Recent, I was involved in assisting a deadlock issue and specifically the exception handling part in an application. Together with a colleague from Visual Studio/Languages team Daniel Sauer, we built a sample application to showcase a deadlock and to demonstrate how developers can handle deadlocks in their code.
Of course the best option is to never encounter deadlocks in the first place. Please refer to Minimizing Deadlocks article in Books Online (Docs).
This blog post is to demonstrate in practice what is described in the Handling Deadlocks article.
So here goes a C# Console app:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;
using System.Data.SqlClient;
using System.Threading;
namespace DeadlockSimulator
{
class Program
{
static string sqlConnectionString = "Server=(local);Database=tempdb;Trusted_Connection=True;Application Name=DeadlockSimulator";
public static int attemps1 = 0;
public static int attemps2 = 0;
static void Main(string[] args)
{
//create a couple of tables and insert some rows
CreateObjects();
Thread t1 = new Thread(new ThreadStart(Routine1));
Thread t2 = new Thread(new ThreadStart(Routine2));
t1.Start();
t2.Start();
t1.Join();
t2.Join();
//drop the objects at the end
DropObjects();
Console.WriteLine("Press Enter to quit...");
Console.Read();
}
static void CreateObjects()
{
string cmdStr = @"use tempdb
if OBJECT_ID('dl_tab1') is not null
drop table dl_tab1;
create table dl_tab1 (col1 int);
if OBJECT_ID('dl_tab2') is not null
drop table dl_tab2;
create table dl_tab2 (col1 int);
insert into dl_tab1 values (10);
insert into dl_tab2 values (101);";
using (SqlConnection connection = new SqlConnection(sqlConnectionString))
{
SqlCommand command = new SqlCommand(cmdStr, connection);
connection.Open();
command.ExecuteNonQuery();
}
Console.WriteLine("CreateObjects completed");
}
static void DropObjects()
{
string cmdStr = @"use tempdb
if OBJECT_ID('dl_tab1') is not null
drop table dl_tab1;
if OBJECT_ID('dl_tab2') is not null
drop table dl_tab2;";
using (SqlConnection connection = new SqlConnection(sqlConnectionString))
{
SqlCommand command = new SqlCommand(cmdStr, connection);
connection.Open();
command.ExecuteNonQuery();
}
Console.WriteLine("DropObjects completed");
}
static void Routine1()
{
int retries = 3;
while (!DoRoutine1())
{
attemps1++;
if (attemps1 >= retries)
{
break;
}
}
}
public static bool DoRoutine1()
{
bool retval = false;
Console.WriteLine("Entered Routine1(). Please be patient there is a 10 sec forced delay");
using (SqlConnection connection = new SqlConnection(sqlConnectionString))
{
SqlTransaction transaction;
SqlCommand command = connection.CreateCommand();
connection.Open();
transaction = connection.BeginTransaction("UpdateTran_Routine1");
command.Connection = connection;
command.Transaction = transaction;
try
{
//update one of the tables
command.CommandText = "update dl_tab1 set col1 = 987";
command.ExecuteNonQuery();
//sleep some so we create a conditions
Thread.Sleep(10000);
//update the other table
command.CommandText = "update dl_tab2 set col1 = 123";
command.ExecuteNonQuery();
transaction.Commit();
Console.WriteLine("Routine1: Both updates were written to database.");
retval = true;
}
catch (SqlException ex)
{
if (ex.Number == 1205)
{
Console.WriteLine("Routine1: Commit Exception Type: {0}", ex.GetType());
Console.WriteLine(" Message: {0}", ex.Message);
// Attempt to roll back the transaction.
try
{
transaction.Rollback();
}
catch (Exception ex2)
{
// This catch block will handle any errors that may have occurred
// on the server that would cause the rollback to fail, such as
// a closed connection.
Console.WriteLine("Rollback Exception Type: {0}", ex2.GetType());
Console.WriteLine(" Message: {0}", ex2.Message);
}
}
}
catch (Exception ex2)//any other .net exception
{
throw;
}
}
Console.WriteLine("Completed Routine1()");
return retval;
}
static void Routine2()
{
int retries = 3;
while (!DoRoutine2())
{
attemps2++;
if (attemps2 >= retries)
{
break;
}
}
}
public static bool DoRoutine2()
{
bool retval = false;
Console.WriteLine("Entered Routine2(). Please be patient there is a 10 sec forced delay");
using (SqlConnection connection = new SqlConnection(sqlConnectionString))
{
SqlTransaction transaction;
SqlCommand command = connection.CreateCommand();
connection.Open();
transaction = connection.BeginTransaction("UpdateTran_Routine2");
command.Connection = connection;
command.Transaction = transaction;
try
{
//update one of the tables
command.CommandText = "update dl_tab2 set col1 = 123";
command.ExecuteNonQuery();
//sleep some so we create a conditions
Thread.Sleep(10000);
//update the other table
command.CommandText = "update dl_tab1 set col1 = 987";
command.ExecuteNonQuery();
transaction.Commit();
Console.WriteLine("Routine2: Both updates were written to database.");
retval = true;
}
catch (SqlException ex)
{
if (ex.Number == 1205)
{
Console.WriteLine("Routine2: Commit Exception Type: {0}", ex.GetType());
Console.WriteLine(" Message: {0}", ex.Message);
// Attempt to roll back the transaction.
try
{
transaction.Rollback();
}
catch (Exception ex2)
{
// This catch block will handle any errors that may have occurred
// on the server that would cause the rollback to fail, such as
// a closed connection.
Console.WriteLine("Rollback Exception Type: {0}", ex2.GetType());
Console.WriteLine(" Message: {0}", ex2.Message);
}
}
}
catch (Exception ex2)//any other .net exception
{
throw;
}
}
Console.WriteLine("Completed Routine2");
return retval;
}
}
}