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POASP .NET WebMethods Synchronization
 

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    <p>I have a webmethod in asp.net .asmx service, wich is supposed to check if there are records in a DB, and if there are no records it should add a record</p> <p>the simplified example of code is like this:</p> <pre><code>object Mutex = new object(); [WebMethod] public void InsertIfNotExists(string CLI) { lock (Mutex) { using (SqlConnection conn = new SqlConnection(ConnectionString)) using (SqlDataAdapter adapter = new SqlDataAdapter()) using (DataSet ds = new System.Data.DataSet()){ { //I log with log4net logger.Debug("InsertIfNotExists: Start Function: CLI:" + CLI); int dummy = 0; string sql = "SELECT * CLI Promote where CLI=" + CLI + " "; adapter.SelectCommand = new SqlCommand(sql, conn); adapter.Fill(ds); DataTable t = ds.Tables[0]; logger.Debug("InsertIfNotExists: " + t.Rows.Count + " records found for CLI:" + CLI); if (t.Rows.Count == 0) { logger.Debug("InsertIfNotExists: starting to add to table: CLI:" + CLI); DataRow dr = t.NewRow(); dr["CLI"] = CLI; dr["DateOfSend"] = DateTime.Now; InsertToTable(t, dr, sql); logger.Debug("InsertIfNotExists: added to table: CLI:" + CLI + ", starting re-check"); //checking if exist more then one lines - one more time sql = "SELECT * CLI Promote where CLI=" + CLI + ""; adapter.SelectCommand = new SqlCommand(sql, conn); adapter.Fill(ds); t = ds.Tables[0]; logger.Debug("InsertIfNotExists: re-check for CLI:" + CLI + ", records count=" + t.Rows.Count); } logger.Debug("InsertIfNotExists: Finish Function for CLI:" + CLI); } } } </code></pre> <p>Actually it does more checks and logic, that's why I implement it in .net and not in the SQL statement itself, but essentially that is it.</p> <p>Most of the time the code works well, but sometimes I get into race conditions because of multithreading, though I use lock. </p> <p>Sample output I got today:</p> <pre><code>2013-09-15 11:47:14,145 [21] DEBUG Namespace.Service1 InsertIfNotExists: Start Function: CLI: 0501234567 2013-09-15 11:47:14,145 [13] DEBUG Namespace.Service1 InsertIfNotExists: Start Function: CLI: 0501234567 2013-09-15 11:47:14,148 [21] DEBUG Namespace.Service1 InsertIfNotExists: 0 records found for CLI: 0501234567 2013-09-15 11:47:14,148 [21] DEBUG Namespace.Service1 InsertIfNotExists: starting to add to table: CLI: 0501234567 2013-09-15 11:47:14,148 [13] DEBUG Namespace.Service1 InsertIfNotExists: 0 records found for CLI: 0501234567 2013-09-15 11:47:14,148 [13] DEBUG Namespace.Service1 InsertIfNotExists: starting to add to table: CLI: 0501234567 2013-09-15 11:47:14,149 [21] DEBUG Namespace.Service1 InsertIfNotExists: added to table: CLI: 0501234567, starting re-check 2013-09-15 11:47:14,149 [13] DEBUG Namespace.Service1 InsertIfNotExists: added to table: CLI: 0501234567, starting re-check 2013-09-15 11:47:14,154 [27] DEBUG Namespace.Service1 InsertIfNotExists: Start Function: CLI: 0501234567 2013-09-15 11:47:14,157 [27] DEBUG Namespace.Service1 InsertIfNotExists: 2 records found for CLI: 0501234567 2013-09-15 11:47:14,157 [27] DEBUG Namespace.Service1 InsertIfNotExists: Finish Function for CLI: 0501234567 2013-09-15 11:47:14,183 [13] DEBUG Namespace.Service1 InsertIfNotExists: re-check for CLI: 0501234567, records count=2 2013-09-15 11:47:14,184 [21] DEBUG Namespace.Service1 InsertIfNotExists: re-check for CLI: 0501234567, records count=2 2013-09-15 11:47:14,185 [13] DEBUG Namespace.Service1 InsertIfNotExists: Finish Function for CLI: 0501234567 2013-09-15 11:49:19,626 [21] DEBUG Namespace.Service1 InsertIfNotExists: Start Function: CLI:0507654321 </code></pre> <p>What we see here that 3 threads attempted to insert CLI 0501234567 to the table in the parallel. Threads 21 and 13 entered the race conditons and each one inserted 1 record. Then thread 27 also tried to insert a record, but found existing records and exited.</p> <p>Why did they do it when locked on mutex?</p> <p>Note: Thread 21 never gets to Finish - I think it is caused by an exception in thread 21, because I try to remove the "additional" rows after the re-check in the real function, and then the second thread which tries to do it should get into an exception. I know it's ugly but that's the only solution I got for now, I'd like to know how to do it properly.</p> <p>Why does asp.net behave in such a way and what is the proper way to accomplish that task without the race conditions?</p>
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