Paul Mison, 12 January 2009
London Ruby User Group
Hi, Iʼm Paul Mison and Iʼm here to talk about Ruby,
In particular, Iʼll be talking about Flickrʼs API. This is
its documentation page. Flickr, as Iʼm sure you all
know, is a site for sharing photos.
Photos like this one, in fact. by Ryan Forsythe
They offer a REST-style API, as well as XML-RPC
and SOAP, which Iʼll ignore. However, you can take
the lessons from this and apply it to many other
popular modern web APIs, which have a similar
The screenshots here are of their API Explorer tool,
which is a nice way to see what arguments a
method takes and what it returns. But you donʼt
want to worry about XML: you just want kitten
RubyForge lists a lot of projects to talk to Flickr.
(This was taken a while ago; I should probably
have searched GitHub.)
In fact, I make out that there are seven libraries
listed. It turns out that a lot of these are
abandoned. Iʼll explain why.
First, hereʼs a Ruby Flickr library called flickr-fu. It
works, and returns objects.
Letʼs try this perfectly valid Flickr API method - in
fact, itʼs one of an entire subset of methods in the
groups namespace - and we see the library is
throwing an error. Why? Well, Thereʼs no groups
method in flickr-fu because nobodyʼs written it yet.
This is part of the code for flickr-fu, for the people
methods. Note all this code thatʼs just copying part
of the response to the Person object. Now, imagine
going through and doing that for the groups
... and there are a few. Daunting, isnʼt it? I can
understand why they havenʼt got around to it yet.
In fact, these are the areas that flickr_fu provides,
according to commonthreadʼs github copy.
r2flickr, an extension of rflickr, which is listed on the
API home page, does better, but even so, itʼs
missing some of the newer parts of the API (such
as the flickr.places methods).
This is the problem
This is the problem with abstraction. Once you start
setting up objects like that, then you find that
thereʼs a lot of work involved to add new ones, and
Flickrʼs API is quite broad. This explains the
abandoned modules- their authors got bored of
writing boilerplate code.
Even when youʼve implemented all the existing
methods, thatʼs not the end. “We added 1 new API
method, and updated 7 others”. If youʼre looking
after an API, thatʼs the last thing you want to see.
... especially since, for now, thereʼs no way to even
track those changes yourself unless you write your
own code to use Flickrʼs reflection methods -
although Flickr are aiming to change that.
Fixing It: The Request
So, we can see why the OO approach leads to
unfinished, or unmaintained, libraries. How do you
Nothing to do with fixing, really, this picture, but I
liked it anyway.
All a request to Flickr is is a signed call. This is
from flickraw, and thatʼs all you need, including
In fact, if you look at flickr-fu earlier, it has a similar
internal method, send_request. If they just exposed
it, I could use it for group or places calls. To be fair,
it is documented as a public method, but you
wouldnʼt think to look for it.
You can be a step cleverer, and offer the nice
methods, even if you donʼt write boilerplate.
This is another part of flickraw, where it uses
Flickrʼs own reflection methods - the ones
mentioned earlier - to dynamically build the list of
methods itʼs able to use. Smart, but takes a bit of
Fixing It: The Response
Now, what about the responses?
Back to flickr-fu. Look at all that code copying from
the rsp XML object to properties of the Person
object. What a drag, but I suppose it saves me
having to do it. Still, surely thereʼs a better way?
Hereʼs some good news. Flickr supports returning
its responses as JSON
The O in JSON stands for Object, and this means
that youʼre able to get things back that you can
easily treat as, well, objects, without any of that
painful XML traversing that we all know and love.
To return an API response in JSON
format, send a parameter “format”
in the request with a value of “json”
Getting back a JSON response rather than an
XML one is trivial; you just change one parameter.
(Sidenote probably not for the talk: you also want
“nocallback” to be set, otherwise the JSON will be
an executable bit of code rather than just an object,
but itʼs still pretty straightforward.)
This is exactly what flickraw does, and as a result,
it can automatically pick up any additions that Flickr
make to their response format. And when you
come to output it, you can just write clean Ruby
So, what have we learnt?
Offer simple access
wrappers to users.
Instead of writing a method per allowed call, first
write a generic method - youʼll probably need it
Abstraction sucks -
unlessdo the work.
Let it youʼre clever
Rubyʼs a dynamic language. Use that to your best
advantage - for example, building convenience
methods on the fly.
Abstraction sucks -
unless youʼre clever
Use JSON, which can be easily turned into an
object, rather than XML, to save yourself work