A Quantum Immortal

The third Alt .Net convention will take place on 26/03 (planning on 25/03).

Read about my impressions from last time, and come register on Facebook. I will not be attending this year because of a ski trip, but you are all encouraged to come.

Edit – just so you know, since writing this I found Shapado and OSQA, which seem well underway to becoming viable Stack Exchange alternative. I don’t think I will continue developing this project, although it has taught me a great deal nonetheless.

I wanted to share a small learning exercise I underwent recently. I decided to learn how to build a website, and share the experience here. Lacking an original idea at the moment, I decided to create yet another StackOverflow clone – not original, but a good exercise nonetheless. The code for the project is hosted at GitHub, although I don’t have a live showcase up at the moment. Yes, I am aware a google search reveals an existing open source SO clone called Stacked – I thought building one from scratch might teach me more than reading someone else’s code base (I could be wrong, but this is how I wanted to roll).

Day 1

The choice of database engine was easy: The one database I had any experience with was mysql, and being open source, free and easy to work with, I went with it. Next, an ORM library. After some digging, I’ve decided to go with NHibernate + Castle ActiveRecord. ActiveRecord is great for easily mapping simple classes and relations, and NHibernate to complement it in ‘advanced queries’. I don’t get any LINQ magic from these ORMs, which is a pity. I then proceeded to create a solution and some projects, added NUnit as a test framework, and setup Castle MicroKernel as an IOC framework.

I tried to found an open hosted source CI server, but didn’t find anything that worked. Oh well, not essential for now.

Day 2

Before proceeding any further, I had to choose a source control provider. If this were a production project I would probably have gone with SVN hosted on Google Code. However, Ken told me about Git long ago, and I thought this was a good chance to experience it. So, on to GitHub. Opening the project was a no brainer, but finding a decent client was more challenging. I had some fun learning about Git’s private keys, and configuring TortoiseGit (I tried GitExtensions, but it doesn’t support Visual Studio 2010 beta 2 yet). Overall, TortoiseGit gets the job done, after some tweaking and .gitignore files.

Git’s distributed source control model is interesting and worth a try.

I created the User, Question & Vote tables, learned about composite keys in AR, and wrote my first NH query:

GetVoteCount – “SELECT Vote, COUNT(*) FROM ” + VotesTableName + ” WHERE PostId = :postId GROUP BY vote”

I currently don’t have any caching on the vote count – am simply storing the votes as relations between users and questions, and counting them on the fly.

Day 3

I would like to implement full features, not write the entire DAL first and then the application logic. So, it’s time to start learning about web development. At work people are using Monorail, but after reading this question I decided to try out ASP.Net MVC instead. So, I read a basic tutorial and starting coding. Some things I learned:

1. I finally got the meaning of Global.asax.cs – it’s simply the ‘main’ of the web application.
2. By default, ASP.Net MVC creates the controllers by itself and does not support IOC. Fixed (remember to setup the Controller’s lifestyle as LifeStyle.PerWebRequest).
3. Some Asp.Net MVC basics:
   1. Use <%= … %> to write to the output stream (that gets sent as the HTML), and <% %> to simply execute code.
   2. Use Html.RenderAction() to create links to other pages (~= Actions)
   3. Your pages are butt ugly without tweaking the CSS

Day 4

• I quickly caught myself duplicating code, and turned to learn about Partial Views, which are reusable View pieces.
• I realized that having my model entities derive from ActiveRecordBase is damn ugly, because it makes my entire application dependant on AR even if I was using repositories to access the data. I switched the repositories to using ActiveRecordMediator instead.
• The magic that is ReturnUrl – an extra request parameter that controllers use to return you to your original page after you login.
• An interesting usage for anonymous object creation syntax in C#, to pass query parameters: new { ReturnUrl = “foo”}
• I decide to create a base class for all my controllers – UserAwareController. This was needed because every controller needs to access the current user, so I put all this logic in the UserAwareController base class.
• Since every View needs the User to render, I created a Model base class that contains the current user. I’m not sure if this is the best way to go here, but it worked (what are your recommended best practices to store the user data?)
• I implemented OpenID login using DotNetOpenAuth, and it was quite a breeze. No need to store user credentials, just store his public open id and let other websites handle the authentication for you.

