DDevelopment & Design

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

— Previous article

Cross-platform mobile apps – possible or not?

Next article —

Mock Retrofit using Dagger and Mockito

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DDevelopment & Design

Spock, Java and Maven

Few months ago I've came across Groovy - powerful language for JVM platform which combines the power of Java with abilities typical for scripting languages (dynamic typing, metaprogramming).

Together with Groovy I've discovered spock framework (https://code.google.com/p/spock/) - specification framework for Groovy (of course you can test Java classes too!). But spock is not only test/specification framework - it also contains powerful mocking tools.

Even though spock is dedicated for Groovy there is no problem with using it for Java classes tests. In this post I'm going to describe how to configure Maven project to build and run spock specifications together with traditional JUnit tests.


Firstly, we need to prepare pom.xml and add necessary dependencies and plugins.

Two obligatory libraries are:
<dependency>
    <groupid>org.spockframework</groupId>
    <artifactid>spock-core</artifactId>
    <version>0.7-groovy-2.0</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupid>org.codehaus.groovy</groupId>
    <artifactid>groovy-all</artifactId>
    <version>${groovy.version}</version>
    <scope>test</scope>
</dependency>
Where groovy.version is property defined in pom.xml for more convenient use and easy version change, just like this: 
<properties>
    <gmaven-plugin.version>1.4</gmaven-plugin.version>
    <groovy.version>2.1.5</groovy.version>
</properties>

I've added property for gmaven-plugin version for the same reason ;)

Besides these two dependencies, we can use few additional ones providing extra functionality:
  • cglib - for class mocking
  • objenesis - enables mocking classes without default constructor
To add them to the project put these lines in <dependencies> section of pom.xml:
<dependency>
    <groupid>cglib</groupId>
    <artifactid>cglib-nodep</artifactId>
    <version>3.0</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupid>org.objenesis</groupId>
    <artifactid>objenesis</artifactId>
    <version>1.3</version>
    <scope>test</scope>
</dependency>

And that's all for dependencies section. Now we will focus on plugins necessary to compile Groovy classes. We need to add gmaven-plugin with gmaven-runtime-2.0 dependency in plugins section:
<plugin>
    <groupid>org.codehaus.gmaven</groupId>
    <artifactid>gmaven-plugin</artifactId>
    <version>${gmaven-plugin.version}</version>
    <configuration>
        <providerselection>2.0</providerSelection>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>compile</goal>
                <goal>testCompile</goal>
            </goals>
        </execution>
    </executions>
    <dependencies>
        <dependency>
            <groupid>org.codehaus.gmaven.runtime</groupId>
            <artifactid>gmaven-runtime-2.0</artifactId>
            <version>${gmaven-plugin.version}</version>
            <exclusions>
                <exclusion>
                    <groupid>org.codehaus.groovy</groupId>
                    <artifactid>groovy-all</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupid>org.codehaus.groovy</groupId>
            <artifactid>groovy-all</artifactId>
            <version>${groovy.version}</version>
        </dependency>
    </dependencies>
</plugin>

With these configuration we can use spock and write our first specifications. But there is one issue: default settings for maven-surefire plugin demand that test classes must end with "..Test" postfix, which is ok when we want to use such naming scheme for our spock tests. But if we want to name them like CommentSpec.groovy or whatever with "..Spec" ending (what in my opinion is much more readable) we need to make little change in surefire plugin configuration:
<plugin>
    <groupid>org.apache.maven.plugins</groupId>
    <artifactid>maven-surefire-plugin</artifactId>
    <version>2.15</version>
    <configuration>
        <includes>
            <include>**/*Test.java</include>
            <include>**/*Spec.java</include>
        </includes>
    </configuration>
</plugin>

As you can see there is a little trick ;) We add include directive for standard Java JUnit test ending with "..Test" postfix, but there is also an entry for spock test ending with "..Spec". And there is a trick: we must write "**/*Spec.java", not "**/*Spec.groovy", otherwise Maven will not run spock tests (which is strange and I've spent some time to figure out why Maven can't run my specs).

Little update: instead of "*.java" postfix for both types of tests we can write "*.class" what is in my opinion more readable and clean:
<include>**/*Test.class</include>
<include>**/*Spec.class</include>
(thanks to Tomek Pęksa for pointing this out!)

With such configuration, we can write either traditional JUnit test and put them in src/test/java directory or groovy spock specifications and place them in src/test/groovy. And both will work together just fine :) In one of my next posts I'll write something about using spock and its mocking abilities in practice, so stay in tune.

DDevelopment & Design

Distributed scans with HBase

HBase is by design a columnar store, that is optimized for random reads. You just ask for a row using rowId as an identifier and you get your data instantaneously. Performing a scan on part or whole table is a completely different thing. First of all, it is sequential. Meaning it is rather slow, because it doesn't use all the RegionServers at the same time. It is implemented that way to realize the contract of Scan command - which has to return results sorted by key. So, how to do this efficiently?HBase is by design a columnar store, that is optimized for random reads. You just ask for a row using rowId as an identifier and you get your data instantaneously. Performing a scan on part or whole table is a completely different thing. First of all, it is sequential. Meaning it is rather slow, because it doesn't use all the RegionServers at the same time. It is implemented that way to realize the contract of Scan command - which has to return results sorted by key. So, how to do this efficiently?