Report from the battlefield #2 - amount of data matters a lot

Home

In the next post from the Report from the battlefield series I'll wrote about a serious mistake that is quite common according to my experience. I'm thinking about a situation when a developer assumes that all data from a database can be processed on the client side. I'll give you 2 examples that I encountered during my reviews.

Case 1

A developer was asked to implement the paging functionality. He created a single page Web application. It looked nice and the paging was working correctly at first glance. I decided to check how it was implemented under the hood. I examined a web service that was used by the application and I was surprised. Why? I didn't find a web method responsible for returning pages. The next step was to dig into a java script code. Unfortunately, I discovered that the paging was implemented only on the client side i.e. the application initially downloaded all data from a database (via Web Service).

Case 2

In the another project the paging was implemented correctly on the server side but a developer made a more subtle mistake. An application had a shopping cart function. Of course it was possible to add and remove products to and from a cart. To do so a web service used by the application had a method GetCart. This method was responsible for retrieving a current content of a cart from a database.

However, it was strange that this method returned only identifiers of products. What's more there was no GetProductDetails web method. It made me curious how the application displays products details to users only knowing its identifiers. It turned out that at the initiation the application was reading details of all products from a database. Having all products on the client side it was easy to find details of a product based on its identifier.

Summary

In both cases applications were fast enough because of a small amount of data. In the case of real-life databases they will not. I think that we should always be prepared for the worst case. Especially, when we participate in a recruitment and we want to show ourselves from the best side. An evaluator shouldn't guess whether we know something or not.

Merry Christmas!

Home

Source: own resources, Authors: Agnieszka and Michał Komorowscy

Giving wishes in a foreign language can be challenging so my wishes will be simple but very sincere. I wish you a Merry, Peaceful Christmas and an Amazing 2016. Let it be at least as good as the past year.

Best wishes,
Michał Komorowski

Report from the battlefield #1 - EF and DTOs

Home

Some time ago, I started doing code reviews of various projects for the recruitment company. It is an interesting experience and I'm learning a lot by this occasion. I also observed that some mistakes are repeated by different authors. Other are not so common but are not obvious. So I came up with the idea to start a new series of posts under the title "Report from the battlefield". In this series I'll describe my observations and findings from my reviews.

Let's start. Recently, I reviewed a project created with AngularJS + ASP.NET Web API + Entity Framework. The code was neither very good nor very bad. However, I noticed that the author decided to use a class generated from the EDMX model as DTO (Data Transfer Object). The reasoning behind this decision was simple - this class had all properties required on the client side so why not to use it. Well there are a few reasons why it is not a good idea.

• With dedicated DTOs it is less possible that changes on the server side will affect the client side.
• With dedicated DTOs we can easily control what will be send to the client side and in what format.
• With dedicated DTOs the server side model can be completely different from the client side model.
• By exposing EF classes to the client side we effectively expose the database model to the client side!

You may agree with my points or not. So, I'll give you a practical example what could happen if we use EF classes as DTOs. Let's assume that there is EDMX model with 3 types of entities:

• Customer with Orders navigation property.
• Orders with Customer and Products navigation properties.
• Products with Orders navigation property.

Now we want to read only 1 customer from a database, serialize it to JSON and send the result to the client side. What could go wrong? Well, because of the navigation properties the JSON serializer that is used by ASP.NET Web API will read from the database and convert to JSON the whole graph of customers, orders and products! To be more specific, I saw 0.5 MB response which should have a few kilobytes for a very small database (it contained small dozens of records in all tables)! I can bet that in the case of a production database a response would have hundreds of megabytes.

Interview Questions for Programmers by MK #6

Home

Question #6
What is the arithmetic overflow and how is it handled in .NET?

Answer #6
It is a situation when the result of an arithmetic operation exceeds (is outside of) the range of a given numeric type. For example the maximum value for byte type in .NET is 255. So in the following example, an operation a+b will cause an overflow:

byte a = 255;
byte b = 20;
byte c = a + b;

The final result depends on the used numeric types:

• For integer types either OverflowException will be thrown or the result will be trimmed/cropped (the default behaviour). It depends on the compiler configuration and usage of checked / unchecked keywords.
• For floating point types OverflowException will never be thrown. Instead the overflow will lead either to the positive or the negative infinity.
• For decimal type OverflowException will be always thrown.

