response)
b) requirement for that link to exist (they still need that link).
Requiring that a client know that to get data Bar, they need to retrieve resource A, follow the link at foo[n]._link to retrieve resource B, and then following the link at _link.bar feels like a total non starter, and actually increases coupling.
According to Roy Fielding[1] (who came up with the concepts of ReST and HATEOAS:
"A REST API should be entered with no prior knowledge beyond the initial URI (bookmark) and set of standardized media types that are appropriate for the intended audience"
So if an application doesn't rely on "standardized media types", and "the client still has to know what to do in advance", then it seems like whatever you're doing, it isn't HATEOAS after all.
[1] http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hyperte...
If you have an extremely dumb interface that just displays fields in API responses, then ok... but that's a very trivial UI that accounts for... basically no useful products ever.