HAL Client Update: Cetacean

After writing my last post, I used the code presented therein in production a few more days and realized I wanted just a smidge more power. Specifically, I wanted it to handle HAL’s embedded resources. So I added that and then the other night, I pulled it out of that project, broke it up into files, named it after sea animals and pushed it to Introducing: Cetacean.

Now, I realize my last post was all, “Do we need HAL clients?” and shit. But let’s be real, even 42 lines is a HAL client. Or… whatever, depending on what your definition of “client”. In fact, the design of Cetacean is basically driven by thinking about what “client” means and what I felt like I needed a tool for with respect to HAL documents.

Basically, Cetacean helps you deal with HAL documents, it doesn’t really help you with hypermedia APIs. The general work flow is that you make a Faraday client (configured however seems best to you) and make a request to some URI. You feed the response to Cetacean and then operate on it. You might get a URI out of it (from a link, say), which you can then feed back to your Faraday client, which response you can feed to Cetacean, again, etc.

Perhaps an example would be helpful:

api ='') do |faraday|
  faraday.headers['Accept'] = 'application/hal+json'

root =

At this juncture, you can ask root about some stuff. It’s an instance of Cetacean::Responce. It proxies some methods back to the Faraday::Response, and otherwise is mostly sugar for getting things out of the HAL document.

root.success? # => true
root.status # => 200
root.response # => The `Faraday::Response` you passed in.
root.body # => '{"foo":"bar","_links":{"self":{"href":"/"},"users":{"href":"..."}}}'
root.hal? # => true
root.attributes # => { 'foo' => 'bar' }
root['foo'] # => 'bar'
root.links # => { 'self' => { 'href' => '/' } }
root.get_uri(:self) # => A `URITemplate` object for you to manipulate.

So if you want to follow the ‘self’ rel, you use get_uri method to get a template, do whatever you want with it (probably to_s and expand are the most common cases). And then… do whatever you want with that. Make another HTTP request? Get it tattooed on your stomach? Cetacean doesn’t really care.

Continuing on, imagine further interactions with an API. Notice that at each stage, Cetacean deals with the HAL, and then gets out of the way.

users =
user = users.embedded(:users).first

Assuming that the ‘users’ rel was some document that embedded an array of documents under the ‘users’ rel, user is an instance of Cetacean::EmbeddedResource. It behaves mostly like a Cetacean::Response except is doesn’t have a request object to proxy or refer to. Ideally, you shouldn’t have to keep track of which came from where once you’re dealing with a document.

important_blog_post = 2)))
interesting_blog_posts = 'interesting')))

Just another couple of examples of following different links, both of these requiring expansion.

This is certainly more wordy than using a more featureful library like Hyperclient or whatever. I feel that this gives you a lot more control over how you deal with the documents you get back. If you unexpectedly get back an invalid body, Cetacean won’t barf trying to parse it like JSON, if you get back 'image/jpeg' instead of 'application/hal+json', Cetacean makes it easy to deal with that however is most appropriate. If you need to do some fancy, weird-ass configuration of Faraday that’s super specific to your use-case, then you don’t have to worry about what a middle-man library might do to your connection configuration.

Anyway, I guess I wrote another gem. If you use it, I’d be interested to hear about it. If you want to help, feel free. It’s on Github.