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Module 8
Designing a Transaction and
Concurrency Strategy
Module Overview
• Guidelines for Defining Transactions
• Defining Isolation Levels
• Guidelines for Designing a Resilient Transaction Strategy
Lesson 1: Guidelines for Defining Transactions
• Guidelines for Creating Transactions
• Considerations for Using Nested Transactions
• Demonstration: How to Create a Nested Transaction
Guidelines for Creating Transactions

Perform a transaction in:
• Explicit Mode
• Autocommit Mode
• Implicit Mode

Manage transactions across multiple platforms

Handle the errors in a transaction by using TRY…CATCH

Use transaction savepoints
Considerations for Using Nested Transactions

Committing inner transactions is ignored by the SQL Server Database
Engine

Each call to COMMIT TRANSACTION or COMMIT WORK applies to the last
executed BEGIN TRANSACTION

The Transaction_name should not refer to the inner transactions of a set of
named nested transactions

The @@TRANCOUNT function records the current
transaction nesting level
Demonstration: How To Create a Nested Transaction
In this demonstration, you will see how to:
Create a nested transaction
Lesson 2: Defining Isolation Levels
• Isolation Levels in the Database Engine
• Comparing Read Committed Isolation with
Snapshot Isolation
• Selecting an Isolation Level for a Database
• Considerations for Offloading Concurrent Activities
Isolation Levels in the Database Engine
READ UNCOMMITTED: Can read data that has been modified by other transactions but
not committed
READ COMMITTED: Cannot read data that has been modified but not committed by
other transactions
REPEATABLE READ: No other transactions can modify data that has been read by the
current transaction until the current transaction completes
SNAPSHOT: Data read by any statement in a transaction will be the transactionally
consistent version of the data that existed at the start of the transaction
SERIALIZABLE: Other transactions cannot insert new rows with key values that would
fall in the range of keys read by any statements in the current transaction until it is
complete
Comparing Read Committed Isolation with
Snapshot Isolation
When to Use Read Committed Isolation
When to Use Snapshot Isolation
Use read committed isolation when:
Use snapshot isolation when:
•
• Optimistic concurrency control is
•
Reader/writer blocking occurs to
the point that concurrency
benefits outweigh increased
overhead of creating and
managing row versions
An application requires absolute
accuracy for long-running
aggregations or queries where
data values must be consistent
to the point in time that a query
starts
desired
• Probability is low that a
transaction would have to be
rolled back because of an
update conflict
• An application needs to
generate reports based on longrunning, multi-statement queries
that must have point-in-time
consistency
Selecting an Isolation Level for a Database
Determine the minimum isolation level
Use alternatives to restrictive isolation levels,
such as:
• Redesigning Transact-SQL logic and tables
• Redesigning how applications access data
• Overriding the default locking strategy
Use row versioning isolation levels
Database Isolation Level
Set the Connection Reset property to True in
the connections string, for connection pools
Considerations for Offloading Concurrent Activities
Offloading concurrent activities helps:
• Avoid contention and blocking problems
• Improve system performance
Methods for Offloading Concurrent Activities

Scale out the
system

Create a data
warehouse

Reschedule
activities
Lesson 3: Guidelines for Designing a Resilient
Transaction Strategy
• Guidelines for Designing an Object Access Strategy
• Guidelines for Transaction Rollback
• Guidelines for Using Optimizer Hints
• Guidelines for Using Locking Hint Functionalities
• Guidelines for Monitoring Transactions
• Guidelines for Troubleshooting Orphaned Transactions
• Demonstration: How to Use Locking Hints
• Discussion: Reducing Deadlock Errors
Guidelines for Designing an Object Access Strategy
1
1
2
2
3
3
Communicate the object access
order and enforce compliance
Design an object access order
based on usage
2
2
1
1
3
Manage conflicts when
accessing objects
3
Discourage ad hoc queries
Guidelines for Transaction Rollback
Minimize errors
Define a rollback strategy
Pre-validate data
Monitor for rollbacks
Define an error-handling strategy
Guidelines for Using Optimizer Hints
The query optimizer enforces a join strategy between two
tables
Join Hints
Query Hints
Query hints enforce query optimizer to execute the select
statement as a whole
Overrides the default behavior of the query
optimizer
Table Hints
Guidelines for Using Locking Hint Functionalities
Locking hint functionalities of SQL Server 2008 include:
• XLOCK at row level
• READPAST
• READCOMMITTEDLOCK
Using locking hints, you can implement the following functionalities:
• Provide finer locking control
• Reduce deadlocking
• Lower the isolation level
Guidelines for Monitoring Transactions
Use the following options to monitor transactions:

sys.dm_tran_database_transactions

DBCC OPENTRAN
Use the following strategies to reduce the impact of large transactions:

Split large transactions into smaller ones

Schedule large transactions to run at off-peak hours
Guidelines for Troubleshooting Orphaned Transactions

Identify the orphaned processes using sp_who

Ignore the orphaned processes or kill them

Check for any improper procedures or general faults

Check whether the Inter-Process Communication (IPC)
session is still active
Demonstration: How To Use Locking Hints
In this demonstration, you will see how to:
Create a locking hint
Discussion: Reducing Deadlock Errors
• What are good practices to prevent deadlock errors?
• How would you use object access strategy to handle deadlock
errors?
Lab 8: Designing a Transaction and
Concurrency Strategy
• Exercise 1: Determining the Database Isolation Level
• Exercise 2: Determining the Order of Object Access
• Exercise 3: Designing Transactions
Logon Information
Virtual machine
NYC-SQL1
User name
Administrator
Password
Pa$$w0rd
Estimated time: 60 minutes
Lab Scenario
You are a lead database designer at QuantamCorp. You are working on the Human
Resources Vacation and Sick Leave Enhancement (HR VASE) project that is
designed to enhance the current HR system of your organization. This system is
based on the QuantamCorp2008 sample database in SQL Server 2008.
The main goals of the HR VASE project are as follows:
• Provide managers with current and historical information about employee vacation
and sick leave.
• Grant view rights to individual employees to view their vacation and sick leave
balances.
• Provide permission to selected employees in the HR department to view and update
the vacation and sick leave details of employees.
• Grant the HR manager with the view and update rights to all the data.
The management team has a requirement to generate performance metric for the
sales staff. For this purpose, you are required to add a new function to the
QuantamCorp HR VASE database that will import information from the CRM
database. The management then will use statistical data to generate the performance
metric report.
Lab Review
• Describe another way to offload the services usage registration
process.
• If you remove the requirement for simultaneous phone number
assignment, what solution would you propose?
Module Review and Takeaways
• Review Questions
• Real-world Issues and Scenarios
• Best Practices