Pedestal is a Clojure library for building server-side web applications. It uses interceptors, which are similar to middleware, to modify the request-response context as it passes through the system. The document shows how to create a basic Pedestal project, define routes and handlers, and use interceptors like body-params to extract parameters from requests. Further reading sections provide links to learn more about Pedestal, interceptors, related concepts, and alternative routing libraries.

1. InterceptorsInterceptors
Into the Core of PedestalInto the Core of Pedestal

2. カマイルカ /laʒenɔʁɛ̃k/カマイルカ /laʒenɔʁɛ̃k/
lagénorhynquelagénorhynque
(defprofile lagénorhynque
:name "Kent OHASHI"
:languages [Clojure Common-Lisp Scheme Haskell
English français]
:interests [programming language-learning law mathematics]
:contributing [github.com/japan-clojurians/clojure-site-ja])

3. 1. Introduction to Pedestal
2. Ring "middleware"
3. Pedestal "interceptor"

4. Introduction to PedestalIntroduction to Pedestal

5. What isWhat is ??
cf. : client-side (no longer developed)
PedestalPedestal
Pedestal is a set of libraries written in
Clojure that aims to bring both the
language and its principles (Simplicity,
Power and Focus) to server-side
development.
pedestal-app

6. Create a Pedestal projectCreate a Pedestal project
cf.
$ lein new pedestal-service hello-pedestal
Generating a pedestal-service application called hello-pedestal.
$ cd hello-pedestal/
lagenorhynque/hello-pedestal

7. $ tree .
.
├── Capstanfile
├── Dockerfile
├── README.md
├── config
│ └── logback.xml
├── project.clj
├── src
│ └── hello_pedestal
│ ├── server.clj
│ └── service.clj
└── test
└── hello_pedestal
└── service_test.clj

8. Run the serverRun the server
$ lein run
INFO org.eclipse.jetty.util.log - Logging initialized @16865ms
to org.eclipse.jetty.util.log.Slf4jLog
Creating your server...
INFO org.eclipse.jetty.server.Server - jetty-9.4.10.v20180503;
built: 2018-05-03T15:56:21.710Z; git: daa59876e6f384329b122929e7
0a80934569428c; jvm 10.0.2+13
INFO o.e.j.server.handler.ContextHandler - Started o.e.j.s.Ser
vletContextHandler@16768389{/,null,AVAILABLE}
INFO o.e.jetty.server.AbstractConnector - Started ServerConnec
tor@22531d51{HTTP/1.1,[http/1.1, h2c]}{localhost:8080}
INFO org.eclipse.jetty.server.Server - Started @17463ms

9. GET http://localhost:8080GET http://localhost:8080
$ curl -i "http://localhost:8080"
HTTP/1.1 200 OK
Date: Tue, 25 Sep 2018 08:41:25 GMT
Strict-Transport-Security: max-age=31536000; includeSubdomains
X-Frame-Options: DENY
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
X-Download-Options: noopen
X-Permitted-Cross-Domain-Policies: none
Content-Security-Policy: object-src 'none'; script-src 'unsafe-i
nline' 'unsafe-eval' 'strict-dynamic' https: http:;
Content-Type: text/html;charset=utf-8
Transfer-Encoding: chunked
Hello World!

10. home­pagehome­page handlerhandler
src/hello_pedestal/service.clj#L13-L16
(defn home-page
[request]
(println request)
(ring-resp/response "Hello World!"))

11. GET http://localhost:8080/aboutGET http://localhost:8080/about
$ curl -i "http://localhost:8080/about"
HTTP/1.1 200 OK
Date: Tue, 25 Sep 2018 08:41:43 GMT
Strict-Transport-Security: max-age=31536000; includeSubdomains
X-Frame-Options: DENY
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
X-Download-Options: noopen
X-Permitted-Cross-Domain-Policies: none
Content-Security-Policy: object-src 'none'; script-src 'unsafe-i
nline' 'unsafe-eval' 'strict-dynamic' https: http:;
Content-Type: text/html;charset=utf-8
Transfer-Encoding: chunked
Clojure 1.9.0 - served from /about

12. about­pageabout­page handlerhandler
src/hello_pedestal/service.clj#L7-L11
(defn about-page
[request]
(ring-resp/response (format "Clojure %s - served from %s"
(clojure-version)
(route/url-for ::about-page))))

13. routesroutes
src/hello_pedestal/service.clj#L18-L25
;; Defines "/" and "/about" routes with their associated :get ha
;; ndlers.
;; The interceptors defined after the verb map (e.g., {:get home
;; -page}
;; apply to / and its children (/about).
(def common-interceptors [(body-params/body-params)
http/html-body])
;; Tabular routes
(def routes #{["/" :get (conj common-interceptors
`home-page)]
["/about" :get (conj common-interceptors
`about-page)]})

14. Ring "middleware"Ring "middleware"

15. request (map)request (map)
spec
{:server-port 8080
:server-name "localhost"
:remote-addr "127.0.0.1"
:uri "/hello"
:query-string "name=World"
:scheme :http
:request-method :get
:headers {"Accept" "application/json"}
:body nil}
(s/def ::request
(s/keys :opt-un [::server-port ::server-name ::remote-addr
::uri ::query-string ::scheme
::request-method ::headers ::body
,,,]))