var b = byte.MaxValue;
//The result will be zero because:
//b = 255 = 1111 1111 
//b++ = 256 = 1 0000 0000
//The result has 9 bits so the result will be trimmed to 8 bits what gives 0000 0000
b++; 
         
checked
{
 b = byte.MaxValue;
 //Exception will be thrown 
 b++; 
}

var f = float.MaxValue;
//The result will be float.PositiveInfinity
f *= 2;  

decimal d = decimal.MaxValue;
//Exception will be thrown
d++; 

TransactionScope and multi-threading

Home

It's my third post about TransactionScope. This time I'll write about using it with multi-threading. Let's start with the following code:

using (var t = new TransactionScope())
{
   var t1 = Task.Factory.StartNew(UpdateDatabase);
   var t2 = Task.Factory.StartNew(UpdateDatabase);
   Task.WaitAll(t1, t2);
   t.Complete();
}

private static void UpdateDatabase()
{
   using (var c = new SqlConnection(connectionString))
   {
      c.Open();

      WriteDebugInfo();

      new SqlCommand(updateCommand, c).ExecuteNonQuery();
   }
}

private static void WriteDebugInfo()
{
   Console.WriteLine("Thread= {0}, LocalIdentifier = {1}, DistributedIdentifier = {2}",
      Thread.CurrentThread.ManagedThreadId,
      Transaction.Current?.TransactionInformation.LocalIdentifier,
      Transaction.Current?.TransactionInformation.DistributedIdentifier);
}

It seems simple but it doesn't work. The problem is that a connection that is created in UpdateDatabase method will not participate in any transaction. We can also observe that WriteDebugInfo will write empty transaction identifiers to the console. It happens because in order to read an ambient transaction (the transaction the code is executed in) TransactionScope uses Transaction.Current property which is thread static (i.e. specific for a thread).

To overcome this issue we have two possibilities. The first one is to use DependentTransacion. However, I'll not show how to do it because since .NET 4.5.1 there is a better way - TransactionScopeAsyncFlowOption enum. Let's try.

using (var t = new TransactionScope(TransactionScopeAsyncFlowOption.Enabled))
{
   ...
}

Unfortunately, there is a big chance that this time we will get TransactionException with the message The operation is not valid for the state of the transaction. in the line with ExecuteNonQuery. The simplified stack trace is:

at System.Transactions.TransactionStatePSPEOperation.get_Status(InternalTransaction tx)
at System.Transactions.TransactionInformation.get_Status()
...
at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()
at Sandbox.Program.UpdateDatabase(Object o)

I read a lot about this but nobody was able to explain why it happens. I also looked into the source code of TransactionStatePSPEOperation class. It was instructive because I learned what is PSPE - Promotable Single Phase Enlistment. However, it also didn't give me an exact answer.

So, I played a little bit with the code and I noticed that the problem occurs when:

• One thread tries to run ExecuteNonQuery.
• Another thread waits for the opening of the connection.

However, when both connections were already opened then the exception wasn't thrown.

At this point it is worth reminding one thing - when there are 2 or more connections opened in a transaction scope at the same time then a transaction is promoted to a distributed one. I'm not 100% sure but I think that the problem occurs because it is not allowed to use a connection which participates in a transaction which is in the middle of promotion to the distributed one. So, the solution is to assure that a transaction will be distributed from the beginning. Here is a fixed code with a magic line (I found it here):

using (var t = new TransactionScope())
{
   //The magic line that makes a transaction distributed
   TransactionInterop.GetTransmitterPropagationToken(Transaction.Current);

   var t1 = Task.Factory.StartNew(UpdateDatabase);
   var t2 = Task.Factory.StartNew(UpdateDatabase);
   Task.WaitAll(t1, t2);
   t.Complete();
}

Nonetheless, the more I think about this the more convinced I'm that using TransactionScope with multi-threading is asking for problems.

How not to use TransactionScope. Another WTF!

Home

This time I will write again about TransactionScope. It is a very useful class and seems to be extremely easy in use. In majority of cases it is true. However, there are also some pitfalls lurking for developers. Especially for these who don't like to waste time for reading MSDN documentation if not really needed i.e. probably vast majority of us ;)

Some time ago, I was analysing more or less the following code:

using(var t = new TransactionScope())
{
   var c = ConnectionProvider.ProvideConnection();
   //Use a connection to update a database
   //...
   t.Complete();
}

ConnectionProvider is a class that hides details of managing connections to a database. There was also a bug in the code responsible for updating a database which caused exceptions. I fixed it and I run tests again. This time an exception was not thrown but something was wrong because a database contained unexpected data. It looked like the transaction was not rollbacked!

