Clojure-Web-Apps.pdf

Web
Applications
Clojure
BERLIN, 22 Feb 2018 
JOY CLARK
❤

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Web Applications Clojure
Joy Clark
Consultant @ innoQ
[email protected]
@iamjoyclark

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Web Applications
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retrieving
a page
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Web Applications ❤ Clojure
Joy Clark / @iamjoyclark

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forms
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redirects

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JavaScript
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Clojure
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• Lisp Variant for the JVM
• Functional Language
• Dynamically Typed
• Immutable Data Structures
• Simplicity - Separation of Data and Behavior

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Data Structures
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“Hello World”
3 3.14 3/2
\a
:first
foo
#“Ch.*se”
(“Hello” :first)
[3 4 3]
{:name “Joy”
:company “INNOQ”} #{3 4 3}

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Functions
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(+ 1 2 3)
> 6
(:city {:name “INNOQ”
:city “Berlin”})
> “Berlin”
(map inc [1 2 3])
> (2 3 4)

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„You’ve just seen it“
- Rich Hickey
Clojure Syntax:

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Web Applications + Clojure =
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❤

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Something that takes a
HTTP Request and returns a
HTTP Response
What’s a Web Application?

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https://github.com/ring-clojure/ring
• HTTP Server abstraction
• Request & Response are data
• Web App is a function
Ring

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{:uri “/”
:request-method :get
:headers { … } 
:params {:name “Joy”}
…}
Ring: Request

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{:status 200
:headers { … } 
:body “Hello!”}
Ring: Response

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(defn example-app [request]
(let [name (get-in request [:params :name])]
{:status 200 
:headers {“Content-Type” “text/plain”}
:body (str “Hello, ” name “!”)})
Ring: Handler

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• Jetty
• Servlet
• http-kit
• Tomcat
…
Ring: Adapter

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(defn wrap-logging [handler]
(fn [request]
(print request) 
(let [response (handler
request)]
(print response)
response)))
Ring: Middleware
webapp
Java HTTP Server
Ring Server Adapter
middleware
middleware

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Ring: Middleware
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• https://github.com/ring-clojure/ring-defaults
• https://github.com/ring-clojure/ring-json

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https://github.com/weavejester/compojure
Compojure

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Excursion… Clojure Macros

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Clojure: Macros
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“(case 3
1 “one”
2 “two”
“more”)”
Reader
Text Data Structures
(case 3
1 “one”
2 “two”
“more”)

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Clojure: Macros
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Expand Data Structures
(case
1 “one”
2 “two”
“more”)
Data Structures
(if (= 3 1)
“one”
(if (= 3 2)
“two”
“more”))

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Back to
Compojure…
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Compojure: Macro
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(GET “/hello” [] “Hello, world!”)
(fn [req]
(if (and (= (:uri req) “/hello”)
(= (:request-method req) :get))
{:body “Hello, world!”}))

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Compojure: Handler
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(defroutes app-routes
(GET “/” request (list-users request))
(POST “/” {params :params :as request}
(add-user request params))
(GET “/:username” [username :as request]
(get-user request username)))

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• https://github.com/juxt/bidi
• http://clojure-liberator.github.io/liberator/
Compojure: Alternatives

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Functional composition!
Putting it all together

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HTML Templating
❤

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Hiccup
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bar

[:div {:id “foo”}
[:span “bar”]]
https://github.com/weavejester/hiccup

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Hiccup
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Make sure to escape HTML when using Hiccup!
(defn render-html [input]
(hiccup.util/escape-html
(hiccup.core/html input)))

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Hiccup: Alternatives
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• https://github.com/cgrand/enlive
• https://github.com/yogthos/Selmer

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Summary

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Summary
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• Functional language ideal for Web Apps
• Clojure is a stable language
• Libraries vs. Frameworks
• Community is healthy and welcoming

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Interesting Libraries
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• https://github.com/technomancy/leiningen
• https://github.com/boot-clj/boot
• https://github.com/weavejester/environ
• https://github.com/krisajenkins/yesql
• https://github.com/stuartsierra/component

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“Frameworks”
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• https://github.com/weavejester/duct
• http://www.luminusweb.net
• https://github.com/metosin/compojure-api

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Live Demo!

https://github.com/innoq/tutorial-clj-webapp

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State in Clojure…

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INNOQ /
„No man ever steps in the same river
twice, for it’s not the same river and
he’s not the same man“
- Heraclitus

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Clojure Identity
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• The world around us is constantly changing.
• We shouldn’t make the world “stop” so that we can
change it.
• Instead…
• We can give the world a function telling it what we
want to change.

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swap!

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CSRF: Cross-Site Request Forgery

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CSRF
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Protect Against CSRF
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• Don't use GET requests to change state!

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CSRF
Protection
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Thank you!
Joy Clark | @iamjoyclark | [email protected]
https://github.com/innoq/tutorial-clj-webapp

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Clojure-Web-Apps.pdf

Clojure-Web-Apps.pdf

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