Question

I have some doubts about optimistic concurrency exception.

Well, For example, I retrieve some data from the database, I modify some registers and then submit the changes. If someone update the information of the registers between my request and my update I get an optimistic exception. The classic concurrency problem.

My first doubt is the following. EF to decide if the information is changed or not, retrieves the data from the database, and compare the original data that I obtain with the data that is retrieved from the database. If exists differences, then the optimistic concurrency exception is thrown.

If when I catch the optimistic concurrency exception, I decide if the client wins or the store wins. in this step EF retrieves again the information or use the data from the first retrieved? Because if retrieve again the data, it would be inefficient.

The second doubt is how to control the optimistic concurrency exception. In the catch block of code, I decide if the client wins or the store wins. If the client wins, then I call again saveChanges. But between the time that I decide that the client wins and the savechanges, other user could change the data, so I get again an optimistic concurrency exception. In theory, It could be an infinity loop.

would it be a good idea to use a transaction (scope) to ensure that the client update the information in the database? Other solution could be use a loop to try N times to update the data, if it is not possible, exit and say it to the user.

would the transaction a good idea? does it consume a lot of resources of the database? Although the transaction blocks for a moment the database, it ensures that the operation of update is finished. The loop of N times to try to complete the operation, call the database N times, and perhaps it could need more resources.

Thanks.
Daimroc.

EDIT: I forgot to ask. is it possible set the context to use client wins by default instead to wait to the concurrency exception?

Answer 1

My first doubt is the following. EF to decide if the information is
changed or not, retrieves the data from the database …

It doesn’t retrieve any additional data from database. It takes original values of your entity used for concurrency handling and use them in where condition of update command. The update command is followed by selecting number of modified rows. If the number is 0 it either means that record doesn’t exists or somebody has changed it.

The second doubt is how to control the optimistic concurrency exception.

You simply call Refresh and SaveChanges. You can repeat pattern few times if needed. If you have so much highly concurrent application that multiple threads are fighting to update same record within fraction of seconds you most probably need to architect your data storage in different way.

Would it be a good idea to use a transaction (scope) to ensure that the client update the information in the database?

SaveChanges always uses database transaction. TransactionScope will not add you any additional value unless you want to use transaction over multiple calls to SaveChanges, distributed transaction or change for example isolation level of the transaction.

Is it possible set the context to use client wins by default instead
to wait to the concurrency exception?

It is set by default. Simply don’t mark any of your properties with ConcurrencyMode.Fixed and you will have no concurrency handling = client wins.