16. response (map)response (map)
spec
{:status 200
:headers {"Content-Type" "application/json"}
:body {:greeting "Hello, World!"}}
(s/def ::response
(s/keys :opt-un [::status ::headers ::body]))

17. handler (function)handler (function)
spec
cf. asynchronous handler
(defn hello [request]
(let [name (get-in request [:params :name])]
{:status 200
:headers {"Content-Type" "application/json"}
:body {:greeting (str "Hello, " name "!")}}))
(s/def ::handler
(s/fspec :args (s/cat :request ::request)
:ret ::response))

18. middlewaremiddleware
cf.
(defn wrap-keyword-params* [handler]
(fn [request]
(handler (update request :params
#(->> %
(map (fn [[k v]] [(keyword k) v]))
(into {}))))))
ring.middleware.keyword-params

19. spec
template
cf. asynchronous middleware
(s/def ::middleware
(s/fspec :args (s/cat :handler ::handler)
:ret ::handler))
(defn some-middleware [handler]
(fn [request]
(let [response (handler (f request))]
(g response))))

20. Ring middlewareRing middleware

21. Pedestal "interceptors"Pedestal "interceptors"

22. context (map)context (map)
spec
{:request {:protocol "HTTP/1.1", :async-supported? true,
:remote-addr "127.0.0.1", ,,,},
:response nil,
:io.pedestal.interceptor.chain/terminators (#object[io.pedestal
.http.impl.servlet_interceptor$terminator_inject$fn__15706 0x257
66941 "io.pedestal.http.impl.servlet_interceptor$terminator_inje
ct$fn__15706@25766941"]),
:io.pedestal.interceptor.chain/queue #object[clojure.lang.Persi
stentQueue 0x37f89872 "clojure.lang.PersistentQueue@72c17787"],
,,,}
(s/def ::context
(s/keys :opt-un [::request ::response]
:opt [:io.pedestal.interceptor.chain/queue
:io.pedestal.interceptor.chain/terminators
,,,]))

23. interceptorinterceptor
cf.
(def keyword-params*
{:name ::keyword-params*
:enter (fn [context]
(update-in context [:request :params]
#(->> %
(map (fn [[k v]] [(keyword k) v]))
(into {}))))})
io.pedestal.http.params

24. spec
template
cf. function returning a core.async channel
(s/def ::enter
(s/fspec :args (s/cat :context ::context)
:ret ::context))
(s/def ::leave
(s/fspec :args (s/cat :context ::context)
:ret ::context))
(s/def ::interceptor
(s/keys :opt-un [::name ::enter ::leave ::error]))
(def some-interceptor
{:name ::some-interceptor
:enter (fn [context]
(f context))
:leave (fn [context]
(g context))})

25. Pedestal interceptorsPedestal interceptors

26. e.g.e.g.
src/hello_pedestal/service.clj
lagenorhynque/hello-pedestallagenorhynque/hello-pedestal
(def common-interceptors [(body-params/body-params) http/html-body
(def routes #{["/" :get (conj common-interceptors `home-page)]
["/about" :get (conj common-interceptors `about-pag

27. e.g.e.g. lagenorhynque/js-frameworks (chat-server)lagenorhynque/js-frameworks (chat-server)
(defmethod ig/init-key ::routes
[_ {:keys [db redis]}]
(let [common-interceptors [(body-params/body-params)
http/json-body
(interceptor/store-session redis)
interceptor/authenticate
interceptor/attach-tx-data
(interceptor/validate validation-sch
(interceptor/attach-database db)]
auth-interceptors [(body-params/body-params)
http/json-body
(interceptor/store-session redis)
interceptor/attach-tx-data
(interceptor/validate validation-schem
(interceptor/attach-database db)]]

28. src/clj/chat_server/routes.clj
#(route/expand-routes
#{["/api/authentication" :get
(conj common-interceptors
`authentication/fetch-user)]
["/api/authentication" :post
(conj auth-interceptors
`authentication/login)]
["/api/authentication" :delete
(conj auth-interceptors
`authentication/logout)]
["/api/channels" :get (conj common-interceptors
`channels/list-channels)]
["/api/channels" :post (conj common-interceptors
`channels/create-channel)]
,,,})))

29. Further ReadingFurther Reading
PedestalPedestal
example codeexample code
Pedestal
GitHub
Microservices with Clojure
lagenorhynque/hello-pedestal
lagenorhynque/js-frameworks (chat-server)

30. interceptorsinterceptors
middlewaremiddleware
Interceptors
Why interceptors? - quanttype
Concepts (Middleware) · ring-clojure/ring Wiki
ring/SPEC · ring-clojure/ring
Asynchronous Ring - Boolean Knot

31. other librariesother libraries
cf.
reitit
sieppari
Reitit, Data-Driven Routing with Clojure(Script)
- Metosin
Welcome Reitit 0.2.0! - Metosin
re-frame