Question

I am using transactionscope to ensure that data is being read to the database correctly. However, I may have a need to select some data (from another page) while the transaction is running. Would it be possible to do this? I’m very noob when it comes to databases.

I am using LinqToSQL and SQL Server 2005(dev)/2008(prod).

Answer 1

Yes, it is possible to still select data from a database while a transaction is running.

Data not affected by your transaction (for instance, rows in a table which are being not updated) can usually be read from other transactions. (In certain situations SQL Server will introduce a table lock that stops reads on all rows in the table but they are unusual and most often a symptom of something else going on in your query or on the server).

You need to look into Transaction Isolation Levels since these control exactly how this behaviour will work.

Here is the C# code to set the isolation level of a transaction scope.

TransactionOptions option = new TransactionOptions();        
options.IsolationLevel = System.Transactions.IsolationLevel.ReadCommitted;        
using (TransactionScope sc = new TransactionScope(TransactionScopeOption.Required, options)
{
    // Code within transaction
}

In general, depending on the transaction isolation level specified on a transaction (or any table hints like NOLOCK) you get different levels of data locking that protect the rest of your application from activity tied up in your transaction. With a transaction isolation level of READUNCOMMITTED for example, you can see the writes within that transaction as they occur. This allows for dirty reads but also prevents (most) locks on data.

The other end of the scale is an isolation level like SERIALIZABLE which ensures that your transaction activity is entirely isolated until it has comitted.