Day 5

After allowing users to login and post questions, the next thing I wanted to implement was voting. So far all the code was server-side, but voting requires javascript because when a user votes you don’t want to refresh the page, but rather just change the vote icon. So:

• I learned about jquery basics and wrote events to handle clicking the voting buttons.
• Sent the vote information to a dedicated controller using JSON. The way JSON requests translate to controller methods is really seamless.
• Initially I had an ‘AddVote’ method, but quickly switched to ‘UpdateVote’, which makes more sense.
• Some css tweaks to make the cursor change to a pointer while on the voting buttons

Day 6

• I finally had to cache vote count on questions & answers. The total vote count / score of a question has to be indexed, because we’ll have pages that get the ‘hottest posts’, and so keeping the User-Question vote relation is not enough.
• So far, all my entities were strictly mapped to database rows. Now, I had to create a new ‘rich entity’ that contained a post and the current users’ vote on this post.
• Finding myself duplicating logic between questions & answers, I create a Post base class and factored the entities and repositories to work on abstract posts.

This is it for now. I hope I didn’t make too many glaring mistakes in the process. As I mentioned, the code is available at GitHub – if you’re interested in helping develop it or have any questions, please let me know.

Lately I need to support task cancellation in a Java process I’m working on. The straightforward options I know to implement this are:

1. Thread.interrupt() – the caller interrupts the worker thread (either directly or using Future.cancel()). Some say this is an erroneous approach, but I still haven’t figured out why. However, it is buggy on some recent versions on the JDK, and it is a bit fragile (what if the worker threads create worker threads that also need to be canceled?).
2. Passing some object (AtomicBoolean?) down to every object you would like to support cancellation. These objects will check the value of this boolean, and should stop if it is false. They can pass the boolean to other objects / tasks. While this works, this boolean cannot be injected, and so must be manually passed along the call stack.

If you want the advantages of the second method, but don’t want to break IOC, here’s how:

First, the usage:

The listener object adds a dependency on ICancelor

public class Foo {
  public Foo(ICancelor cancelor) {
    this.cancelor = cancelor;
    ...
}

It then checks the cancellation state every now and then:

if (cancelor.wasTaskCanceled("TakeOverTheWorld"))
   return;

The top-level thread that wishes to cancel a task simply calls

cancelor.cancelTask("TakeOverTheWorld");

And whenever a task is started, you should call

cancelor.resetTask("TakeOverTheWorld");

I’ll admit using strings for task names is a bit ugly, but this is not a terrible price to pay, assuming you have a few core tasks you intend to support. All that remains is the cancellation service itself:

/**
 * A cancellation service.
 */
public interface ICancelor {
    /**
     * Resets a task to "Not canceled" state
     */
    void resetTask(String name);
 
    /**
     * Returns true iff the a cancelTask was called, and no resetTask was called afterwards.
     */
    boolean wasTaskCanceled(String name);
 
    /**
     * Cancel a task
     */
    void cancelTask(String name);
}
 
public class Cancelor implements ICancelor {
  private final ConcurrentHashMap tasks = new ConcurrentHashMap();
 
    public void resetTask(String name) {
        tasks.put(name, true);
    }
 
    public boolean wasTaskCanceled(String name) {
        Boolean value = tasks.get(name);
        return value != null & value;
    }
 
    public void cancelTask(String name) {
        tasks.put(name, false);
    }
}

Because we rely on task names, there is an assumption here that all classes that play in the cancellation game belong to the same task semantically. If a class is a common class that doesn’t belong to a single task or flow, this approach does not work – in fact, I cannot think of an approach that will work in this case with dependency injection. The common class has to accept the cancellation signal somehow, it must either get an boolean explicit and not from the IOC container, or must check its interrupted state (or some other thread-local state) itself. Any smart ideas on how to solve this problem?

Come hear about .Net tools in a “no bullshit” evening (more details here).

