Push messages to browser/clients with NodeJS WebSocket

Hi Folks, off late I have been trying to do a POC on pushing messages from server to all the connected clients and found that websocket can be used for it in conjunction with NodeJS. So I am putting it all over to this post.

Following are the prerequisites in order to run the setup:

1. NodeJS Installation (We need npm to install packages) Download Link
2. NPM Packages (websocket, http-server, finalhandler, serve-static).

NPM package installation commands:

• npm install websocket
• npm install http-server
• npm install finalhandler
• npm install serve-static

After successful installation of NodeJS and above mentioned npm packages we can start off writing the code. We need following three files hosted on a folder (All the npm package installation commands will be executed on this folder).

1. frontend.html.
2. frontend.js (NodeJS Frontend).
3. backend.js (NodeJS Backend).

Following is the source code of above files:

frontend.html

frontend.js

backend.js

Following is the directory structure of project:

Limit typing numbers in input type 'number' HTML

Recently I have started front-end development using AngularJS where I had the following requirement:

1. Open a modal window.
2. Paint form on the modal.
3. Form will have a text field of type "number" with following criteria:
1. User can put only positive numbers in the text field nothing else.
2. User can put numbers only from 0-999.

At first above requirement seems to be very easy to implement with min and max properties of input type "number". However there is a catch that min and max restricts only the spinner (increase and decrease) but user can still type numbers which does not belong to 0-999 (Even characters).

To address first criteria we can use onkeypress event and pass it a function which will listen to 0-9 number key presses only and discard the rest:

onkeypress="return (event.charCode == 8 || event.charCode == 0) ? null : event.charCode >= 48 && event.charCode <= 57"

To address second criteria we can use oninput event and pass it a function which will clip/remove the characters after 3rd position:

oninput="this.value.length > 3 ? this.value= this.value.slice(0,3) : this.value"

Note: I have seen suggestions of an alternate approach to accomplish the above using onkeydown event however I have found that if we use onkeydown event it screws up the spinner and also allows text to be pasted and dragged having length more than 3 characters whereas oninput handles all these cases.

Following is the complete input tag:

<input type="number"
       name="sampleTextbox"
       min="0"
       max="999"
       oninput="this.value.length > 3 ? this.value= this.value.slice(0,3) : this.value"
       onkeypress="return (event.charCode == 8 || event.charCode == 0) ? null : event.charCode >= 48 && event.charCode <= 57"/>

Frequency of occurrence of Strings/Words in a list.

I had a problem where I have to keep a track of the frequency of occurrence of a given string in a record (row). I was thinking of doing this using a Map (Probably the best candidate here) where the key is the string and value is the frequency of occurrence.

But there was a catch here, what should we do on the first occurrence of the string as it won't be present in the Map. Below is the solution I came up with:

import java.util.HashMap;

public class MyHashMap {
       private HashMap<String, Integer> myHashMap;

       public MyHashMap(final HashMap<String, Integer> myHashMap) {
           this.myHashMap = myHashMap;
       }

       public void put(String key) {
           Integer integer = myHashMap.get(key);

           if (integer == null) {
                myHashMap.put(key, new Integer(1));
           } else {
                myHashMap.put(key, ++integer);
          }
        }

        @Override
        public String toString() {
            return "MyHashMap [myHashMap=" + myHashMap + "]";
         }

        public static void main(String[] args) {
             MyHashMap m = new MyHashMap(new HashMap<>());

             m.put("Steve");
             m.put("Steve");
             m.put("Simon");

             System.out.println(m);
       }
}

Output:

        MyHashMap [myHashMap={Simon=1, Steve=2}]

Above implementation works fine however you can see we had to do first occurrence handling in put method from our side which I wanted to avoid so I was looking at some other solution. No problem Java 8 Stream API is here and does this thing very smoothly in one liner.

Following is the Java 8 code snippet :

import java.util.ArrayList;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

import java.util.stream.Stream;

public class StreamsDemo {

public static void main(String[] args) {

List<String> list = new ArrayList<>();

list.add("Steve");

list.add("Steve");

list.add("Frank");

list.add("Tom");

list.add("Steve");

Stream<String> stream = list.stream();

Map<String, Long> collect = stream.collect(Collectors.groupingBy(e -> e, Collectors.counting()));

System.out.println(collect);

}

}

Output:

{Frank=1, Tom=1, Steve=3}

Above implementation which uses Java 8 Stream API does the trick as if we are executing a aggregate function COUNT(*) with GROUP BY. Cool !!

Create executable jar with Maven

If you are using Maven and wants to create an executable jar file but not getting an exact solution (Google is providing too many results and nothing is straightforward :-)). So let me get this straight and there is a plugin for it which is maven-assembly-plugin.

