Micro services on the JVM part 1 – Clojure

Micro services could be a buzzword of 2014 for me. Few months ago I was curious to try Dropwizard framework as a separate backend, but didn’t get the whole idea yet. But then I watched a mind-blowing “Micro-Services Architecture” talk by Fred George. Also, the 4.0 release notes of Spring covers microservices as an important rising trend as well. After 10 years of having SOA in mind, but still developing monoliths, it’s a really tempting idea to try to decouple systems into a set of independently developed and deployed RESTful services. Micro services could be a buzzword of 2014 for me. Few months ago I was curious to try Dropwizard framework as a separate backend, but didn’t get the whole idea yet. But then I watched a mind-blowing “Micro-Services Architecture” talk by Fred George. Also, the 4.0 release notes of Spring covers microservices as an important rising trend as well. After 10 years of having SOA in mind, but still developing monoliths, it’s a really tempting idea to try to decouple systems into a set of independently developed and deployed RESTful services.

Micro services could be a buzzword of 2014 for me. Few months ago I was curious to try Dropwizard framework as a separate backend, but didn’t get the whole idea yet. But then I watched a mind-blowing “Micro-Services Architecture” talk by Fred George. Also, the 4.0 release notes of Spring covers microservices as an important rising trend as well. After 10 years of having SOA in mind, but still developing monoliths, it’s a really tempting idea to try to decouple systems into a set of independently developed and deployed RESTful services.

So when I decided to write a simple API for my DevRates.com website, instead of adding some code to existing codebase, I wanted to build a separate tiny app. But what’s the best stack for micro-services? In this series of posts I’ll try to compare various JVM technology stacks for this approach.

Here is my list of must-have features for the stack:

  • declarative REST support (no manual URL parsing)
  • native JSON support (bidirectional JSON-object mapping)
  • single “fat” jar packaging, no web container needed
  • fast development feedback loop (eg. runtime code reloading)
  • Swagger and Metrics integration

In this post I’ll try to cover Clojure with Ring and Compojure.

TL;DR

You can find all the covered concepts in the following GitHub examples:

Basic setup

There is an excellent Zaiste’s tutorial showing how to kickstart REST app with Compojure, just follow these few simple steps (the rest of the post assumes compojure-rest as the app name).

My sample route from handler.clj:

(defroutes app-routes (GET "/messages/:name" [name] {:body {:message (str "Hello World" " " name)}}) (route/resources "/") (route/not-found "Not Found"))

Fat jar

In a simple setup, Compojure app is being run through lein ring plugin. To enable running it as a standalone command-line app, you have to write a main method which starts Jetty server.

project.clj

:dependencies ... [ring/ring-jetty-adapter "1.2.0"] .. :main compojure-rest.handler

handler.clj

To build a single “fat” jar just run lein uberjar, and then java -jar target/compojure-rest-0.1.0-SNAPSHOT-standalone.jar runs the app.

(ns compojure-rest.handler ... (:require ... [ring.adapter.jetty :refer (run-jetty)]) (:gen-class)) ... (defn -main [& args] (run-jetty app {:port 3000 :join? false }))

Swagger

The nice thing about Compojure is that you can easy expose Swagger documentation by using swag library. There are some conflicts between swag and ring lein plugin, so just look at the compojure-swag for a working example.

Here is a typical snippet from handler.clj:

(set-base "http://localhost:3000") (defroutes- messages {:path "/messages" :description "Messages management"} (GET- "/messages/:name" [^:string name] {:nickname "getMessages" :summary "Get message"} {:body {:message (str "Hello World" " " name)}}) (route/resources "/") (route/not-found "Not Found"))

So, swag introduces defroutes-, GET-, POST- which take additional metadata as parameters to generate Swagger docs. If you’re little scared with this ^:string fragment – check metadata section from Clojure manual. Swagger-compatible definition should be available at http://localhost:3000/api-docs.json after running the app.