I recently downloaded Visual Studio 2010 beta, and took the chance to play with PLINQ. PLINQ, for those of you in the dark ages of .Net Framework 2.0, is parallel LINQ - an extension to the famous query language that makes it easy to write parallel code (essential to programming in the 21th century, in the age of the many-core).

A code sample, as usual, is the best demonstration:

public static int CountPrimes(IEnumerable<int> input)
{
    return input.AsParallel().Where(IsPrime).Count();
}
 
private static bool IsPrime(int n)
{
    for (int i = 2; i*i < n; ++i)
        if (n % i == 0)
            return false;
    return true;
}

This code sample, regardless of using an inefficient primality test, is fully parallel. PLINQ will utilize all your cores when running the above code, and I didn't have to use any locks, queues, threadpools or any of the more complex tools of the trade. Just tell PLINQ "AsParallel()", and it works.

I hit some gotcha when I tried to compare the parallel performance with the sequential one. Do you spot the problem in the following code?

public static void CountPrimesTest(IEnumerable<int> input)
{
    // parallel benchmark 
    var timer = new Stopwatch();
    timer.Start();
    CountPrimes(input.AsParallel());
    timer.Stop();
    Console.WriteLine("Counted primes in parallel took " + timer.Elapsed);
 
    // sequential benchmark
    timer = new Stopwatch();
    timer.Start();
    CountPrimes(input);
    timer.Stop();
    Console.WriteLine("Counted primes sequentially took " + timer.Elapsed);
}

This is all fine and dandy when the task at hand is CPU bound, but works pretty miserabbly when your task is IO bound, like downloading a bunch of web pages. Next, I simulated some IO-bound tasks (I used Sleep() to emulate IO - basically not using a lot of CPU for every task):

[ThreadStatic]
private static Random _random;
 
public static List<string> FindInterestingDomains(IEnumerable<string> urls)
{
    // select all the domains of the interesting URLs
    return urls.AsParallel().Where(SexFilter).
                Select(url => new Uri(url).Host).ToList();
}
 
public static bool SexFilter(string url)
{
    if (_random == null)
        _random = new Random();
 
    // simulate a download
    Thread.Sleep(1000);
    var html = "<html>" + _random.Next() + "</html>";
    return html.Contains("69");
}

Testing this with a list of 10 URLs took 5 seconds, meaning LINQ again spun only two cores, which is the number of cores on my machine. This really sucks for IO bound tasks, because most of the time the threads are idle, waiting on IO. Let's see if we can speed this up:

// Use WithDegreeOfParallelism to specify the number of threads to run
return urls.AsParallel().WithDegreeOfParallelism(10).Where(SexFilter).
              Select(url => new Uri(url).Host).ToList();

This appeared not to work at first, because WithDegreeOfParallelism is just a recommendation or upper bound. You can ask PLINQ nicely to run with ten threads, but it will only allocate two if it so chooses. This is yet another example of C# being more magical than Java - compared to Java's rich ExecutorService, PLINQ offers less fine grained control.

However, further testing revealed the damage is not so horrible. This is what happened when I put the above code in a while(true):

Tested 10 URLs in 00:00:05.0576333
Tested 10 URLs in 00:00:03.0018617
Tested 10 URLs in 00:00:03.0013939
Tested 10 URLs in 00:00:03.0013175
Tested 10 URLs in 00:00:04.0018983
Tested 10 URLs in 00:00:03.0024044
Tested 10 URLs in 00:00:01.0004407
Tested 10 URLs in 00:00:01.0007645
Tested 10 URLs in 00:00:01.0007280
Tested 10 URLs in 00:00:01.0003358
Tested 10 URLs in 00:00:01.0003347
Tested 10 URLs in 00:00:01.0002470

After some trial and error, PLINQ found that the optimal number of threads needed to run this task under its concurrency guidelines is ten. I imagine that if at some point in the future the optimal number of threads change, it will adapt.

P.S.
If you found this interesting, wait till you read about DryadLINQ - it's LINQ taken to the extreme, run over a cluster of computers.

I have been programming in C# for several years now, and recently made the switch to Java (at least for now). I noticed that Java, as a language, is “less magical” than C#.

