Pedestal is all about interceptors, they are integral to how Pedestal applications are constructed and composed.

Each endpoint function may define a set of interceptors specific to that function, using :interceptors metadata.

Ultimately, Rook wraps endpoints functions so that they are Pedestal interceptors.

Interceptors may be inherited from the namespace and elsewhere.

Interceptor Values

The interceptor values can be any of the values that Pedestal accepts as an interceptor:

  • An interceptor, as by io.pedestal.interceptor/interceptor.
  • A map, which is converted to an interceptor
  • A bunch of other things that make sense in terms of Pedestal’s deprecated terse routing syntax

Rook adds one additional option, a keyword.

The keyword references the :interceptor-defs option (passed to io.aviso.rook/gen-table-routes).

A matching value must exist (otherwise, an exception is thrown).

The value is typically a configured interceptor value.

Alternately, the value might be a function, which acts as an interceptor generator.

Interceptor Generators

An interceptor generator is a function that creates an interceptor customized to the particular endpoint.

It is passed a map that describes the endpoint, and returns an interceptor.

The endpoint description has the following keys:

The Clojure Var containing the endpoint function.
The metadata map for the endpoint function.
A string version of the fully qualified name of the endpoint function

In this way, the interceptor can use the details of the particular endpoint to generate a custom interceptor.

For example, an interceptor that did some special validation, or authentication, might use metadata on the endpoint function to determine what validations and authentications are necessary for that particular endpoint.

Here’s a more concrete example, part of Rook’s test suite:

(ns sample.dynamic-interceptors
  (:require [io.pedestal.interceptor :refer [interceptor]]))

(defn endpoint-labeler
  (let [{:keys [endpoint-name]} endpoint]
    (interceptor {:name ::endpoint-labeler
                  :leave (fn [context]
                           (assoc-in context
                                     [:response :headers "Endpoint"]

This generator returns an interceptor that operates during the leave phase, when there’s a response map. It adds the Endpoint header to the response. This same interceptor generator could be added to any number of endpoints; a unique interceptor instance will be generated for each endpoint.

Applying Interceptors

When a namespace provides :interceptor metadata, that’s a list of interceptors to add to every endpoint in the namespace, and in any nested namespaces.


This can cast a wider net than is desirable; for example, including the io.aviso.rook.interceptors/keywordized-form interceptor at the namespace level will add it to all endpoints in the namespace, even those that do not include a POSTed form.

However, each individual endpoint will ultimately end up with its own individual interceptor list.

Further, none of those interceptors will actually execute in a request unless routing selects that particular endpoint to handle the request.

digraph {
    incoming [label="Incoming Request"];
    definterceptors [label="Default Interceptors"];
    routing [label="Pedestal Routing"];
    endpoint [label="Endpoint Selected"];
    interceptors [label="Endpoint's Interceptors"];
    fn [label="Endpoint Function"];

    incoming -> definterceptors -> routing -> endpoint -> interceptors -> fn;


The default interceptors are usually provided by io.pedestal.http/create-server. These cover a number of cases such as handling requests for unmapped URIs, logging, preventing cross-site scripting forgery, and so forth.

The :interceptor metadata in namespaces and elsewhere simply builds up the Endpoint’s Interceptors stage.