Specifying Locking Hints in a SQL Server Database

[ Team LiB ]Recipe 6.14 Specifying Locking Hints in a SQL Server Database Problem You need to pessimistically lock rows in an underlying SQL Server database.. A locking hint can be spe

[ Team LiB ]

Recipe 6.14 Specifying Locking Hints in a SQL Server Database

Problem

You need to pessimistically lock rows in an underlying SQL Server database

Solution

Use SQL Server locking hints from ADO.NET

The sample code contains three event handlers:

Start Tran Button.Click

Creates a SQL SELECT statement to retrieve the Orders table from the Northwind database A locking hint, either UPDLOCK or HOLDLOCK, is added to the statement as specified A Connection is opened and a Transaction started on it with an isolation level of ReadCommitted A DataAdapter is used on the

transacted connection to fill a DataTable A CommandBuilder is created to

generate updating logic The default view of the table is bound to the data grid on the form

Cancel Button.Click

Clears the data grid, rolls back the transaction, and closes the connection

Form.Closing

Rolls back the transaction if it exists and closes the connection

The C# code is shown in Example 6-39

Example 6-39 File: UsingLockingHintsForPessimisticLockingForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Data;

using System.Data.SqlClient;

private SqlConnection conn;

private SqlTransaction tran;

//

private void startButton_Click(object sender, System.EventArgs e) {

startButton.Enabled = false;

String sqlText = "SELECT * FROM Orders WITH ";

// Add pessimistic locking as specified by user

if(updLockRadioButton.Checked)

sqlText += "(UPDLOCK)";

else if(holdLockRadioButton.Checked)

sqlText += "(HOLDLOCK)";

// Create connection

conn = new SqlConnection(

ConfigurationSettings.AppSettings["Sql_ConnectString"]); conn.Open( );

// Start the transaction

tran = conn.BeginTransaction(IsolationLevel.ReadCommitted); // Create the command

SqlCommand cmd = new SqlCommand(sqlText, conn, tran); // Create the DataAdapter and CommandBuilder

SqlDataAdapter da = new SqlDataAdapter(cmd);

SqlCommandBuilder cb = new SqlCommandBuilder(da);

// Fill table using the DataAdapter

DataTable dt = new DataTable( );

da.Fill(dt);

// Bind the default view of the table to the grid

dataGrid.DataSource = dt.DefaultView;

cancelButton.Enabled = true;

dataGrid.ReadOnly = false;

}

private void cancelButton_Click(object sender, System.EventArgs e) {

cancelButton.Enabled = false;

// Unbind the table from the grid

dataGrid.DataSource = null;

// Roll back the transaction and close the connection

tran.Rollback( );

conn.Close( );

startButton.Enabled = true;

}

Discussion

A lock is an object indicating that a user has a dependency on a resource Locks ensure transactional integrity and database consistency by preventing other users from changing data being read by a user and preventing users from reading data being changed by a user Locks are acquired and released by user actions; they are managed internally by database software

A locking hint can be specified with SELECT, INSERT, DELETE, and UPDATE

statements to instruct SQL Server as to the type of lock to use You can use locking hints when you need control over locks acquired on objects The SQL Server Optimizer

automatically determines correct locking; hints should be used only when necessary Locking hints override the current transaction isolation level for the session

A locking hint is specified following the FROM clause using a WITH clause The hint is specified within parentheses and multiple hints are separated by commas

Tables Table 6-21, Table 6-22, and Table 6-23 describe the different locking hints that you can use, categorized according to their function

Table 6-21 SQL Server locking hints for isolation level

HOLDLOCK

Hold a shared lock until the transaction is completed instead

of releasing it as soon as the required object—table, row, or data page—is no longer needed

NOLOCK Do not issue shared locks and do not recognize exclusive

locks Applies only to the SELECT statement

READCOMMITTED Use the same locking as a transaction with an isolation level

of READ COMMITTED

READUNCOMMITTED Same as NOLOCK

REPEATABLEREAD Use the same locking as a transaction with an isolation level

of REPEATABLE READ

SERIALIZABLE Use the same locking as a transaction with an isolation level

of SERIALIZABLE

Table 6-22 SQL Server locking hints for granularity Locking

NOLOCK Do not issue shared locks and do not recognize exclusive locks Applies

only to the SELECT statement

PAGLOCK Use page locks where a single table lock would normally be used

ROWLOCK Use row-level locking instead of page-level and table-level locking TABLOCK Use table-level locking instead of row-level and page-level locking By

default, the lock is held until the end of the statement

TABLOCKX

Use an exclusive table lock preventing other users from reading or updating the table By default, the lock is held until the end of the statement

Table 6-23 SQL Server Locking Hints for Other Functions Locking

READPAST

Skip locked rows that would ordinarily appear in the result set rather than blocking the transaction by waiting for other transactions to release locks

on those rows Applies only to transactions with an isolation level of

READ COMMITTED Applies only to the SELECT statement

UPDLOCK

Use update locks instead of shared locks when reading a table This allows you to read data and later update it with a guarantee that it has not changed since you last read it while other users are not blocked from reading the data Cannot be used with NOLOCK or XLOCK

XLOCK

Use an exclusive lock that is held until the end of the transaction on all data processed by the statement Can be specified with either PAGLOCK

or TABLOCK granularity Cannot be used with either NOLOCK or UPDLOCK

There are a number ways to get information about database locks:

• The system stored procedure sp_lock returns a result set containing all active

locks

• The syslockinfo table in the master database contains information about all

granted, converting, and waiting lock requests It is a denormalized view of the

data structures used internally by the lock manager

• The SQL Server Profiler can be used to monitor and record locking information

• The Windows Performance Monitor has a SQL Server Locks Object counter that can be used to monitor lock activity

For more information about database locks, using locking hints, or monitoring database locks, see Microsoft SQL Server Books Online

[ Team LiB ]