Metrics

To expose basic metrics of your REST API calls just use Ring-compatible metrics-clojure-ring library.

project.clj

:dependencies ... [metrics-clojure-ring "1.0.1"] ...

handler.clj

(ns compojure-rest.handler ... (:require ... [metrics.ring.expose :refer [expose-metrics-as-json]] [metrics.ring.instrument :refer [instrument]])) ... (def app (expose-metrics-as-json (instrument app) "/stats/"))

After generating some load by eg. wrk, you can check the collected stats by visiting http://localhost:3000/stats/.

ring.requests.rate.GET: { type: "meter", rates: { 1: 189.5836593065824, 5: 39.21602480726734, 15: 13.146759983907245 } }

Some random Clojure thoughts

  • The best newbie guide to Clojure is Kyle Kingsbury’s “Clojure from the ground up” series.
  • Leiningen is probably the best build tool for the JVM. Easy to install, fast, simple, no XML – just doing it right. And the “new” project templates is what’s Maven been missing from ages (anyone using archetypes?).
  • Lighttable is great! I’m really impressed with the fast feedback loop by just ctrl+entering the expressions.
  • Also, live reloading with ring server works fine. Just change the change code and see the changes immediately. Rapid!
  • Unlike other recently popular languages, Clojure has no killer-framework. Rails, Play/Akka, Grails/Gradle – all of these are key parts of Ruby, Scala and Groovy ecosystems. What about Clojure? A collection of small (micro?) libraries doing one thing well and working great together – just like Unix commands.
  • It may be true that Clojure is not good for large projects. With all the complex contructs (meta or ) and no control of the visibility, it could be hard to maintain large codebase. But it’s not a first-class problem in a micro-services world..

Resources

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CasperJS for Java developers

Why CasperJS

Being a Java developer is kinda hard these days. Java may not be dead yet, but when keeping in sync with all the hipster JavaScript frameworks could make us feel a bit outside the playground. It’s even hard to list JavaScript frameworks with latest releases on one website.

In my current project, we are using AngularJS. It’a a nice abstraction of MV* pattern in frontend layer of any web application (we use Grails underneath). Here is a nice article with an 8-point Win List of Angular way of handling AJAX calls and updating the view. So it’s not only a funny new framework but a truly helper of keeping your code clean and neat.

But there is also another area when you can put helpful JS framework in place of plan-old-java one - functional tests. Especially when you are dealing with one page app with lots of asynchronous REST/JSON communication.

Selenium and Geb

In Java/JVM project the typical is to use Selenium with some wrapper like Geb. So you start your project, setup your CI-functional testing pipeline and… after 1 month of coding your tests stop working and being maintainable. The frameworks itselves are not bad, but the typical setup is so heavy and has so many points of failure that keeping it working in a real life project is really hard.

Here is my list of common myths about Selenium: * It allows you to record test scripts via handy GUI - maybe some static request/response sites. In modern web applications with asynchronous REST/JSON communication your tests must contain a lot of “waitFor” statements and you cannot automate where these should be included. * It allows you to test your web app against many browsers - don’t try to automate IE tests! You have to manually open your app in IE to see how it actually bahaves! * It integrates well with continuous integration servers like Jenkins - you have to setup Selenium Grid on server with X installed to run tests on Chrome or Firefox and a Windows server for IE. And the headless HtmlUnit driver lacks a lot of JS support.

So I decided to try something different and introduce a bit of JavaScript tooling in our project by using CasperJS.

Introduction

CasperJS is simple but powerful navigation scripting & testing utility for PhantomJS - scritable headless WebKit (which is an rendering engine used by Safari and Chrome). In short - CasperJS allows you to navigate and make assertions about web pages as they’d been rendered in Google Chrome. It is enough for me to automate the functional tests of my application.

If you want a gentle introduction to the world of CasperJS I suggest you to read: * Official website, especially installation guide and API * Introductionary article from CasperJS creator Nicolas Perriault * Highlevel testing with CasperJS by Kevin van Zonneveld * grails-angular-scaffolding plugin by Rob Fletcher with some working CasperJS tests

