Re: Concurrency exceptions with only one user? [modified] - Visual Basic Discussion Boards

Not sure if I can help but you might want to google a bit.

"Real programmers just throw a bunch of 1s and 0s at the computer to see what sticks" - Pete O'Hanlon

hi all,

i have a System DSN connection where i have to create an exe to check a particular table in the database with some status.

can any body let me know.

thanks in advance

M.MohanVamsi

What have you tried ? Do you use google to look for information ?

Christian Graus - Microsoft MVP - C++

"also I don't think "TranslateOneToTwoBillion OneHundredAndFortySevenMillion FourHundredAndEightyThreeThousand SixHundredAndFortySeven()" is a very good choice for a function name" - SpacixOne ( offering help to someone who really needed it ) ( spaces added for the benefit of people running at < 1280x1024 )

i tried to look into google but i'm not getting proper stuff.

i already have a system dsn.

just i want to connect with visual basic and run some query on the tables

can you please let me know how to connect.

M.MohanVamsi

Do you have a connection string ?

Christian Graus - Microsoft MVP - C++

"also I don't think "TranslateOneToTwoBillion OneHundredAndFortySevenMillion FourHundredAndEightyThreeThousand SixHundredAndFortySeven()" is a very good choice for a function name" - SpacixOne ( offering help to someone who really needed it ) ( spaces added for the benefit of people running at < 1280x1024 )

I suggest that you follow this tutorial through from the beginning.
The end of the tutorial introduces databases:
Clicketty

You always pass failure on the way to success.

Hello. I designed a scheduling application for my office in Visual Basic 2005 Express. It connects intermittently to an MS Access mdb database (on our office share drive) to populate datasets and make changes and updates to tables in the database. Occasionally it is used my multiple users at once but most of the time it is used by one user at a time. What baffles me is that I get concurrency exceptions even with one user. (As I had previously understood it - obviously incorrectly - concurrency errors were caused by two users trying to change the same row of data at the same time.) I'd really like to get to the root of what's causing these errors. It's not enough for me to Try - Catch these errors and display a messagebox; I'd like to understand what's causing the errors and handle them more intelligently.

In general, what could be causing single user concurrency errors? How can I effectively search for the cause of the errors? (It seems that they occur at very random times.) Any advice from someone who knows more about concurrency than me (or where could I learn more)? Thanks, all.

Regards,

Craig

"That rug really tied the room together."

modified on Thursday, December 20, 2007 12:05:22 PM

craigmg78 wrote:
As I had previously understood it - obviously incorrectly - concurrency errors were caused by two users trying to change the same row of data at the same time.)

That's true - you got that wrong. A concurrency failure occurs when an application goes to update a row in a database and the data in that row has changed since the application read the row OR the application made a mistake in identifying the row to be updated. For instance, when your application gets a row from the database, it sees something like:

ID  FirstName  LastName   PhoneNumber
 1   Joe        Schmoe      555-1212

Say you are trying to change the phone number. You make the change in the application and the app is trying to write the new data back to database. Well, id order to identify the record, the code is using an SQL query that looks something like this:

UPDATE ContactsTable
SET FirstName = 'Joe', LastName = 'Schmoe', PhoneNumber = '123-4567'
WHERE ID = 1 AND FirstName = 'Joe' AND LastName = 'Schmoe' AND PhoneNumber = '555-1212'

Notice that the WHERE clause in the SQL statement mentions all the fields in the table and gives the values of the fields as they are supposed to be now, when the application read this record. This is how the code matches up the record it intends to update with the record in the table. If any of these fields do not match, the record is not found, and hence cannot be updated. The "number of rows affected" by this SQL statement should return 1 if the change is successfully written.

If the "number of rows affected" comes back 0, or greater than 1, this throws the concurrency exception in your code. Now, what are the possible causes of "rows affected" returning a non-1 value?

1) The record in the database has been changed by an outside source. This is either another user updated the same record, the database changed the record in response to some other application, or some problem with the database.

2) The data in the WHERE clause of the SQL UPDATE statement is not correct. What the application read before is not what the app is now submitting in the WHERE clause to uniquely identify this record.