Firstly, I though that it is some kind of magic. However, as usual in this kind of cases it wasn't. I digged into ConnectionProvider and I found out that this class was performing some kind of pooling and a connection wasn't opening every time. It was a big problem because connections opened outside a transaction scope do not participate in a transaction. The solution of this problem is to explicitly enlist a connection in an existing transaction scope with the EnlistTransaction method.

It is also worth highlighting that the described problem won't occur if ConnectionProvider doesn't try to implement polling on its own. In general we don't have to do it because .NET do it for us. The problem will also not occur if using statement is used to close a connection returned by a provider.

TransactionScope + Ninject + a small mistake = WTF

Home

Sometimes one stupid mistake can cost a lot of time. A few days ago my application (AngularJS + ASP.NET Web API) started crashing because of the following error:

MSDTC on server 'XXX' is unavailable

It was strange. I wasn't aware of any distributed transactions in my application. To be honest, I was using TransacionScope but I was sure that there was no reason to promote a lightweight transaction into a distributed one. To make things more strange the error wasn't reported every time. When I tried to update data for the first time everything was ok. However, the second attempt (and every next) was failing.

It took me some time to examine all recent changes but finally I found a problem. It was quite tricky so I decided to write about it. Let's start with the fact that I use Ninject as a dependency injection container. Among others Ninject allow us to control a lifetime of objects (instances). Particularly, in the case of web applications, we can use:

• InRequestScope method - it tells Ninject that one object of a particular type should be created for each individual request.
• InSingletonScope method - it tells Ninject that one object of a particular type should be created for all requests.

For example:

kernel.Bind(x => x
   .FromAssembliesMatching("test.dll")
   .SelectAllClasses().InheritedFrom(typeof(IInterfaace))
   .BindAllInterfaces()
   .Configure(z => z.InSingletonScope()));

The problem was that accidentally I mixed InSingletonScope and InRequestScope. For example, let's assume that each request requires objects of two classes A and B. Objects of type A are within the request scope and objects of type B are within the singleton scope.

Both objects perform updates/inserts/deletes and are used inside TransacionScope. For the first request it is not a problem. Both objects are initialized within the same request and use the same database connection. It means that a lightweight transaction is used.

However, for the second (and every next) request an object of type B is re-used whereas a new object of type A is created. Object of type B was initiated in the previous request and it uses a different connection than the one used by an object of type A. It means that a distributed transaction will be used in this case.

To sum up:

• DI containers give a great power but with the power comes great responsibility.
• Be careful when using objects of a different scope together. Especially when these objects require data access.
• Be careful when using multiple connections inside TransacionScope. In the case of MSSQL 2005 in this situation a distributed transaction will be always used. In the case of MSSQL 2008 or newer it is possible to use more than one connection inside TransacionScope without automatic promotion. However, if and only if these connections are not opened at the same time.
• TransactionScope automatically escalating to MSDTC on some machines? is a great source of knowledge about TransacionScope and about the process of promoting lightweight transactions into distributed ones.

Do you know OUTPUT clause?

Home

Today, I'll write about using OUTPUT clause together with INSERT statements. It seems to be that it is not a very well known syntax. However, it is especially useful when use Identity columns to generate keys. Let's start with a simple table:

CREATE TABLE dbo.Main
( 
 Id int Identity (1,1) PRIMARY KEY,
 Code varchar(10),
 UpperCode AS Upper(Code)
);

The old fashioned approach to retrieve a value of Identity column for a new row is to use SCOPE_IDENTITY(). For example:

INSERT INTO dbo.Main (Code) VALUES ('aaa');
SELECT SCOPE_IDENTITY();

With OUTPUT clause it will look in the following way:

DECLARE @InsertedIdentity TABLE(Id int);
INSERT INTO dbo.Main (Code) OUTPUT INSERTED.Id INTO @InsertedIdentity VALUES ('aaa')
SELECT TOP(1) * FROM @InsertedIdentity

You can say wait a minute. If I want to use OUTPUT I have to declare a table variable first and then use SELECT. It is more complex than just using SCOPE_IDENTITY().

Well, the first benefit is that with OUTPUT clause we can read values from many columns, including these that are computed (as it was shown above). However, the real power of OUTPUT clause can be observed if we want to insert many rows into a table:

DECLARE @ToBeInserted TABLE(Code varchar(10), Name varchar(100));

INSERT INTO @ToBeInserted
VALUES ('aaa','1111111111'), ('ddd','2222222222'), ('ccc','3333333333');

DECLARE @Inserted TABLE(Id int, Code varchar(10), UpperCode varchar(10));

INSERT INTO dbo.Main (Code)
OUTPUT INSERTED.Id, INSERTED.Code, INSERTED.UpperCode INTO @Inserted
SELECT Code
FROM @ToBeInserted;

SELECT * FROM @Inserted;

Without OUTPUT we would have to write a nasty loop!

