+1 to all of Richard's points here. Making something easier for you to develop is no justification for making it harder to consume or deviating from well supported standards.

[Robert]
>  You can't 
> just put a file in the file system, unlike with separate URIs for 
> distinct representations where it just works, instead you need server 
> side processing.

If we introduce languages into the negotiation, this won't scale.

[Robert]
> This also makes it much harder to cache the 
> responses, as the cache needs to determine whether or not the 
> representation has changed -- the cache also needs to parse the 
> headers rather than just comparing URI and content.  

Don't know caches intimately, but I don't see why that's algorithmically difficult. Just look at the Content-type of the response. Is it harder for caches to examine headers than content or URI? (That's an earnest, perhaps naïve, question.)

If we are talking about caching on the client here (not caching proxies), I would think in most cases requests are issued with the same Accept-* headers, so caching will work as expected anyway.

[Robert]
> Link headers 
> can be added with a simple apache configuration rule, and as they're 
> static are easy to cache. So the server side is easy, and the client side is trivial.

Hadn't heard of these. (They are on Wikipedia so they must be real.) What do they offer over HTML <link> elements populated from the Dublin Core Element Set?

---

My ideal setup would be to maintain a canonical URL that always serves the clients' flavour of representation (format/language), which could vary, but points to other representations (and versions for that matter) at separate URLs through a mechanism like HTML link elements.

My whatever it's worth . great topic, though, thanks Robert :)

Cheers

Hugh Barnes
Digital Access Coordinator
Library, Teaching and Learning
Lincoln University
Christchurch
New Zealand
p +64 3 423 0357

-----Original Message-----
From: Code for Libraries [mailto:[log in to unmask]] On Behalf Of Richard Wallis
Sent: Monday, 2 December 2013 12:26 p.m.
To: [log in to unmask]
Subject: Re: [CODE4LIB] The lie of the API

"It's harder to implement Content Negotiation than your own API, because you get to define your own API whereas you have to follow someone else's rules"

Don't wish your implementation problems on the consumers of your data.
There are [you would hope] far more of them than of you ;-)

Content-negotiation is an already established mechanism - why invent a new, and different, one just for *your* data?

Put your self in the place of your consumer having to get their head around yet another site specific API pattern.

As to discovering then using the (currently implemented) URI returned from a content-negotiated call  - The standard http libraries take care of that, like any other http redirects (301,303, etc) plus you are protected from any future backend server implementation changes.


~Richard


On 1 December 2013 20:51, LeVan,Ralph <[log in to unmask]> wrote:

> I'm confused about the supposed distinction between content 
> negotiation and explicit content request in a URL.  The reason I'm 
> confused is that the response to content negotiation is supposed to be 
> a content location header with a URL that is guaranteed to return the 
> negotiated content.  In other words, there *must* be a form of the URL that bypasses content negotiation.
>  If you can do content negotiation, then you should have a URL form 
> that doesn't require content negotiation.
>
> Ralph
> ________________________________________
> From: Code for Libraries <[log in to unmask]> on behalf of 
> Robert Sanderson <[log in to unmask]>
> Sent: Friday, November 29, 2013 2:44 PM
> To: [log in to unmask]
> Subject: Re: The lie of the API
>
> (posted in the comments on the blog and reposted here for further 
> discussion, if interest)
>
>
> While I couldn't agree more with the post's starting point -- URIs 
> identify
> (concepts) and use HTTP as your API -- I couldn't disagree more with 
> the "use content negotiation" conclusion.
>
> I'm with Dan Cohen in his comment regarding using different URIs for 
> different representations for several reasons below.
>
> It's harder to implement Content Negotiation than your own API, 
> because you get to define your own API whereas you have to follow 
> someone else's rules when you implement conneg.  You can't get your 
> own API wrong.  I agree with Ruben that HTTP is better than rolling 
> your own proprietary API, we disagree that conneg is the correct 
> solution.  The choice is between conneg or regular HTTP, not conneg or a proprietary API.
>
> Secondly, you need to look at the HTTP headers and parse quite a 
> complex structure to determine what is being requested.  You can't 
> just put a file in the file system, unlike with separate URIs for 
> distinct representations where it just works, instead you need server 
> side processing.  This also makes it much harder to cache the 
> responses, as the cache needs to determine whether or not the 
> representation has changed -- the cache also needs to parse the 
> headers rather than just comparing URI and content.  For large scale 
> systems like DPLA and Europeana, caching is essential for quality of service.
>
> How do you find our which formats are supported by conneg? By reading 
> the documentation. Which could just say "add .json on the end". The 
> Vary header tells you that negotiation in the format dimension is 
> possible, just not what to do to actually get anything back. There 
> isn't a way to find this out from HTTP automatically,so now you need 
> to read both the site's docs AND the HTTP docs.  APIs can, on the 
> other hand, do this.  Consider OAI-PMH's ListMetadataFormats and SRU's Explain response.
>
> Instead you can have a separate URI for each representation and link 
> them with Link headers, or just a simple rule like add '.json' on the 
> end. No need for complicated content negotiation at all.  Link headers 
> can be added with a simple apache configuration rule, and as they're 
> static are easy to cache. So the server side is easy, and the client side is trivial.
>  Compared to being difficult at both ends with content negotiation.
>
> It can be useful to make statements about the different 
> representations, and especially if you need to annotate the structure 
> or content.  Or share it -- you can't email someone a link that 
> includes the right Accept headers to send -- as in the post, you need 
> to send them a command line like curl with -H.
>
> An experiment for fans of content negotiation: Have both .json and 302 
> style conneg from your original URI to that .json file. Advertise 
> both. See how many people do the conneg. If it's non-zero, I'll be 
> extremely surprised.
>
> And a challenge: Even with libraries there's still complexity to 
> figuring out how and what to serve. Find me sites that correctly 
> implement * based fallbacks. Or even process q values. I'll bet I can 
> find 10 that do content negotiation wrong, for every 1 that does it correctly.  I'll start:
> dx.doi.org touts its content negotiation for metadata, yet doesn't 
> implement q values or *s. You have to go to the documentation to 
> figure out what Accept headers it will do string equality tests against.
>
> Rob
>
>
>
> On Fri, Nov 29, 2013 at 6:24 AM, Seth van Hooland <[log in to unmask]>
> wrote:
> >
> > Dear all,
> >
> > I guess some of you will be interested in the blogpost of my 
> > colleague
> and co-author Ruben regarding the misunderstandings on the use and 
> abuse of APIs in a digital libraries context, including a description 
> of both good and bad practices from Europeana, DPLA and the Cooper Hewitt museum:
> >
> > http://ruben.verborgh.org/blog/2013/11/29/the-lie-of-the-api/
> >
> > Kind regards,
> >
> > Seth van Hooland
> > Président du Master en Sciences et Technologies de l'Information et 
> > de la
> Communication (MaSTIC)
> > Université Libre de Bruxelles
> > Av. F.D. Roosevelt, 50 CP 123  | 1050 Bruxelles 
> > http://homepages.ulb.ac.be/~svhoolan/
> > http://twitter.com/#!/sethvanhooland
> > http://mastic.ulb.ac.be
> > 0032 2 650 4765
> > Office: DC11.102
>



--
Richard Wallis
Founder, Data Liberate
http://dataliberate.com
Tel: +44 (0)7767 886 005

Linkedin: http://www.linkedin.com/in/richardwallis
Skype: richard.wallis1
Twitter: @rjw

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