DDevelopment & Design

Vert.x on Raspberry Pi

Lightweight JVM web server

Some people say that using Java on Raspberry Pi is a stupid idea. Sure, JVM uses a lot of system resources but when you can watch HD movies on RPi why don’t run a tiny Java app? But I do mean really tiny – no Tomcat nor any other application server involved, no war and deployment stuff. That’s why I came out with idea of using vert.x – a relatively new framework for writing modern web application. In this post I’ll try to show lessons learned after developing a sample app using vert.x and then running it on the Raspberry Pi.

Working with vert.x on RPi

Vert.x is node.js on steroids for JVM. In addition to all the libs for the JVM it offers you a lot more:

  • polyglot – write your application in any language that runs on the JVM (or mix a few)
  • really simple programming model
  • inter-app communication via event bus and shared data
  • ease of scalability and concurrency handling

If you want a gentle start in the world of vert.x I definitely suggest you reading the manual on project website.

Installation

First of all – install Java 8 early access build for ARM from Oracle website. Vert.x indeed requires Java 7, but the 8 build has a real hard float support.

The easiest way to manage vert.x installation is to use GVM:

curl -s get.gvmtool.net | bash
gvm install vertx

Now you should be able to run vertx from the command line.

Development process

In my approach I’m developing the app on my laptop and want to have straight path of running the code on RPI. Since I love the Intellij Idea 12 I put my sources into common maven/gradle structure and use Verticle interface to get full benefits of my IDE: code completion, refactoring and testing support. I use Groovy as a language for my verticles but I want the sources to be compiled on my laptop instead of RPI which a bit improves startup time of the app.

The examples for vert.x distribution are not reaaly development-friendly but I’ve found great example of Groovy starter project on GitHub which introduces two huge ideas: separating configuration of the app in single JSON file and a runVertx gradle task to build and run the app in a single step. I also use the web-server vert.x module to serve static web files of the app frontend. So I have SockJS and REST verticles in the backend and JavaScript frontend in a single project root.

Here is the final layout explained:

src/main/groovy - verticles code
    App.groovy - app lancher
src/main/test - verticles tests
web - frontend stuff
build.gradle - gradle script with runVertx task
run.sh - script for running app on RPI (no compilation)

And a typical development process looks like this:

  • write verticles code (*)
  • write unit tests for verticles (*)
  • run the app on laptop and modify the frontend
  • copy binaries to RPi and run the app from command line

(*) change the order of these two for TDD ;)

The only thing I’m only struggling from is lack of runtime reloading of verticles. Maybe using JRebel could help there but I ain’t got the licence (yet).

You can see my example project here.

Benchmark

I also did some benchmarking of vert.x app on RPI throughput. Sure, you cannot expect that single RPI could handle a lot of transactions per sec, so the test was just for fun. In the test I served a static CSS (bootstrap) and invoked a really simple REST resource (example details/Joe/123 mapping from RouteMatcher). I used siege with 2 and 200 consumers and compared the results between the very.x (1 instance), node.js express and nginx (just for static CSS). I installed node.js 0.8.18 from binaries found here.

  • It is nice that serving static files is quite as fast as in the nginx. There is a benchmark showing that Monkey web server is a bit faster, but in my environment I wasn’t able to achieve more than 45 req/sec on nginx, so ~40 req/sec on vert.x is really good result.
  • Vert.x seems to be about 2-2.5 times faster than node.js with express. This is similar to Tim Fox’ benchmark with both node.js and vert.x in single instance. (BTW: I have no idea how to get 30k req/sec on a workstation as in Tim’s test).
  • The benchmark shows that RPi is indeed a toy PC – on my workstation it’s really easy to achieve thousands of requests per second. So forget about handling hundreds of requests per seconds on a single RPi instance.

(a bit) Speeding up startup time

The thing that really puts node.js in front is the startup time. Java is slow at both JVM startup and while classloading. My first application deployed on RPi took about 30 sec to start, while on my laptop it was about 2-3 seconds. It is possible to cut it down to 18 seconds, here are a few hints:

  • don’t compile sources directly on RPi – build a jar on your workstation. In my setup I run the server from gradle on my laptop but also have a bash script to run it on raspberry which passes vert.x a -cp parameter pointing to builded jar.
  • don’t run a SockJS server when not needed. It tooks about 5 sec to start. You can disable it by setting bridge to false in mod-web configuration.
  • you can write your own mod-web in just a few lines of code. You will save some milliseconds but also this setup allows you to pass custom RESTful routes to RouteMatcher. Look at this example:
def rm = new RouteMatcher()
rm.get('/details/:user/:id', { HttpServerRequest req ->
    req.response.end "User: ${req.params()['user']} ID: ${req.params()['id']}"
} as Handler<HttpServerRequest>)

// Catch / - serve the index page
rm.get('/', { HttpServerRequest req ->
    req.response.sendFile("web/index.html")
} as Handler<HttpServerRequest>)

// Catch all - just send the file from web directory
rm.getWithRegEx('.*', { HttpServerRequest req ->
    req.response.sendFile "web/${req.path}"
} as Handler<HttpServerRequest>)

To debug the most time consuming steps on app startup, you should configure log4j logging. Vert.x outputs some info about deploying verticles, you can also put your own logging messages. Just copy log4j.jar to vert.x lib directory and add

-Dorg.vertx.logger-delegate-factory-class-name=org.vertx.java.core.logging.impl.Log4jLogDelegateFactory

parameter in vert.x startup script.