What do I mean by that is that in C# things are usually done for you, behind the scenes, magically, while Java is much more explicit in the toolset it provides. For example, take thread-local storage. The concept is identical in both langauges – there is often a need for a copy of a member variable that’s unique to the current thread, so it can be used without any locks or fear of concurrency problems.

The implementation in C# is based on attributes. You basically take a static field, annotate it with [ThreadStatic], and that’s it:

[ThreadStatic]
private static ThreadUnsafeClass foo = null;
 
private ThreadUnsafeClass Foo
{
  get
  {
    if (foo != null)
      foo = new ThreadUnsafeClass(...);
 
    // no other thread will have access to this copy of foo
    // note - foo is still static, so it will be shared between instances of this class.
    return foo;
  }
}

How does it work? Magic. Sure, one can find the implementation if he digs deep enough, but the first time I encountered it I just had to try it to make sure it actually works, because it seemed too mysterious.

Let’s take a look at Java’s equivalent, ThreadLocal. This is how it works (amusingly enough, from a documentation bug report):

public class SerialNum {
     // The next serial number to be assigned
     private static int nextSerialNum = 0;
 
     private static ThreadLocal<Integer> serialNum = new ThreadLocal<Integer>() {
         protected synchronized Integer initialValue() {
             return new Integer(nextSerialNum++);
         }
     };
 
     public static int get() {
         return serialNum.get();
     }
 }

No magic is involved here – get() gets the value from a map, stored on the calling Thread object (source code here, but the real beauty is that’s it’s available from inside your IDE without any special effort to install it).

Let’s look at another example – closures.

In C#, you can write this useful piece of code:

var list = new List<int>();
...
// find an element larger than 10
list.Find(x => x > 10);

You can also make this mistake:

var printers = new List<Action>();
...
foreach (var item in list)
{
  printers.Add(() => Console.WriteLine(item));
}
Parallel.Foreach(printers, p => p())

An innocent reader might think this prints all the items in list, but actually this only prints the last items list.Count times. This is how closures work. This happens because the item referred to in the closure is not a new copy of item, it’s actually the same item that’s being modified by the loop. A workaround is to add a new temporary variable like this:

foreach (var item in list)
{
  int tempItem = item;
  printers.add(() => Console.WriteLine(tempItem));
}

And in Java? Instead of closures, one uses anonymous classes. In fact, this is how they are implemented under the hood in C#. Here the same example, in Java:

for (Integer item : list)
{
  final int tempItem = item;
  printers.add(new Action(){
    public void doAction()
    {
      // can't reference item here because it's not final.
      // this would have been a compilation error
      // system.out.println(item);
      System.out.println(tempItem);
    });
}
...

Notice it’s impossible to make the mistake and capture the loop variable instead of a copy of it, because Java requires it to be final. So … less powerful perhaps than C#, but more predictable. As a side note, Resharper catches the ill-advised capturing of local variables and warns about it.

I myself rather prefer the magic of C#, because it does save a lot of the trouble. Lambdas, properties, auto-typing variables… all these are so convenient it’s addictive. But I have to give Java a bit of credit, as the explicit way of doing stuff sometimes teaches you things that you just wouldn’t have learn cruising away in C# land.

Check out the idcc, register (free), vote on the topics, and attend.

Q.E.D.

P.S.

Actually, registration costs 100 NIS.

Especially when extending a 3rd party base class.

This is a known best practice, but when I read about it I natrually assumed I was smarter than the author of the best practice. The reason not to use synchronized methods (or lock(this)) is that other code might lock on your object too, thus causing nasty deadlocks.

I figured this wouldn’t happen because ‘who would just lock on my object, there’s no chance of that’. Well, this is obviously not safe, but especially so when extending a 3rd-party base class. In my case, I was extending log4j’s AppenderSkeleton, and found out the hard way that log4j obtains locks on the appenders.

The solution:

1. Use a private lock object (duh), seperating your intended lock semantics from whatever evil outside code will use
2. Stop assuming that I know best and ‘it will never happen’