Ferhat KUL

Monday, June 16, 2014

A Good Resource about Java Generics

I just hit a good resource about generic classes and methods in java, fun to read : http://math.hws.edu/javanotes/c10/s5.html

Stinging Lessons Learned Today

Don't use static SimpleDateFormat as class member. SimpleDateFormat is not thread safe and multiple threads reaching to a single instance of SimpleDateFormat at the same time may result in strange time formatting results. Use it inside of synchronized block or even better use apache date utils and apache date format those are thread safe and already tested and quality passed solutions :)

Know that elements inherited from interface are static although they are not declared as static inside interface.

import java.text.SimpleDateFormat;

public interface MyIntf {
 public SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy");
}

Implementation:

public class MyImpl implements MyIntf {
 public static void main(String[] args) {
  System.out.println(new MyImpl().dateFormat);
 }
}

Each MyImpl instance will inherit same dateFormat static instance. It is better not to use object with an interface, just leave it as an interface.

To get unique identity of an object, use System.identityHashCode(anObject);

Monday, April 21, 2014

Small Simple Safe (SSS Rule)

I have to devote this blog entry to my teacher in Marmara University Prof. Akif Eyler, I heard small-simple-safe from him first time. This entry is an opportunity to thank him for all valuable teachings he gave us professionals or non-professionals, "tesekkurler hocam".

I have used this principle "small-simple-safe" in my technical career frequently since I heard from him and tried to look problem through it. Here is web page of SSS and here is a great cartoon picturing the term.

Thursday, February 13, 2014

Google Guava RateLimiter

Uncontrolled power is not power. Here how throttling can be done easily using Google Guava RateLimiter

create a RateLimiter with TPS value at line 15.

Acquire RateLimiter at line 21 before submitting a job into an ExecutorService. Thats all :)

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

import com.google.common.util.concurrent.RateLimiter;


public class Test {
 static long start;
 static AtomicInteger doneCounter = new AtomicInteger(0);
 static int joblimit = 10000;
 static int TPS = 1000;
 
 public static void main(String[] args) {
  RateLimiter rateLimiter = RateLimiter.create(TPS);

  ExecutorService executorService = Executors.newCachedThreadPool();//newFixedThreadPool(10);
  start = System.currentTimeMillis();
  
  for (int i = 0; i < joblimit; i++){
   rateLimiter.acquire(); // may wait
   
   executorService.execute(new Runnable() {
    
    @Override
    public void run() {
     System.out.println("doing by " + Thread.currentThread().getName());
     int index = doneCounter.incrementAndGet();
     
     if (index == joblimit){
      System.out.println(joblimit + " job is completed in " + 
         (System.currentTimeMillis() - start) + " msec with " + TPS + " tps");
     }
    }
   });
  }
 }
  }

Wednesday, October 23, 2013

How to design good UI

Here are some good ideas about how to design good UI

http://goodui.org/

AJAX Demo

Here is a sample AJAX demo deployed on google app engine. When link is clicked, request is sent to a backend service (http://kulferhat1.appspot.com/time) and result is displayed asynchronously without posting whole page. An ajax loading image is displayed until response is received.

Html code is below

<html>
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>

<script type="text/javascript">
function createXMLHttpRequest(){
  // See http://en.wikipedia.org/wiki/XMLHttpRequest
  // Provide the XMLHttpRequest class for IE 5.x-6.x:
  if( typeof XMLHttpRequest == "undefined" ) XMLHttpRequest = function() {
    try { return new ActiveXObject("Msxml2.XMLHTTP.6.0") } catch(e) {}
    try { return new ActiveXObject("Msxml2.XMLHTTP.3.0") } catch(e) {}
    try { return new ActiveXObject("Msxml2.XMLHTTP") } catch(e) {}
    try { return new ActiveXObject("Microsoft.XMLHTTP") } catch(e) {}
    throw new Error( "This browser does not support XMLHttpRequest." )
  };
  return new XMLHttpRequest();
}

