Oliver Hine


September 29, 2016

I've worked quite a lot with APIs and HTTP in particular, and I've come to some conclusions:

  • I feel like I keep writing the same code on every project
  • It's still surprisingly hard to get right
  • I like defining what
  • I don't want to have to care about how

When creating an API there will normally be two parts to consider: the interface and the application layer. The interface consists of the operations, parameters and data that gets returned, and is what makes your API unique and useful. The application layer is how two processes talk to each other, HTTP for example, which has its own nomenclature, idioms and behaviours which are abstract and generic. Layers in networking - physical, transport etc - should be opaque to each other. Your interface in turn should be opaque to HTTP, but too often we fall at the final hurdle and complect the definition of our interface with implementation details like verbs and distinctions between minutiae like route parameters and query parameters. It's not something you should necessarily care about - as an application developer you want that sort of thing to just work, so you can get on with using data and operations to do interesting and valuable things.

Fortunately, if you want to provide an API libraries like pedestal-api, yada and compojure-api can take care of HTTP and let you simply describe your interface using data structures. With interceptors or middleware you can easily add cross-cutting aspects like metrics, logging and security. You even get a flashy Swagger UI for free to show your friends how awesome your API is. Sweet!

(defhandler create-pet
  {:summary     "Create a pet"
   :parameters  {:body-params {:name s/Str
                               :type s/Str
                               :age s/Int}}
   :responses   {201 {:body {:id s/Int}}}}
 (fn [request] ... ))))

But what about consuming APIs as a client? Client HTTP libraries like httpkit, clj-http and cljs-http let you make HTTP calls but you still have to know what parameters go where in the url, query string, headers or body, how to serialise and deserialise the body, what method to use for each request and so on; your concise description of your API operation has been lost, all complected with HTTP. Cross-cutting requirements are also hard to implement in a uniform manner - you'll have to pass your metrics registry or credentials configuration around to wrap any code that might need to make an HTTP call.

At this point you reach an interesting dichotomy - how come the Swagger UI makes using the API easy for a human, but all your code calling it is messy? Aren't machines meant to be better at talking to other machines than people?

Martian uses descriptions of APIs to hide the HTTP application layer and leave behind just the operations and parameters of the interface. You're back to calling operations with parameters and getting return values, just like normal code. Here's a minimal example of using Martian against an API described by Swagger:

(require '[martian.core :as martian]
         '[martian.clj-http :as martian-http])

(let [m (martian-http/bootstrap-swagger "https://pedestal-api.herokuapp.com/swagger.json")]
  (martian/response-for m :create-pet {:name "Doggy McDogFace" :type "Dog" :age 3})
  ;; => {:status 201 :body {:id 123}}

  (martian/response-for m :get-pet {:id 123}))
  ;; => {:status 200 :body {:name "Doggy McDogFace" :type "Dog" :age 3}}

Nice and simple, looks like the interface we described on the server side, and bootstrapping at runtime even allows the underlying HTTP implementation to be refactored without your code needing to change. Martian maps the parameters to the right place and chooses the most efficient serialisation the API supports without any of it leaking into your code. This is the separation of interface and application layer that we've been aiming for.

Martian offers a few other features to further speed the progress of writing your client code:

;; explore the endpoints
(explore m)
=> [[:get-pet "Loads a pet by id"]
    [:create-pet "Creates a pet"]]

;; explore the :get-pet endpoint
(explore m :get-pet)
=> {:summary "Loads a pet by id"
    :parameters {:id s/Int}}

;; build the url for a request
(url-for m :get-pet {:id 123})
=> https://pedestal-api.herokuapp.com/pets/123

;; build the request map for a request
(request-for :get-pet {:id 123})
=> {:method :get
    :url "https://pedestal-api.herokuapp.com/pets/123"
    :headers {"Accept" "application/transit+msgpack"
    :as :byte-array}

What about those cross-cutting, non-functional aspects like authentication or metrics? Martian uses interceptors, just like Pedestal, to allow you to customise behaviour of the call either before the request is made, after the response is received, or both. Let's add some timing to our requests:

(require '[martian.core :as martian]
         '[martian.clj-http :as martian-http])

(def request-timer
  {:name ::request-timer
   :enter (fn [ctx]
            (assoc ctx ::start-time (System/currentTimeMillis)))
   :leave (fn [ctx]
            (->> ctx ::start-time
                 (- (System/currentTimeMillis))
                 (format "Request to %s took %sms" (get-in ctx [:handler :route-name]))

(let [m (martian-http/bootstrap-swagger
               {:interceptors (concat martian/default-interceptors

        (martian/response-for m :all-pets {:id 123}))
        ;; Request to :all-pets took 38ms
        ;; => {:status 200 :body {:pets []}}

Martian uses interceptors for everything, including the making of the request, and allows you to write your own and configure them as you wish. As a result, you can use any HTTP library you please if you don't like the implementations provided for httpkit, clj-http and cljs-http. For testing, you may even want to mock out HTTP and use schema validation and generative responses - martian-test does just this, and is covered in the next article, along with mapping APIs without Swagger and more advanced composition. Continue reading at Advanced Martian and watch a talk I gave at ClojureX Bytes explaining the rationale.

Martian is on Github, feedback and contributions are very welcome.

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