Parallel.ForEach - Add object to Db If not exists - multithreading

Question

Hi guys and girls:) I have a question about parralelism and ms sql in C#
I have method that looks into Db for specific object. If it not exists it will add it to Db. Unfortunately it is done with Parallel.ForEach, so I have had some situation, using thread A and B:
A: look for entity with code 'xxx' - result: Not Exist
B: look for entity with code 'xxx' - result: Not Exist
A: Add entity to Db - result OK
B: Add entity to Db - result: "Violation of UNIQUE KEY constraint (...) The duplicate key value is 'xxx' "
what should I do to avoid that situation ?

Answer 1

For do not have this duplicate error, you want to catch here
try{
//execute you insert in base
}catch(Exception ex){
// If your constraint is not respected, an error is thrown.
console.WriteLine("db error : "+ex.Message);
}
But, it's temporary, It's functionnaly, but it's bad, it's not proper...
For have a proper code, you want to create a spooler:
class Spooler{
public System.Collections.Generic.List<String> RequestList = new System.Collections.Generic.List<String>();
public Spooler(){
// Open you Database
// Start you thread will be verify if a request adding in the collection
SpoolThread = new Thread(new ThreadStart(SpoolerRunner));
SpoolThread.Start();
}
public createRequestDb(String DbRequest){
RequestList.Add(DbRequest);
}
private void SpoolerRunner()
{
while (true)
{
if (RequestList.Count() >= 1){
Foreach (String request in RequestList){
// Here, you want to verify your request, if args already exist
// And add request in Database
}
}
// Verify is request exist in the collection every 30 seconds..
Thread.Sleep(30000);
}
}
}
Why using a spooler?
just because, when you initialize you spooler before call you threads, you want to call spooler in every threah, and, for each request, you adding request in collection, and, the spooler will process one after another... and not in the same time in every different threads...
EDIT:
This spooler is a sample, for insert a string request one by one in your database,
you can create a spooler with a collection for your object in you want, and insert in db if not exist... It's just a sample for a solution, when you have many threads, to have a treatments one after another ^^