Below is the pom.xml which makes use of maven-assembly-plugin to generate executable jar:

<?xml version="1.0" encoding="UTF-8"?>
<project
    xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 
       http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.example</groupId>
    <artifactId>test-project</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <properties>
        <targetJdk>1.7</targetJdk>
        <project.build.sourceEncoding>UTF-8
                          </project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8
                          </project.reporting.outputEncoding>
    </properties>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>1.7</source>
                    <target>1.7</target>
                </configuration>
            </plugin>
            <plugin>
                <artifactId>maven-assembly-plugin</artifactId>
                <version>2.4</version>
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <mainClass>com.test.App</mainClass>
                        </manifest>
                    </archive>
                    <descriptorRefs>
                        <descriptorRef>jar-with-dependencies
                                                </descriptorRef>
                    </descriptorRefs>
                </configuration>
                <executions>
                    <execution>
                        <id>make-jar-with-dependencies</id>
                        <phase>package</phase>
                        <goals>
                            <goal>single</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
    <dependencies>
        <dependency>
            ......
            ......
        </dependency>
    </dependencies>
</project>

Now run "Maven install" and following executable jar file will be created : test-project-0.0.1-SNAPSHOT-jar-with-dependencies.jar

Monitoring java process running under jetty using JConsole

I came across a situation recently where an application running under jetty were having some performance issues which eventually resulted in high latency. After spending quite sometime troubleshooting the problem with no conclusion we had to profile the application to see how many threads are running, how much heap is getting used etc.

To profile the application we had chosen JConsole. Now at first it seemed to be a straightforward setup (After all its just a connection to a given host and port) however it took us a while. Hence sharing the steps here which should be followed to have a hassle free setup.

Step 1:

Add following parameters inside start.ini file found in jetty distribution directory/folder:

-Dcom.sun.management.jmxremote
-Dcom.sun.management.jmxremote.ssl=false
-Dcom.sun.management.jmxremote.authenticate=false
-Dcom.sun.management.jmxremote.port=1099 # 1099 is default JMX Port.
-Djava.rmi.server.hostname=Your_Machine_IP

Step 2:

Make below changes inside (Adding a connector for remote JMX connection) etc/jetty-jmx.xml as per https://wiki.eclipse.org/Jetty/Tutorial/JMX:

<new class="org.eclipse.jetty.jmx.ConnectorServer" id="ConnectorServer">
<arg>
<new class="javax.management.remote.JMXServiceURL">
<arg type="java.lang.String">rmi</arg>
<arg type="java.lang.String">
<arg type="java.lang.Integer"><systemproperty default="1099" name="jetty.jmxrmiport"></systemproperty></arg>
<arg type="java.lang.String">/jndi/rmi://<systemproperty default="localhost" name="jetty.jmxrmihost">:<systemproperty default="1099" name="jetty.jmxrmiport">/jmxrmi</systemproperty></systemproperty></arg>
</arg></new>
</arg>
<arg>org.eclipse.jetty.jmx:name=rmiconnectorserver</arg>
<call name="start">
</call></new>

Note: Above xml configuration is already present inside etc/jetty-jmx.xml file but its commented by default. So just uncomment the xml tag.

Now we are all done. Open the JConsole application and connect to the application using below URL:

service:jmx:rmi:///jndi/rmi://Your_Machine_IP:1099/jmxrmi

Now we should be able to see the real-time application activity in terms of JConsole graphs.

UI for Zookeeper - zk-web

I have been using Zookeeper for the configuration management of my application. There are usually various configurations stored in zookeeper and sometimes it happens that we have to check the configuration values. Using zk-cli command line utility we can accomplish this however zk-cli is a command line tool and you need to learn commands in order get the information out of it.

So I was looking for a GUI Tool which would be simple and cleaner and luckily I found one known as zk-web (https://github.com/qiuxiafei/zk-web).

All the installation instruction can be found here (https://github.com/qiuxiafei/zk-web), However while installing it I faced some issues and thought it would be worth sharing all the steps to have a hassle free installation.

Prerequisites:

• Java ( > Java 6 update 40 )
• Git
• Lien

Installing git:

sudo yum install git (My machine was amazon ec2 Linux)
sudo apt-get install git (If the machine is having ubuntu Linux)

NOTE: Make sure you are running on java version higher than java 6 update 40 If not upgrade the java version.

Cloning git repository for zk-web:

git clone git://github.com/qiuxiafei/zk-web.git

Configuration File:

cd zk-web/conf/

[ec2-user@ip-xx-xxx-x-xxx conf]$ cat zk-web-conf.clj
{
:server-port 8080
:users {"admin" "hello"}
:default-node ""
}

Installing lein:

wget https://raw.githubusercontent.com/technomancy/leiningen/stable/bin/lein
wget --no-check-certificate https://raw.githubusercontent.com/technomancy/leiningen/stable/bin/lein

Move lein to ~/bin so that it will be available on PATH.

Grant execute permissions to lein:

cd ~/bin/
chmod 755 lein

To start zk-web run the below commands:

lein deps
lein run

Now the zk-web should be available at Your_IP:8080 URL.

Most of the time we want zk-web to run as a background process so that we can use it without the need to start it. In this scenario there is an option to generate a jar file using lein:

[ec2-user@ip-xx-xxx-x-xxx zk-web]$ cat project.clj
(defproject zk-web "0.1.0-SNAPSHOT"
     :description "FIXME: write this!"
     :dependencies [[org.clojure/clojure "1.4.0"]
       [noir "1.3.0-beta3"]
       [com.netflix.curator/curator-framework "1.1.16"]
       [com.netflix.curator/curator-test "1.1.16"]]
     :main zk-web.server)

Now using lein we have to build this clj(closure) file.

lein uberjar

This will build 2 jar files:

zk-web-0.1.0-SNAPSHOT.jar
zk-web-0.1.0-SNAPSHOT-standalone.jar

We have to use the second one which is a standalone jar. Below is command we can use to run this jar in background:

nohup java -jar target/zk-web-0.1.0-SNAPSHOT-standalone.jar &

As an alternative, we can put this command to /etc/rc.local file to start zk-web on machine reboot.