Paul Lou's blog

TF401189: The source branch has been modified since the last merge attempt

Paul Lou — Sat, 20 Feb 2016 22:51:00 GMT

If you are using Visual Studio Online and setup your project using TFS doing a code review is straightforward in visual studio.

However ff you are using Visual Studio Online and setup your project Git you might wonder how you can conduct a code review. You may do a quick search and then try ‘new pull request’ . You might get a weird error message you don’t understand.

TF401189: The source branch has been modified since the last merge attempt

Then you do another search (bing or google) you won’t find anything useful.

After trial and error I found that the root cause is that I didn’t use a topic branch. It is that simple.

https://paullou.com/2016/02/20/tf401189-the-source-branch-has-been-modified-since-the-last-merge-attempt/

Debugging a weird process crash issue

Paul Lou — Tue, 22 Dec 2015 15:18:31 GMT

I recently debugged a very interesting process crash issue. The code was running in the cloud so I could not just attach the debugger as easily as on my dev box. Besides using debugger should be the last option in my opinion. So I logged on to the machine. Well before that I had to go a through a process to get permission and get the environment ready for security/compliance reason. I observed that the process was recycling by using taskmgr. So I did the below things
1. Checked the log. No exceptions. No errors. No logs related to the crash
2. Checked crash dump files. No files were there
3. Checked OS eventlog. No crash events. Normally for any process crash there is at least one event

I never saw a process crash like this. But I knew the related code which caused this. It was a background task kicked off by calling Task.Run(). I thought that there must be some unhandled exceptions but I checked the code. There was already a catch block which caught all exceptions. However it was still possible that the catch block might throw exception. So I put another catch all inside the catch block to make sure that no unhandled exception was thrown.

However it did not work. The process was still crashing. I asked around about how a process could crash since I was relatively new to the code base. I got some pointers but none of them could match what I observed.

Then I had to turn to the last option. I copied windbg to all the nodes (remember that this is distributed system and there are multiple replicas) and hooked it up with the process. Boom! I got an exception in the debugger and then the process crashed.

I looked at the exception which was a normal .net exception. I could not think of a reason how it could cause crash. Then I looked into the call stack and saw the below line.
000007f9`8538bb27 : 0000001b`9c29cf80 0000001c`2c50c880 0000001c`2c50c888 0000001c`2c50c890 : mscorlib_ni!System.Environment.Exit(Int32)+0x7b

That was really weird. Please check my other blog to see the end result.

https://paullou.com/2015/12/22/debugging-a-weird-process-crash-issue/

StackOverflow Exception, Long time no see

Paul Lou — Fri, 30 Oct 2015 15:24:37 GMT

I have not seen a stackoverflow exception for a long time. Today I happened to bump into one in random test code. I will explain how it happened. What the code is used for and why it was written this way are not interesting to me and are not in the scope of this post.

public class ChildClass : BaseClass
{
    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            base.Dispose();
            //DO something
        }
    }
}
 
public class BaseClass
{
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }
 
    protected virtual void Dispose(bool disposing)
    {
        if (disposing)
        {
            //DO something
        }
    }
}

Persistent MAC Address in Cloud (Microsoft Azure /Amazon AWS / Google GCP / OpenStack)

Paul Lou — Wed, 28 Oct 2015 13:27:53 GMT

What is a NIC? It is the network adaptor on your machine. MAC address is the physical address of the adaptor. You can consider it as fixed on your machine for this discussion. However the NIC you get on a VM in cloud is a virtualized one. If you delete a VM and create a new one you will probably get a new MAC address. Why do we need a persistent MAC address? Many licensed software rely on MAC for the licenses. If you want to move licenses from one VM to another persistent MAC address becomes very useful.

It looks like a small feature. Let’s look at a few major cloud provides to see how they deal with this.

Microsoft Azure
NIC https://azure.microsoft.com/en-us/documentation/articles/resource-groups-networking/#nic

It is not available yet. According to the post it is already planned.
https://feedback.azure.com/forums/216843-virtual-machines/suggestions/5217933-static-mac-address

Amazon AWS
It is already supported. The below statement is from the official AWS documentation.
The interface maintains its private IP addresses, Elastic IP addresses, and MAC address.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html

When did AWS start supporting this? I have no idea. From this post it was not supported yet in 3/2011 but it was supported in 2/2012.https://forums.aws.amazon.com/thread.jspa?

Please continue to read at https://paullou.com/2015/10/11/persistent-mac-address-in-cloud-microsoft-azure-amazon-aws-google-gcp-openstack/

Forgot password for your Azure VM?

Paul Lou — Sun, 25 Jan 2015 18:45:26 GMT

Unfortunately Azure Portal doesn’t have this simple feature. The only solution I know is using Azure Powershell Cmdlets. There are different articles talking about this. But they are not concise at all. To me I just need simple steps to reset password for one single VM. So here you go. Just “8 simple steps” you can get your VM back.

Read on https://paullou.com/ for the steps.

