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By default (as guided by the server s machineconfig file), the data-aware services use SQL providers that work against a SQL Express database (by default aspnetdbmdf in the App_Data folder) The relevant part of the machineconfig containing the default connection strings is as follows: <connectionStrings> <clear /> <add name="LocalSqlServer" connectionString="data source=\SQLEXPRESS;Integrated Security=SSPI; AttachDBFilename=|DataDirectory|aspnetdbmdf;User Instance=true" providerName="SystemDataSqlClient" /> </connectionStrings> This defines a LocalSqlServer connection string that all data-centric providers use by default, which points to the aspnetdbmdf SQL Express database in the web data directory (Note how the database server is set to be running as a SQL Express named instance) This database file is automatically created upon first use or by triggering the website administration console.

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Update t set y=10 where y=5;

Update t Set x=x+1 Where y=5;

It is important to remember that these default settings will work only with SQL Express, but on a typical development machine with a standard SQL Express installation, everything works like a charm out of the box Even if you are using SQL Express and all the default providers, you should (re)define your connection string settings in your site webconfig file, mostly because it is more portable that way and because this simply becomes necessary if you use a different database server, say SQL Server At the time of writing, SQL Server does not allow attaching database files to the server using the AttachDBFilename property, so you will need to use a regular database and refer to it using the Initial Catalog or the Database keywords in your connection string.

Using consistent reads, this will find the record session 1 modified, but it won t be able to update it since session 1 has it locked. Session 2 will block and wait for this row. This releases session 2; session 2 becomes unblocked. It can finally do the current read on the block containing this row, where Y was equal to 5 when session 1 began its update. The current read will show that Y is now equal to 10, not 5 anymore.

Commit;

If for some reason you cannot automatically generate the default database when using SQL Server, you can simply build it by hand by first creating an empty database (for instance, by right-clicking Server Explorer/Data Connections and selecting Create New SQL Server Database) and running aspnet_regsqlexe (found in your NET installation folder) in wizard mode with the information on your newly created database This tool generates all the data tables and stored procedures needed by the default providers You can then change your webconfig file to include a reference to this new database: <connectionStrings> <remove name="LocalSqlServer" /> <add name="LocalSqlServer" connectionString="Data Source=localhost;Integrated Security=SSPI; Initial Catalog=YourDatabase" providerName="SystemDataSqlClient" /> </connectionStrings> Here we used localhost for SQL Server and the YourDatabase database within You may need to change the host when you deploy your application to point to the right database server.

So the record that was Y=5 when you began the UPDATE is no longer Y=5 The consistent read component of the UPDATE says, You want to update this record because Y was 5 when we began, but the current version of the block makes you think, Oh, no, I can t update this row because Y isn t 5 anymore It would be wrong If we just skipped this record at this point and ignored it, then we would have a nondeterministic update It would be throwing data consistency and integrity out the window The outcome of the update (how many and which rows were modified) would depend on the order in which rows got hit in the table and what other activity just happened to be going on.

You could take the same exact set of rows and in two different databases, each one running the transactions in exactly the same mix, you could observe different results, just because the rows were in different places on the disk In this case, Oracle will choose to restart the update When the row that was Y=5 when you started is found to contain the value Y=10, Oracle will silently roll back your update (just the update, not any other part of the transaction) and restart it, assuming you are using READ COMMITTED isolation If you are using SERIALIZABLE isolation, then at this point you would receive an ORA-08177: can't serialize access for this transaction error In READ COMMITTED mode, after the transaction rolls back your update, the database will restart the update (ie.