An introduction to the Ring and Compojure libraries for clojure. Presentation from the February meeting of the Austin Clojure Meetup.

1. Clojure and the Web
Nick Bailey

2. Who is this guy?
Nick Bailey
nick@datastax.com
@nickmbailey

3. Clojure at DataStax
OpsCenter - Monitoring tool for Apache Cassandra
Server agents written in clojure
Agent communication done using http ( Ring and Compojure)

4. What is Compojure?
https://github.com/weavejester/compojure/wiki
“Compojure is a small web framework for the Clojure
programming language. It uses a concise DSL to generate
Ring handler functions.”

5. Ok, What is Ring?
https://github.com/mmcgrana/ring
“Ring is a Clojure web applications library inspired by
Python's WSGI and Ruby's Rack. By abstracting the details of
HTTP into a simple, unified API, Ring allows web applications
to be constructed of modular components that can be shared
among a variety of applications, web servers, and web
frameworks.”

6. Ring Architecture
Adapters
Handlers
Middleware
Request Map
Response Map

7. Adapters
Responsible for implementing the HTTP protocol.
Basically, the server.
Don’t worry, you don’t need to write your own
Ring uses Jetty

8. Handlers
Your application
Process request, return a response

9. Middleware
Augment the functionality of a Handler.
Can:
Modify Requests
Modify Responses

10. Code or GTFO
(ns compojure-talk.core
(:use ring.adapter.jetty))
; Middleware Uppercase the response
(defn wrap-upcase [app]
(fn [req] from the handler
(let [orig-resp (app req)]
(assoc orig-resp
:body
(.toUpperCase
(:body orig-resp))))))
; Handler
(defn app [req] Basic, ‘Hello World’ Handler
{:status 200
:headers {"Content-Type" "text/html"}
:body "Hello World from Ring"})
(def upcase-app (wrap-upcase app))
; Run the adapter
(run-jetty upcase-app {:port 8080}) Run our adapter

11. But Wait, There’s More
See: ring.middleware
Serve static files -- ring.middleware.file
Browser cookies -- ring.middleware.cookies
User sessions -- ring.middleware.session
And much more: https://github.com/mmcgrana/ring

12. Note for Developing
ring.middleware.reload
Automatically reload handler before each request.
Allows for easy testing

13. Ok, What about Compojure
From earlier: “It uses a concise DSL to generate Ring handler
functions.”
An easy an concise way to generate the handler part of your ring
application.

14. The Compojure DSL
defroutes
Macro taking a sequence of routes and the code to process
requests that match those routes.
(defroutes app
(<route>)
(<route>)
(<route>)

15. Anatomy of a Route
Each route has 4 components
(<HTTP method> <URI> <Req. Destructuring> <response>)

16. HTTP Method
One of the http method types defined by compojure
GET, POST, PUT, DELETE, HEAD, ANY
(GET <URI> <Req. Destructuring> <response>)

17. URI
Simple the URI this route should match
(GET “/hello” <Req. Destructuring> <response>)

18. Request Destructuring
Dynamic URIs, URL Params, POST bodies
We’ll come back to this
(GET “/hello” [] <response>)

19. Response
How we want to handle the request
(GET “/hello” [] “Hello World from Compojure”)

20. Hello World, Revisited
(ns compojure-talk.core
(:use ring.adapter.jetty))
; Middleware Note we still have our
(defn wrap-upcase [app]
(fn [req] custom middleware
(let [orig-resp (app req)]
(assoc orig-resp
:body
(.toUpperCase
(:body orig-resp))))))
; Handler We use defroutes to
(defroutes app
(GET "/hello" [] "Hello World from Compojure")) create a handler
(def upcase-app (wrap-upcase app))
; Run the adapter
(run-jetty upcase-app {:port 8080})

21. Back to Destructuring
Access to URI parameters and request body
In the basic form, bind parameter maps and body value to local
vars
(GET “/test” {params :params} (println params))
Take full advantage of clojure destructuring
(GET “/hello” {{user :user} :params} (str “Hello” user “!”))

22. URI Destrcturing
Match dynamic URIs
(GET “/hello/:user” [user] (str “Hello” user “!”))
Note: the destructuring syntax above is short for
“{{user :user} :params}”
Also allows for advanced pattern matching in the URI

23. Our Final Hello
(ns compojure-talk.compojure
(:use compojure.core, ring.adapter.jetty, compojure.handler))
(defroutes app
(GET "/hello/:user" [user] (str "Hello " user "!"))
(GET "/hello" {{user :user} :params}
(if user
(str "Hello " user "!")
"Hello from Compojure!")))
; Run the adapter
(run-jetty (site app) {:port 8080})

24. Links
https://github.com/mmcgrana/ring
https://github.com/weavejester/compojure

25. QUESTIONS?