For example, your application read the record in the example above (1, Joe, Schmoe, 555-1212), but, for some reason, when it build the SQL statement to update the record with new data, the data it used in the WHERE clause doesn't match what it read. It might be doing something like:

UPDATE ContactsTable
SET FirstName = 'Joe', LastName = 'Schmoe', PhoneNumber = '123-4567'
WHERE ID = 4 AND FirstName = 'Frank' AND LastName = 'Schmoe' AND PhoneNumber = '555-1212'

Notice the ID and FirstName fields in the WHERE clause no longer match what was originally read. This too can cause a concurrency violation, without another user using the database.

A guide to posting questions on CodeProject[^]

Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007

modified on Thursday, December 20, 2007 2:01:13 PM

That has given me some real food for thought. Now I just have to hunt for cases in my code where I have done something like you mentioned! Thank you for your detailed response.

- Craig

"That rug really tied the room together."

hi,
i use windows application I want validation to create texbox write arabic only

Ahmed hassan

Hi,

I think if you will set the Font property of that particular textbox to Arabic font, then whatever text is entered in the textbox will be in Arabic only and you wont need to validate it.

Regards,

Ujjaval Modi Smile | :)

Manpower moves wrenches, horsepower moves cars, and the power of the mind moves the world.

please send to me code

Ahmed hassan

Please don't ask twice. Also, there is no code. DId you understand the answer ? I was wondering myself, if a computer is set up to be arabic, what other characters would you expect ? Or do you just mean to reject numbers and symbols ?

Christian Graus - Microsoft MVP - C++

"also I don't think "TranslateOneToTwoBillion OneHundredAndFortySevenMillion FourHundredAndEightyThreeThousand SixHundredAndFortySeven()" is a very good choice for a function name" - SpacixOne ( offering help to someone who really needed it ) ( spaces added for the benefit of people running at < 1280x1024 )

You already asked before. Don't keep asking the same thing over and over. It is rude.

"Real programmers just throw a bunch of 1s and 0s at the computer to see what sticks" - Pete O'Hanlon

Hi,

As the title says. I need to directly connect two computers or devices that have my software using IP addreses. And to share files between the two computers.

Example.

I select from a list IP address that I previously obtained, I connect to that IP adress and upload or download some files from it.

How to handle such a transfer?

Any ideas?

Thank you!

One machine must be running some kind of server to handle it's end of the file transfer and the other must be running a client that talks to that server.

There are a ton of different ways to do this, but to get any kind of an answer you're going to have to be more specific about what you're doing. Are you transferring files as part of a peer-to-peer chat application? Is this a central server that's going to hold these files? Can you use a normal FTP server as part of your app? Do you know anything about writing app's that use sockets?

A guide to posting questions on CodeProject[^]

Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007

Good afternoon All

i have an Application in an Multiuser evironment, i have a Concurency Problem. i would like someone to Explain to me how to handle Concurency using thread, i would like to lock the Update Function when one of users is updating the records and unlock it when the process is done . here is an example of my code

Public Function SaveProperties_Lis(ByVal dsDAta As PropDAL.Dssearchlis) As PropDAL.Dssearchlis

Dim ws As PropDAL.Properties 
Try

ws = New PropDAL.Properties 
'check the Business Rules 

Me.CheckRules_Lis(dsDAta) 
'Updating the dataset 

dsDAta.EndInit()

ws.UpdateLis_search(dsDAta)

Catch ex As Exception 
MsgBox(ex.Message)

End Try

Return dsDAta 
End Function

.

i have this Function in BLL(Business Logic Layer). i want to lock it , when another user is using it.
The Bll is running on the Client machine and its a DLL that is deployed with the Application. i need to trap the Concurency situation in the BLL. The code above is retreaving the data from the DAL(data Access Layer), so the BLL will create an object of a DAL and use the DAL functions, and the Client application will call the BLL for service by creating an object of BLL and use the methods of BLL that will use the methods of DAL, i have not prevented Concurency to a database,and thats what i want to do. i want to know how to achieve this in a multi user Environment.