var AJAX = createXMLHttpRequest();

function handler() {
  
  if(AJAX.readyState == 4 && AJAX.status == 200) {
      var json = eval('(' + AJAX.responseText +')');      
   document.getElementById("resultdata").innerHTML='Success. Result: time => ' + json.time; 
  }else if (AJAX.readyState == 4 && AJAX.status != 200) {
    document.getElementById("resultdata").innerHTML = 'Something went wrong...';
  }
}

function show(){
  AJAX.onreadystatechange = handler;
  AJAX.open("GET", "/time");
  AJAX.send("");
  document.getElementById("resultdata").innerHTML = '<img src="img/Ajax-loader.gif" />';
};

function clearResultData(){
 document.getElementById("resultdata").innerHTML = '';
}
</script>

<body>
  <a href="#" onclick="javascript:show();"> Click here to get time data from the server side</a>
  <a href="#" onclick="javascript:clearResultData();"> Clear </a>
  <div id="resultdata"></div>
</html>

References:

Sunday, September 29, 2013

Apache Camel Prototype

I have just uploaded apache camel prototype application into github. Basically, you can prepare camel routes at runtime and execute them. You can check documentation about details. I should say, it is a prototype and might miss some validation and error handling issues :)

Wednesday, September 18, 2013

Good Tips about JQuery

Here is a list of good tips about JQuery. Especially number 2 and 3 is considerable about why to use google hosted jquery instead of local one and fallback scenario

Bonus of the day: Take a look at Bootstrap to see how it is easy to create fancy web applications, here is a easy style tutorials

Saturday, September 07, 2013

MongoDB as a NoSQL, when to use or not

Here is a great stackoverflow about when to use MongoDb versus RDBMS

http://stackoverflow.com/questions/1476295/when-to-use-mongodb-or-other-document-oriented-database-systems

The most crucial sentence is that:

NoSQL is a great tool, but it’s certainly not going to be your competitive edge, it’s not going to make your app hot, and most of all, your users won’t give a shit about any of this.

After that killing sentence :), lets go with some advantage and disadvantage of NoSQL, specifically MongoDB

Advantages:

Document oriented and schemaless

This is the most prominent advantage of using NoSQL databases. They are good to store unstructured data,

Unlike relational DBs, MongoDB stores all your data in collections of BSON documents and has no schema

It worths for rapid software development where you basically can’t afford to spend too much time doing schema design

Horizontal Scalability and High Availability

MongoDB allows to build a clustered topology with replication and sharding, where the former provides fail-over and eventually consistent read scaling and the latter facilitates scaling out writes(and reads as well).

Just add up nodes and it can scale horizontally quite well.

Fast writes in the fire-and-forget mode. Which is quite useful for collecting various statistics where possible loss of some data is acceptable in favor of a shorter response time.

It’s Free and OpenSource.

Disadvantages:

No SQL = No Joins.

You end up writing jobs to do things like joining data from different tables/collections, something that an RDBMS would do for you automatically.

MongoDb queries can retrieve data from only one collection and take advantage of only one index. n many scenarios, this means more round-trips to the server to find related records. And then you start de-normalizing data - which means background jobs.

No foreign key constraint to ensure data consistency

MongoDB does not support transactions, so it is not suitable where it is needed. You can imagine money transfer or other complex operations.

Well suited for the following situations

Scaling out
Caching
High volume traffic

Less suited for the following situations

High transactional applications
Ad-hoc business intelligence
Problems which require a sql solution

In Summary

Need fast? consider
Need horizontal scaling? consider
Need fast programmability / agile? consider
Need some atomicity? consider
Need complex transactional semantics? no
Need SQL? no
Lots of reporting? a little reporting: fine. real time stats: great. lots of traditional reports: SQL GROUP BY is very powerful. but see also scale.