Sample project

You can check these concepts by cloning a top monitor GitHub project. It is a Dashing-inspired web app showing results from calling a top command by lucid JQuery knobs.
Every 2 seconds stats from the top call are pushed through eventBus (and internally by SockJS) to frontend.

The project itself it not smashing, you can just check the structure and the tests when I’m mocking ARM-specific top command results. Enjoy!

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DDevelopment & Design

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!

DDevelopment & Design

Grails session timeout without XML

This article shows clean, non hacky way of configuring featureful event listeners for Grails application servlet context. Feat. HttpSessionListener as a Spring bean example with session timeout depending on whether user account is premium or not.

Common approaches

Speaking of session timeout config in Grails, a default approach is to install templates with a command. This way we got direct access to web.xml file. Also more unnecessary files are created. Despite that unnecessary files are unnecessary, we should also remember some other common knowledge: XML is not for humans.

Another, a bit more hacky, way is to create mysterious scripts/_Events.groovy file. Inside of which, by using not less enigmatic closure: eventWebXmlEnd = { filename -> ... }, we can parse and hack into web.xml with a help of XmlSlurper.
Even though lot of Grails plugins do it similar way, still it’s not really straightforward, is it? Besides, where’s the IDE support? Hello!?

Examples of both above ways can be seen on StackOverflow.

Simpler and cleaner way

By adding just a single line to the already generated init closure we have it done:
class BootStrap {


    def init = { servletContext ->    

        servletContext.addListener(OurListenerClass)    

    }    

}

Allrighty, this is enough to avoid XML. Sweets are served after the main course though :)

Listener as a Spring bean

Let us assume we have a requirement. Set a longer session timeout for premium user account.
Users are authenticated upon session creation through SSO.

To easy meet the requirements just instantiate the CustomTimeoutSessionListener as Spring bean at resources.groovy. We also going to need some source of the user custom session timeout. Let say a ConfigService.
beans = {    

    customTimeoutSessionListener(CustomTimeoutSessionListener) {    

        configService = ref('configService')    

    }    

}

With such approach BootStrap.groovy has to by slightly modified. To keep control on listener instantation, instead of passing listener class type, Spring bean is injected by Grails and the instance passed:
class BootStrap {


    def customTimeoutSessionListener


    def init = { servletContext ->    

        servletContext.addListener(customTimeoutSessionListener)

    }    

}

An example CustomTimeoutSessionListener implementation can look like:
import javax.servlet.http.HttpSessionEvent    

import javax.servlet.http.HttpSessionListener    

import your.app.ConfigService    


class CustomTimeoutSessionListener implements HttpSessionListener {    


    ConfigService configService


    @Override    

    void sessionCreated(HttpSessionEvent httpSessionEvent) {    

        httpSessionEvent.session.maxInactiveInterval = configService.sessionTimeoutSeconds

    }    


    @Override    

    void sessionDestroyed(HttpSessionEvent httpSessionEvent) { /* nothing to implement */ }    

}
Having at hand all power of the Spring IoC this is surely a good place to load some persisted user’s account stuff into the session or to notify any other adequate bean about user presence.

Wait, what about the user context?

Honest answer is: that depends on your case. Yet here’s an example of getSessionTimeoutMinutes() implementation using Spring Security:
import org.springframework.security.core.context.SecurityContextHolder    


class ConfigService {


    static final int 3H = 3 * 60 * 60

    static final int QUARTER = 15 * 60


    int getSessionTimeoutSeconds() {    


        String username = SecurityContextHolder.context?.authentication?.principal    

        def account = Account.findByUsername(username)    


        return account?.premium ? 3H : QUARTER

    }    

}
This example is simplified. Does not contain much of defensive programming. Just an assumption that principal is already set and is a String - unique username. Thanks to Grails convention our ConfigService is transactional so the Account domain class can use GORM dynamic finder.
OK, config fetching implementation details are out of scope here anyway. You can get, load, fetch, obtain from wherever you like to. Domain persistence, principal object, role config, external file and so on...

Any gotchas?

There is one. When running grails test command, servletContext comes as some mocked class instance without addListener method. Thus we going to have a MissingMethodException when running tests :(

Solution is typical:
def init = { servletContext ->

    if (Environment.current != Environment.TEST) {    

        servletContext.addListener(customTimeoutSessionListener)    

    }    

}
An unnecessary obstacle if you ask me. Should I submit a Jira issue about that?

TL;DR

Just implement a HttpSessionListener. Create a Spring bean of the listener. Inject it into BootStrap.groovy and call servletContext.addListener(injectedListener).