Transforming the Interceptor Chain

There is an extra option in http-router (actually, in the underlying interceptor-router): :reitit.interceptor/transform to transform the interceptor chain per endpoint. Value should be a function or a vector of functions that get a vector of compiled interceptors and should return a new vector of interceptors.

Note: the last interceptor in the chain is usually the handler, compiled into an Interceptor. Applying a transformation clojure.core/reverse would put this interceptor into first in the chain, making the rest of the interceptors effectively unreachable. There is a helper reitit.interceptor/transform-butlast to transform all but the last interceptor.

Example Application

(require '[reitit.http :as http])
(require '[reitit.interceptor.sieppari :as sieppari])

(defn interceptor [message]
  {:enter (fn [ctx] (update-in ctx [:request :message] (fnil conj []) message))})

(defn handler [req]
  {:status 200
   :body (select-keys req [:message])})

(def app
  (http/ring-handler
    (http/router
      ["/api" {:interceptors [(interceptor 1) (interceptor 2)]}
       ["/ping" {:get {:interceptors [(interceptor 3)]
                       :handler handler}}]])
    {:executor sieppari/executor}))

(app {:request-method :get, :uri "/api/ping"})
; {:status 200, :body {:message [1 2 3]}}

Reversing the Interceptor Chain

(def app
  (http/ring-handler
    (http/router
      ["/api" {:interceptors [(interceptor 1) (interceptor 2)]}
       ["/ping" {:get {:interceptors [(interceptor 3)]
                       :handler handler}}]]
      {::interceptor/transform (interceptor/transform-butlast reverse)})
    {:executor sieppari/executor}))

(app {:request-method :get, :uri "/api/ping"})
; {:status 200, :body {:message [3 2 1]}}

Interleaving Interceptors

(def app
  (http/ring-handler
    (http/router
      ["/api" {:interceptors [(interceptor 1) (interceptor 2)]}
       ["/ping" {:get {:interceptors [(interceptor 3)]
                       :handler handler}}]]
      {::interceptor/transform #(interleave % (repeat (interceptor :debug)))})
    {:executor sieppari/executor}))

(app {:request-method :get, :uri "/api/ping"})
; {:status 200, :body {:message [1 :debug 2 :debug 3 :debug]}}

Printing Context Diffs

[metosin/reitit-interceptors "0.3.9"]

Using reitit.http.interceptors.dev/print-context-diffs transformation, the context diffs between each interceptor are printed out to the console. To use it, add the following router option:

:reitit.interceptor/transform reitit.http.interceptor.dev/print-context-diffs

Sample output:

Http Context Diff

Sample applications (uncomment the option to see the diffs):

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