Pluggable Coercion

Basic coercion is explained in detail in the Coercion Guide. With Ring, both request parameters (:query, :body, :form, :header and :path) and response :body can be coerced.

To enable coercion, the following things need to be done:

• Define a reitit.coercion/Coercion for the routes
• Define types for the parameters and/or responses
• Mount Coercion Middleware to apply to coercion
• Use the coerced parameters in a handler/middleware

Define coercion

reitit.coercion/Coercion is a protocol defining how types are defined, coerced and inventoried.

Reitit ships with the following coercion modules:

• reitit.coercion.schema/coercion for plumatic schema
• reitit.coercion.spec/coercion for both clojure.spec and data-specs

Coercion can be attached to route data under :coercion key. There can be multiple Coercion implementations within a single router, normal scoping rules apply.

Defining parameters and responses

Parameters are defined in :parameters key and responses in :responses.

Below is an example with Plumatic Schema. It defines input schemas for :query, :body and :path parameters and a schema for a successful response :body.

Handler can access the coerced parameters can be read under :parameters key in the request.

(require '[reitit.coercion.schema])
(require '[schema.core :as s])

(def PositiveInt (s/constrained s/Int pos? 'PositiveInt))

(def plus-endpoint
  {:coercion reitit.coercion.schema/coercion
   :parameters {:query {:x s/Int}
                :body {:y s/Int}
                :path {:z s/Int}}
   :responses {200 {:body {:total PositiveInt}}}
   :handler (fn [{:keys [parameters]}]
              (let [total (+ (-> parameters :query :x)
                             (-> parameters :body :y)
                             (-> parameters :path :z))]
                {:status 200
                 :body {:total total}}))})

Coercion Middleware

Defining a coercion for a route data doesn't do anything, as it's just data. We have to attach some code to apply the actual coercion. We can use the middleware from reitit.ring.coercion:

• coerce-request-middleware to apply the parameter coercion
• coerce-response-middleware to apply the response coercion
• coerce-exceptions-middleware to transform coercion exceptions into pretty responses

Full example

Here's an full example for applying coercion with Reitit, Ring and Schema:

(require '[reitit.ring.coercion :as rrc])
(require '[reitit.coercion.schema])
(require '[reitit.ring :as ring])
(require '[schema.core :as s])

(def PositiveInt (s/constrained s/Int pos? 'PositiveInt))

(def app
  (ring/ring-handler
    (ring/router
      ["/api"
       ["/ping" {:name ::ping
                 :get (fn [_]
                        {:status 200
                         :body "pong"})}]
       ["/plus/:z" {:name ::plus
                    :post {:coercion reitit.coercion.schema/coercion
                           :parameters {:query {:x s/Int}
                                        :body {:y s/Int}
                                        :path {:z s/Int}}
                           :responses {200 {:body {:total PositiveInt}}}
                           :handler (fn [{:keys [parameters]}]
                                      (let [total (+ (-> parameters :query :x)
                                                     (-> parameters :body :y)
                                                     (-> parameters :path :z))]
                                        {:status 200
                                         :body {:total total}}))}}]]
      {:data {:middleware [rrc/coerce-exceptions-middleware
                           rrc/coerce-request-middleware
                           rrc/coerce-response-middleware]}})))

Valid request:

(app {:request-method :post
      :uri "/api/plus/3"
      :query-params {"x" "1"}
      :body-params {:y 2}})
; {:status 200, :body {:total 6}}

Invalid request:

(app {:request-method :post
      :uri "/api/plus/3"
      :query-params {"x" "abba"}
      :body-params {:y 2}})
; {:status 400,
;  :body {:schema {:x "Int", "Any" "Any"},
;         :errors {:x "(not (integer? \"abba\"))"},
;         :type :reitit.coercion/request-coercion,
;         :coercion :schema,
;         :value {:x "abba"},
;         :in [:request :query-params]}}

Invalid response:

(app {:request-method :post
      :uri "/api/plus/3"
      :query-params {"x" "1"}
      :body-params {:y -10}})
; {:status 500,
;  :body {:schema {:total "(constrained Int PositiveInt)"},
;         :errors {:total "(not (PositiveInt -6))"},
;         :type :reitit.coercion/response-coercion,
;         :coercion :schema,
;         :value {:total -6},
;         :in [:response :body]}}

Pretty printing spec errors

Spec problems are exposed as-is into request & response coercion errors, enabling pretty-printers like expound to be used:

(require '[reitit.ring :as ring])
(require '[reitit.ring.middleware.exception :as exception])
(require '[reitit.ring.coercion :as coercion])
(require '[expound.alpha :as expound])

(defn coercion-error-handler [status]
  (let [printer (expound/custom-printer {:theme :figwheel-theme, :print-specs? false})
        handler (exception/create-coercion-handler status)]
    (fn [exception request]
      (printer (-> exception ex-data :problems))
      (handler exception request))))

(def app
  (ring/ring-handler
    (ring/router
      ["/plus"
       {:get
        {:parameters {:query {:x int?, :y int?}}
         :responses {200 {:body {:total pos-int?}}}
         :handler (fn [{{{:keys [x y]} :query} :parameters}]
                    {:status 200, :body {:total (+ x y)}})}}]
      {:data {:coercion reitit.coercion.spec/coercion
              :middleware [(exception/create-exception-middleware
                             (merge
                               exception/default-handlers
                               {:reitit.coercion/request-coercion (coercion-error-handler 400)
                                :reitit.coercion/response-coercion (coercion-error-handler 500)}))
                           coercion/coerce-request-middleware
                           coercion/coerce-response-middleware]}})))

(app
  {:uri "/plus"
   :request-method :get
   :query-params {"x" "1", "y" "fail"}})
; => ...
; -- Spec failed --------------------
;
;   {:x ..., :y "fail"}
;                ^^^^^^
;
; should satisfy
;
;   int?



(app
  {:uri "/plus"
   :request-method :get
   :query-params {"x" "1", "y" "-2"}})
; => ...
;-- Spec failed --------------------
;
;   {:total -1}
;           ^^
;
; should satisfy
;
;   pos-int?

Optimizations

The coercion middleware are compiled againts a route. In the middleware compilation step the actual coercer implementations are constructed for the defined models. Also, the middleware doesn't mount itself if a route doesn't have :coercion and :parameters or :responses defined.

We can query the compiled middleware chain for the routes:

(require '[reitit.core :as r])

(-> (ring/get-router app)
    (r/match-by-name ::plus)
    :result :post :middleware
    (->> (mapv :name)))
; [::mw/coerce-exceptions
;  ::mw/coerce-request
;  ::mw/coerce-response]

Route without coercion defined:

(app {:request-method :get, :uri "/api/ping"})
; {:status 200, :body "pong"}

Has no mounted middleware:

(-> (ring/get-router app)
    (r/match-by-name ::ping)
    :result :get :middleware
    (->> (mapv :name)))
; []