Thanks

Vuyiswa Maseko,

Sorrow is Better than Laughter, it may Sadden your Face, but It sharpens your Understanding

VB.NET/SQL7/2000/2005
http://vuyiswamb.007ihost.com
http://Ecadre.007ihost.com
vuyiswam@tshwane.gov.za

Theading has nothing to do with concurrency issues in the database. This question is not answerable, as you've posted it.

Concurrency is a situation that your app must handle according to your business and data rules. Noone can tell you how to write an "all encompassing solution" to a problem you haven't explained yet.

Locking a table is NOT a good option since noone will be able to read that table until it's unlocked. This too will genereate it's own set of problems for your app. If you wanted to kill your SQL server's performance from your applications point of view, this is the way to do it.

From your description, it looks like you don't have a firm grasp of what data concurrency is, what situations cause concurrency violations, nor do you understand what your options are in handling various concurrency issues and what the implications are for each solution. I highly suggest reading up on data concurrency before you start throwing locks around that you really don't need.

Think about this situation. Say one copy of your app places a lock on the table, then crashes. When does the lock get removed??

You can read up more on Data Access and Concurrency issues in this[^], this[^], and these[^].

A guide to posting questions on CodeProject[^]

Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007

Hi!
I've the following problem:
I've two bitmap variables, and I want to check that they contain the same image or not.
If I try to use the "=" operator, I get an error message.
I wrote a function what examines every pixel (by setpixel and getpixel methods), but it makes the program very slow.
Isn't there a faster way?

Thanks in advance.

Because VB sucks, no. But, if you did this in C# and called it from a dll, you could use direct pixel access, like I do in my image processing articles, which would be faster.

Christian Graus - Microsoft MVP - C++

"also I don't think "TranslateOneToTwoBillion OneHundredAndFortySevenMillion FourHundredAndEightyThreeThousand SixHundredAndFortySeven()" is a very good choice for a function name" - SpacixOne ( offering help to someone who really needed it ) ( spaces added for the benefit of people running at < 1280x1024 )

Thanks

Yes I had the same problem with get/setpixel. Here is what you do, create a file stream for each file then compare the file streams.

If you need an example just let me know (don't worry it's real simple). I don't have the exact commands for it memorized but I have code for something similar on my laptop and I can give it to you later if you want me to.

I'll be very grateful if you send me the code.
Much thanks

A few notes. This will compare 2 files of ANY format not just bitmaps. path1 and path2 are your file paths. This will only work if the files are the same format(if you compare a 32 bit and 24 bit bitmap it will always return false). Also I have not tested this but I think it is correct but if you get an error or problem just let me know. I don't think there is much overhead with filestreams but if it isn't real fast then try using the read command at the bottom to read it into arrays then compare the arrays instead of the filestreams. If you need to compare different formats or if you are looking for a way to get or set pixels then I know how to do that but it is a little more involved than this since you have to match up pixels and so forth and it requires some knowledge of the bitmap format.

    Public Function CompareFiles(ByVal path1 As String, ByVal path2 As String) As Boolean<br />
        Dim n As Integer, issame As Boolean = True, fstream1 As IO.FileStream, fstream2 As IO.FileStream<br />
<br />
        'create streams<br />
        fstream1 = New IO.FileStream(path1, IO.FileMode.Open, IO.FileAccess.Read)<br />
        fstream2 = New IO.FileStream(path2, IO.FileMode.Open, IO.FileAccess.Read)<br />
<br />
        'compares the streams<br />
        If fstream1.Length = fstream2.Length Then<br />
            For n = 0 To fstream1.Length<br />
                If fstream1.ReadByte <> fstream2.ReadByte Then<br />
                    issame = False<br />
                    Exit For<br />
                End If<br />
            Next<br />
        Else<br />
            issame = False<br />
        End If<br />
<br />
        fstream1.Close()<br />
        fstream2.Close()<br />
<br />
        Return issame<br />
    End Function

The file stream is basically like an array of bytes. You can manipulate any file with file streams and if you want to copy data to or from an array use these commands.

fstream1.Read(yourarray, youroffset, yourcount)<br />
fstream1.Write(yourarray, youroffset, yourcount)

Thanks