Here is one more example. Let's assume that we have an additional table that references dbo.Main.

CREATE TABLE dbo.Child
( 
 MainId int,
 Name varchar(100),
 CONSTRAINT [FK_Child_Main] FOREIGN KEY(MainId)REFERENCES dbo.Main (Id)
);

We want to insert a few rows into dbo.Main and then related rows to dbo.Child. It is quite easy if we use OUTPUT clause.

INSERT INTO dbo.Child (MainId, Name)
SELECT i.Id, tbi.Name
FROM @ToBeInserted tbi
 JOIN @Inserted i ON i.Code = tbi.Code;

Extremely useful thing that you must know!

At the end it is worth mentioning that OUTPUT clause can be also used together with UPDATE, DELETE or MERGE statements.

Oracle VM VirtualBox and Windows 8.1

Home

In my day to day work I use a 64 bit version of Windows 8.1 Pro N. I needed a virtualization software so I decided to use a free Oracle VM Virtual Box. Everything was ok up to the moment when I wanted to install a x64 version of an operating system on a fresh virtual machine. To my surprise VirtualBox reported the following error:

VT-x/AMD-V hardware acceleration has been enabled, but is not operational. Your 64-bit guest will fail to detect a 64-bit CPU and will not be able to boot.

Please ensure that you have enabled VT-x/AMD-V properly in the BIOS of your host computer.

After some time I noticed that VirtualBox stopped showing 64 bit versions in the Version list. Well, it was actually good because I couldn't use a64 bit virtual machines anyway ;) But, I still didn't know why it happened.

I checked BIOS settings and it seemed ok. I searched Internet for the answer but everyone were recommended to verify BIOS configuration what I've already done. I needed a new VM quickly so at that point I installed a x86 version of Windows.

A few days later my colleague Przemek suggested that the problem may be in the conflict between Hyper-V and VirtualBox and that I should disable Hyper-V. It was strange because I've never installed Hyper-V. However, I checked and I discovered that Hyper-V features were enabled on my computer. It seems to me that they are installed by default with the operating system.



The solution was easy. I pressed Win+S and typed Turn windows feature on or off. Then, I cleared a box next to Hyper-V and restarted computer. After that I was able to install a x64 version of a operating system on a virtual machine.

To sum up, if:

• Your host system is a x64 version of Windows 8.1.
• Virtualization is enabled in BIOS.
• You use VirtualBox.

And you cannot install x64 operating system on a virtual machine then try to disable Hyper-V.

A hint how to use TaskCompletionSource<T>

Home

Some time ago I wrote about using TaskCompletionSource<T> class in order to take advantage of async/await keywords. In that post I included the following code:

public async Task<Stream> ProcessFileAsync(string key, string secret, string path)
{
   var client = new DropNetClient(key, secret);
   //...
   var tcs = new TaskCompletionSource<Stream>();
   client.GetFileAsync(path, response => tcs.SetResult(new MemoryStream(response.RawBytes)), tcs.SetException);
   return tcs.Task;
}

Now, Let's assume that we want to provide a possibility to cancel a task returned from ProcessFileAsync method. We can do something like that:

public async Task<Stream> ProcessFileAsync(string key, string secret, string path, CancellationToken ct)
{
   var client = new DropNetClient(key, secret);
   //...
   var tcs = new TaskCompletionSource<Stream>();

   ct.Value.Register(tcs.SetCanceled);

   client.GetFileAsync(path, response => tcs.SetResult(new MemoryStream(response.RawBytes)), tcs.SetException);
   return tcs.Task;
}

I used CancellationToken.Register method in order to register a callback that will be executed when a token is canceled. This callback is responsible for notifying TaskCompletionSource<T> that underlying task should be cancelled.

You may say that it is not enough because this code doesn't inform DropNetClient that an action should be cancelled. You are right. However, according to my knowledge DropNet API doesn't provide such a possibility.

It leads to the situation when a task is cancelled but DropNetClient continues processing and finnaly TaskCompletionSource.SetResult method will be executed. This will cause ObjectDisposedException because this method cannot be executed for a disposed task. What can we do in this case?

