DB的长期连接还是按需连接？

Question

I was assigned to implement an application (in C++) to evaluate pending submissions (a submission is a programming algorithm to a given problem). A site (in ASP.NET MVC) posts problems and allows the users to submit their answers, then marks the submissions as "pending to evaluation" on the database (SQL Server 2008R2) and that is when my work begins:

I'll have 3 (or maybe more) instances of my application running as services. Each instance has to check if any pending submissions exists in the DB every 2 seconds. If it exists I retrieve and compile it, after successful compilation I execute it and finally, after execution, check the correctness of the answer. Then I update that submission setting the results and deleting it from the pending table. I need to specify in the DB the current status of the pending submission (compiling, running, judging).

The time to evaluate a submition is ~(1-3)s and the same instance never evaluates more that one submission at the same time.

My problem is: How to connect to the DB server? I have 3 possibles solutions and I need to know what should be better (in order to increase efficiency) and why: 1 - Establish a connection to the DB once I instantiate the application and never close it (close it when I delete the instance or shut down the server, that theoretically never will happen.) 2 - Open a connection each 2s in order to get the pending submission (if any one exists) wait for the full evaluation process to end, sets the evaluations results and then close the connection. 3 - Same as 2, but closing the connection when I retrieve the submission, when the compilation finish, open it again and update pending submission's status, close it, when the execution finish, open it again and update pending submission's status, close it, finally when the judging finish open it and set the evaluation result.

Answer 1

You don't say what database access library you are using (ODBC, ado.net, other?). Opening and closing database connections is a relatively expensive operation. You should be using some sort of connection pooling scheme in your db access framework. A pool of connections is opened for a period of time, and when your app opens a connection it will get handed an already open connection from a pool. That will make it more efficient. Go read about connection pooling for SQL Server