I was happy to read this blog post, because it contain lots of very
important statements, but as one of the developers of Europeana API I
would like to mention some points.
The idea of content negotiation is nice, but it also adds some
additional burden for the API users. In some tools and programming
languages it is easy to modify HTTP headers, in others it is not that
trivial. For non tech people it is a burden. In an environment such as
Europeana not only tech people would like to see and check the non
HTML output, but it also has a meaning for metadata experts, marketing
people, ingestion team members and so on.
Europeana has a history, and even the API and the metadata model
behind has its own history. When we released the new API which
reflects the new metadata structure, it was evident, that we did not
want to break existing client side applications. So we had to
introduce versioning. With versioning we had the same choces as with
content type: we can make it transparent in the URL or use hypermedia
versioning via HTTP headers. This lead to the same problem as we had,
so we choosed URL approach.
Finally, when creating an API there are lots of different aspect we
should consider. Beside technological, scientific or aesthetic aspects
there are lots of other ones as well. We follow a way, which has good
and bad points, but as I see the same is true for Ruben's suggestions.
It is not true, that our way is driven by simle ignorance. We never
claimed, that we created RESTFul and pedantic API. We did a practical
one, and we keep improving it gradually, considering such a feedbacks
as this post.
2013/11/29 Robert Sanderson <[log in to unmask]>:
> (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
> 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.
> On Fri, Nov 29, 2013 at 6:24 AM, Seth van Hooland <[log in to unmask]>
>> 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:
>> 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
>> 0032 2 650 4765
>> Office: DC11.102
Europeana - http://europeana.eu
eXtensible Catalog - http://eXtensibleCatalog.org