The first solution is to check if a task is cancelled before calling SetResult method. However, it can still happen that a task will be cancelled after this check but before calling SetResult method.

My proposition is to use methods from TaskCompletionSource.Try* family. They don't throw exceptions for disposed tasks.

public async Task<Stream> ProcessFileAsync(string key, string secret, string path, CancellationToken ct)
{
   var client = new DropNetClient(key, secret);
   //...
   var tcs = new TaskCompletionSource<Stream>();

   ct.Value.Register(tcs.SetCanceled);

   client.GetFileAsync(path, response => tcs.TrySetResult(new MemoryStream(response.RawBytes)), tcs.TrySetException);
   return tcs.Task;
}

I'm aware that it is not a perfect solution because it actually does not cancel processing. However, without modifying DropNet code it is not possible. It the case of my application it is an acceptable solution but it is not a rule.

Interview Questions for Programmers by MK #5

Home

Question #5
Here you have a very simple implementation of Template method pattern.

public abstract class BaseAlgorithm
{
   protected SomeObject Resource { get; set; }
   //Other resources

   public void Start()
   {
      // Configure
      Resource = new SomeObject();
      //...
      try
      {
         InnerStart();
      }
      finally
      {
         // Clean up
         Resource.Dispose();
         Resource= null;               
         //...
      }
   }

   protected abstract void InnerStart();
}

public class Algorithm1: BaseAlgorithm
{
   protected override void InnerStart()
   {
      //Do something with allocated resources
   }  
}

At some point someone decided to create a new class Algorithm2 derived from BaseAlgorithm. The difference between the new class and the previous one is that Algorithm2 starts an asynchronous operation. A programmer decided to use async/await keywords to handle this scenario. What do you think about this approach? What could possibly go wrong?

public class Algorithm2: BaseAlgorithm
{
   protected async override void InnerStart()
   {
      var task = DoAsyncCalculations();
      await task;

      //Do something with allocated resources
   }

   private Task DoAsyncCalculations()
   {
      //Let's simulate asynchronous operation
      return Task.Factory.StartNew(() => Thread.Sleep(1000));
   }
}

Answer #5
I think that the developer who created Algorithm2 doesn't understand well how async/await keywords work. The main problem is that finally block inside Start method will be executed before DoAsyncCalculations method will end calculations. In other words resources will be disposed in the middle of calculations and this will cause an exception. Sequence of events will be as follows:

• Start method begins.
• SomeObject is created.
• InnerStart method begins.
• InnerStart method starts an asynchronous operation and uses await to suspend its progress.
• This causes that control returns to Start method.
• Start method cleanups resources.
• When the asynchronous operation is finished InnerStart method continues processing. It tries to use resources, that have been already disposed, what leads to an exception.

It is also not recommended to have async void methods (except event handlers). If an async method doesn't return a task it cannot be awaited. It is also easier to handle exceptions if an async method returns a task. For details see also this article.

To fix a problem BaseAlgorithm must be aware of asynchronous nature of calculations. For example InnerStart method can return a task which will be awaited inside try block. However, it also means that synchronous version of InnerStart method in Algorithm1 will have to be changed. It may not be acceptable. Generally, providing asynchronous wrappers for synchronous methods is debatable and should be carefully considered.

In this case, I'll consider to have separated implementations of Template method pattern for synchronous and asynchronous algorithms.

Interview Questions for Programmers by MK #4

Home

Question #4
You have to implement a very fast, time critical, network communication between nodes of a distributed system. You decided to use good old sockets. However, you haven't decided yet whether to use TCP or UDP protocol. Which one would you choose and why?

Answer #4
If a speed is the only one important factor, I'd choose UDP. UDP is faster than TCP because it has a smaller overhead. In comparison to TCP it is a connection-less, unreliable protocol that doesn't provide features like retransmission, acknowledgment or ordering of messages.

However, it also means that usage of UDP might be more difficult and will require additional coding in some cases. For example, if we have to assure that sent messages have been delivered. In this case, I'd certainly use TCP.

Finally, there is one more thing in favor of UDP. It provides broadcasting and multicasting. So, if it is required I'd also use UDP instead of TCP.

What every blogger should do if using someone else's code #3

Home

Recently, I've written a post about integration with Dropbox. This one is also about integration but this time with Skydrive. I spent some time on investigating how to implement this feature in a desktop application. I was even considering to write what I need from the scratch on my own.

Fortunately, it turned out that someone did it for me. I found a nice piece of code prepared by Microsoft. It includes sample applications showing how to do everything step by step. You can download it from github.