Full example

I run my test suite via following script:

casperjs test --direct --log-level=debug --testhost=localhost:8080 --includes=test/casper/includes/casper-angular.coffee,test/casper/includes/pages.coffee test/casper/specs/

casper-angular.coffe

casper.test.on "fail", (failure) ->
    casper.capture(screenshot)

testhost   = casper.cli.get "testhost"
screenshot = 'test-fail.png'

casper
    .log("Using testhost: #{testhost}", "info")
    .log("Using screenshot: #{screenshot}", "info")

casper.waitUntilVisible = (selector, message, callback) ->
    @waitFor ->
        @visible selector
    , callback, (timeout) ->
        @log("Selector [#{selector}] not visible, failing")
        withParentSelector selector, (parent) ->
            casper.log("Output of parent selector [#{parent}]")
            casper.debugHTML(parent)
        @echo message, "RED_BAR"
        @capture(screenshot)
        @test.fail(f("Wait timeout occured (%dms)", timeout))

withParentSelector = (selector, callback) ->
    if selector.lastIndexOf(" ") > 0
       parent = selector[0..selector.lastIndexOf(" ")-1]
       callback(parent)

Sample pages.coffee:

x = require('casper').selectXPath

class EditDocumentPage

    assertAt: ->
        casper.test.assertSelectorExists("div.customerAccountInfo", 'at EditDocumentPage')

    templatesTreeFirstCategory: 'ul.tree li label'
    templatesTreeFirstTemplate: 'ul.tree li a'
    closePreview: '.closePreview a'
    smallPreview: '.smallPreviewContent img'
    bigPreview: 'img.previewImage'
    confirmDelete: x("//div[@class='modal-footer']/a[1]")

casper.editDocument = new EditDocumentPage()

End a test script:

testhost = casper.cli.get "testhost" or 'localhost:8080'

casper.start "http://#{testhost}/app", ->
    @test.assertHttpStatus 302
    @test.assertUrlMatch /\/fakeLogin/, 'auto login'
    @test.assert @visible('input#Create'), 'mock login button'
    @click 'input#Create'

casper.then ->
    @test.assertUrlMatch /document#\/edit/, 'new document'
    @editDocument.assertAt()
    @waitUntilVisible @editDocument.templatesTreeFirstCategory, 'template categories not visible', ->
        @click @editDocument.templatesTreeFirstCategory
        @waitUntilVisible @editDocument.templatesTreeFirstTemplate, 'template not visible', ->
            @click @editDocument.templatesTreeFirstTemplate

casper.then ->
    @waitUntilVisible @editDocument.smallPreview, 'small preview not visible', ->
        # could be dblclick / whatever
        @mouseEvent('click', @editDocument.smallPreview)

casper.then ->
    @waitUntilVisible @editDocument.bigPreview, 'big preview should be visible', ->
        @test.assertEvalEquals ->
            $('.pageCounter').text()
        , '1/1', 'page counter should be visible'
        @click @editDocument.closePreview

casper.then ->
    @click 'button.cancel'
    @waitUntilVisible '.modal-footer', 'delete confirmation not visible', ->
        @click @editDocument.confirmDelete

casper.run ->
    @test.done()

Here is a list of CasperJS features/caveats used here:

  • Using CoffeeScript is a huge win for your test code to look neat
  • When using casper test command, beware of different (than above articles) logging setup. You can pass --direct --log-level=debug from commandline for best results. Logging is essential here since Phantom often exists without any error and you do want to know what just happened.
  • Extract your helper code into separate files and include them by using --includes switch.
  • When passing server URL as a commandline switch remember that in CoffeeScript variables are not visible between multiple source files (unless getting them via window object)
  • It’s good to override standard waitUntilVisible with capting a screenshot and making a proper log statement. In my version I also look for a parent selector and debugHTML the content of it - great for debugging what is actually rendered by the browser.
  • Selenium and Geb have a nice concept of Page Objects - an abstract models of pages rendered by your application. Using CoffeeScript you can write your own classes, bind selectors to properties and use then in your code script. Assigning the objects to casper instance will end up with quite nice syntax like @editDocument.assertAt().
  • There is some issue with CSS :first and :last selectors. I cannot get them working (but maybe I’m doing something wrong?). But in CasperJS you can also use XPath selectors which are fine for matching n-th child of some element (x("//div[@class='modal-footer']/a[1]")).
    Update: :first and :last are not CSS3 selectors, but JQuery ones. Here is a list of CSS3 selectors, all of these are supported by CasperJS. So you can use nth-child(1) is this case. Thanks Andy and Nicolas for the comments!

Working with CasperJS can lead you to a few hour stall, but after getting things working you have a new, cool tool in your box!