How can I lock a table on read, using Entity Framework?

Question

I have a SQL Server (2012) which I access using Entity Framework (4.1). In the database I have a table called URL into which an independent process feeds new URLs. An entry in the URL table can be in state "New", "In Process" or "Processed".

I need to access the URL table from different computers, check for URL entries with status "New", take the first one and mark it as "In Process".

var newUrl = dbEntity.URLs.FirstOrDefault(url => url.StatusID == (int) URLStatus.New);
if(newUrl != null)
{
    newUrl.StatusID = (int) URLStatus.InProcess;
    dbEntity.SaveChanges();
}
//Process the URL

Since the query and update are not atomic, I can have two different computers read and update the same URL entry in the database.

Is there a way to make the select-then-update sequence atomic to avoid such clashes?

Answer 1

I was only able to really accomplish this by manually issuing a lock statement to a table. This does a complete table lock, so be careful with it! In my case it was useful for creating a queue that I didn't want multiple processes touching at once.

using (Entities entities = new Entities())
using (TransactionScope scope = new TransactionScope())
{
    //Lock the table during this transaction
    entities.Database.ExecuteSqlCommand("SELECT TOP 1 KeyColumn FROM MyTable WITH (TABLOCKX, HOLDLOCK)");

    //Do your work with the locked table here...

    //Complete the scope here to commit, otherwise it will rollback
    //The table lock will be released after we exit the TransactionScope block
    scope.Complete();
}

Update - In Entity Framework 6, especially with async / await code, you need to handle the transactions differently. This was crashing for us after some conversions.

using (Entities entities = new Entities())
using (DbContextTransaction scope = entities.Database.BeginTransaction())
{
    //Lock the table during this transaction
    entities.Database.ExecuteSqlCommand("SELECT TOP 1 KeyColumn FROM MyTable WITH (TABLOCKX, HOLDLOCK)");

    //Do your work with the locked table here...

    //Complete the scope here to commit, otherwise it will rollback
    //The table lock will be released after we exit the TransactionScope block
    scope.Commit();
}