Two very useful small tools in Windows

Paul Lou — Fri, 29 Aug 2014 18:36:27 GMT

There are two small tools in Windows which saved my day multiple times. I didn't bother to write a blog post since I thought that it was not worth it but the two tools got me out of a big problem today (again). To appreciate the two tools I decided to write it down briefly.

With any further ado let me introduce them as below. The nice thing is that they are in the box.
Takeown.exe - take ownership of files/directories
Cacls.exe - change ACLs of files/directories

My case today
My system hanged when I did a certain operation which unfortunately I need to do regularly. This was very annoying since I couldn't work effectively. To fix it I need to copy a system file to a system folder. This is not an easy operation. You will get access denied by default. If you don't believe it open the folder c:\windows\winsxs (replace c: with your system drive) try to create a new file/folder or delete anything or copy anything there.

What I did is very simple. I just ran the below command to take control of that folder and copied the file there. Then I could solve the problem.
takeown /f [foldername]
cacls [foldername] /G [username]:F

If you run into any problem related to access denied when doing any file/directory access operation feel free to give these two cute tools a try. It won't disappoint you.

A design pattern for caching – two tier caching with strong consistency between servers

Paul Lou — Thu, 03 Jul 2014 16:50:27 GMT

This is a very interesting and useful pattern in practice. I haven’t seen any books for classic patterns to talk about this. But I think that it should be a classic one for caching. To make it clear I think that it is an old pattern. I do not invent it and I do not discover it. I designed the solution for our problem then I realized that it should be a pattern. I just want to write it down since I didn’t know it is a pattern before designing the solution and the pattern is useful in real world.

Problem

You have a not scalable data source which has relatively stable data (e.g. config or metadata) but the data does change.
You have many servers (front ends and backends) to access that data. Since the data source is not scalable you need caching.
You want strong consistency between the servers. i.e. when one server sees a new version of data all other servers need to see the new version or newer version(s). Simple put it is shared caching.

Please read the full text at https://paullou.com/2014/07/03/a-design-pattern-for-caching-two-tier-caching-with-strong-consistency-between-servers-2/

Unable to retrieve metadata for ''. Ambiguous match found

Paul Lou — Tue, 24 Jun 2014 15:37:00 GMT

Right now I am working with Visual Studio 2013, ASP.NET MVC 5.0 and Entity Framework 6.1.1. One of the things I like about Visual Studio is the ability to generate scaffolded items. e.g. if I want to manage student records I just need to create a c# class named Student and Visual Studio can generate all the CRUD views and controller actions for me. This will save a lot of time and trouble for me.

However things might not always work as expected. I used this code generation feature quite a few times and it worked like a charm. But today it just didn't work. I got the below error.
There was an error running the selected code generator:
‘Unable to retrieve metadata for ‘HomeHome Models. ServiceproviderReview’. Ambiguous match found’

I did some research and tried the below things.
1. Examine all the fields and properties to make sure that they don't have the same names with different casing
2. Comment out most properties and keep only two properties (int:Id and string:Name)
3. Rename the model, re-compile, regenerate the database
4. Uninstall entity framework 6.1.1 and install 6.0.0
5. Close visual studio and start again
6. Clean the solution and rebuild
7. Create another simple model class to just see what's going on

Unfortunately all of them did not work! In the end I was so frustrated after a couple of hours of trying all kind of things. I was almost going to restart my dev machine. Then I decided to do one last attempt by creating a new DbContext. To my surprise it worked! I think that there must be some bug with Visual Studio / Entity Framework. After that I removed the new DbContext & the new connection string in web.config and switched to use the old DbContext. I was happy to see it still worked.

Column '' in table '' is of a type that is invalid for use as a key column in an index

Paul Lou — Sat, 21 Jun 2014 12:36:35 GMT

I decided to change my blogging style a little bit. Instead of writing lengthy blog posts in a more systematic way I want to write things about all the different kinds of issue I got when building stuff and learning new things. I am very experienced in some areas but new on many more areas so the issues I got might be very basic and simple. I think that it is useful to just record it for myself and also for other people since they might get into the same situation.

I just started using entity framework 6.1.1 and tried to define an index on a column called ZipCode as below but I got the below error when doing 'Update-Database' by using code first migrations.

public string ZipCode { get; set; }

Column 'ZipCode' in table 'dbo.ServiceProviders' is of a type that is invalid for use as a key column in an index.

It turned out that it is a simple issue. The max length for an index column is 900 bytes. The fix is as simple as below.

 [MaxLength(20)]
 public string ZipCode { get; set; }

Adding an index on a column in entity framework

Paul Lou — Fri, 20 Jun 2014 22:22:00 GMT

Adding an index column in entity framework was not easy until entity framework 6.1. Today (6/20/2014) the entity frame 6.1.1 got released officially. I just downloaded through nuget package manager in visual studio 2013 and liked the simplicity of doing this job. I am surprised that the EF6.1.1 has been downloaded so many times on the first day. it shows how popular it is.

 [IndexAttribute]
 public string ZipCode { get; set; }