Deep copy & Shallow copy - Why basics matter

Recently, we ran into an issue in our application, a weird bug that was not easily reproducible. It goes like this:

When I try to look at data for User X, the application sometimes pulls up the details of X and sometimes it gets me the data for some other random User Y. There was no relation between X and Y. This was happening more under load and it was pretty difficult to reproduce.

After digging through the freaking code for hours and hours we finally figured out the problem.It happened for two reasons:

First Reason:
The programmer who wrote the code was careless or lacked a clear understanding of programming fundamentals.

Second Reason:
A shallow copy of the data was being returned instead of a deep copy.

Explanation:
We were using the memory cache to cache the common user metadata from the database so as to cut down the load on the database servers. We then created a class (a Singleton) to encapsulate the access to the cache. This class has an instance of a cache object that talks to the actual memory. The getters in this instance returned shallow copies of data i.e actual references to memory. It was the responsibility of the Singleton class to create a deep copy of the data before returning it to the calling code.

This critical piece of code was missing and so the Singleton was essentially returning a reference to the data in cache.The code that called the Singleton was then modifying the shallow copy of the metadata, unaware that it was actually modifying the values in cache. So when User Y requested data, since the class was a singleton(and therefore using the same cache instance as well), it was returning a modified version of the metadata. When this metadata was combined with other data, specific to user Y, it resulted in weird values and as a result the user ended up seeing some other data.

Solution:
The fix was very simple. In the Singleton class, we had to create a deep copy of the object returned from the cache, before passing it to the calling code, so any changes made to that object did not impact the original values in cache.

throw and throw ex

There are some things in life which come back to bite you in the face(haha...) if you do not pay enough attention to them. In my case one such thing was NOT knowing the difference between throw and throw ex when dealing with exceptions in C#(other languages probably do the same). I didnt know that the stack trace was going to be different depending what whether I did a throw or a throw ex and as a result I always ended up looking up in the wrong places whenever there were exceptions deep inside the call stack.Let me give you an example of what I am talking:

Look at the code below. There are three classes Level 1, Level 2, Level 3. Level makes a call to method HelloLevel2()  in Level2 which in turn makes a call to a method HelloLevel3() in Level 3. The method in Level 3 throws an exception.Pay attention to the text in Red.

public partial class Level1 : System.Web.UI.Page
    {        
        protected void Page_Load(object sender, EventArgs e)
        {            
            Level2 objlevel2 = new Level2();
            objlevel2.HelloLevel2();

        }
    }

 public class Level2
    {
        public void HelloLevel2()
        {
            try
            {
                int x = 10;
                Level3 objlevel3 = new Level3();
                objlevel3.HelloLevel3();
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
    }

public class Level3
    {
        public void HelloLevel3()
        {
            try
            {                
                throw new Exception("Oops....Something got messed up here");
            }
            catch (Exception ex)
            {
                throw ex;
            }

        }
    }

If you look at the stack trace, this is how it looks. If you notice the text in Red you will notice that even though the error occured in HelloLevel3() there is no indication of that. This is happening because the method HelloLevel2() is doing a throw ex inside it's catch block instead of doing a  throw

[Exception: Oops....Something got messed up here]
   WebApplication1.Level2.HelloLevel2() in C:\VisualStudioProjects\WebApplication1\WebApplication1\Level2.cs:20
   WebApplication1._Default.Page_Load(Object sender, EventArgs e) in C:\VisualStudioProjects\WebApplication1\WebApplication1\Default.aspx.cs:29
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
   System.Web.UI.Control.OnLoad(EventArgs e) +91
   System.Web.UI.Control.LoadRecursive() +74
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2207

Now, as shown below, I have modified the code in HelloLevel2() to as shown below

 public class Level2
    {
        public void HelloLevel2()
        {
            try
            {
                int x = 10;
                Level3 objlevel3 = new Level3();
                objlevel3.HelloLevel3();
            }
            catch (Exception ex)
            {
                throw;
            }
        }
    }

If we run the application,the stack now looks as shown below:

[Exception: Oops....Something got messed up here]
WebApplication1.Level3.HelloLevel3() in C:\VisualStudioProjects\WebApplication1\WebApplication1\Level3.cs:18

WebApplication1.Level2.HelloLevel2() in C:\VisualStudioProjects\WebApplication1\WebApplication1\Level2.cs:20
WebApplication1._Default.Page_Load(Object sender, EventArgs e) in C:\VisualStudioProjects\WebApplication1\WebApplication1\Default.aspx.cs:29
System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
System.Web.UI.Control.OnLoad(EventArgs e) +91
System.Web.UI.Control.LoadRecursive() +74
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2207

As you can see it tells you that the exception actually occured in HelloLevel3(). This information is extremely useful and can save hours of troubleshooting in the wrong direction.

Hope this helps! Peace.

Address Verification Software

Last month, I was working on an Order processing application in which the customers had to provide their shipping addresses as a part of the final step. My boss mentioned that prior to shipping, these addresses had to be verified to ensure they were valid deliverable addresses.

My initial thoughts were "Verify addresses? really? How do you do that?".I actually found the idea pretty amusing. Ignorance is bliss I guess.

As I started researching I realized that inaccurate addresses posed a big problem for businesses, especially those that rely on mailings for their sales. Companies that did not have an address verification solution in place were losing millions of dollars in postage, lost merchandise, lost invoices and all the additional time spent by employees in double checking all existing procedures to ensure accuracy. As a result this had opened up a big market for address verification software. A number of companies of varying sizes and shapes offer this software. My next task was to find the one that best suited us.

I poured over a number of "How To" guides on choosing address verification software and also spent a considerable amount of time reading the websites of the various companies suggested in those guides. Finally I narrowed down my choices to the following five companies: 

• MelissaData
• Intelligent Search Technology
• QAS
• USPS
• Sartoris Software  

I called each of these companies and spoke to their sales people. I noted down the various options provided by each and also the cost associated with each option. Most of the Companies except USPS offered two options:

• Hosting the software in-house
•  You get an installation CD that contains the API and all the addresses. You also get a list of updated addresses every two months  
• Accessing the software via web services 
• You pass the address to their web service that will perform the validation and return the results. This option was not offered by USPS. 
• This option is a lot cheaper compared to the first option.  

In the end we decided to go with the web services option provided by Intelligent Search Technology. They had a good API that satisfied our requirements and the cost was very affordable. Their web services were secure (https based) and used a 128 bit algorithm to encrypt the incoming and outgoing data. I created a proxy and tested their API quite extensively and so far the address verification has been pretty consistent and accurate.

Overall, It has been an amusing and enlightening experience and I hope this information proves useful to people out there looking for address verification software.

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