TAILIEUCHUNG - The Real MTCS SQL Server 2008 Exam 70/432 Prep Kit- P119

The Real MTCS SQL Server 2008 Exam 70/432 Prep Kit- P119: Congratulations on your journey to become certified in SQL Server 2008. This book will help prepare you for your exam and give you a practical view of working with SQL Server 2008. | 572 Chapter 13 Performance Tuning other already has locked a vicious circle occurs where neither can complete without the other first releasing its locks. Locking Locking is responsible for ensuring database users don t interfere with each other and is an essential component of any Relational Database Management system. Locking provides concurrency control within the database engine. A transaction is one or more actions treated as a unit of work. There are a set of database properties that help understand transactions and the importance of locking Atomic An atomic transaction must either complete or fail. No aspect of the transaction must be allowed to succeed or fail without all changes succeeding or failing. Should one modification fail the whole transaction should be rolled back. Consistent Data must be consistent when a transaction completes. Data integrity must not be compromised by a transaction. Isolated User transactions should be isolated from each other. Data affected by one transaction can be accessed by other transactions only in the state it was before the transaction began or after the transaction completes. Durable Once a transaction has completed the changes must persist even if there is a crash or system failure. SQL Server uses careful transaction management to enforce Atomicity and Consistency. The Transaction Log ensures transaction durability since all changes are first written to the transaction log. Finally Isolation is provided by locking within the database engine. Blocking We should consider locking to be completely normal activity within the database engine. When there are lots of users working on a SQL Server database or in a situation with a poor index strategy or badly written queries there can be many thousands of locks. If a transaction holds many locks or holds locks for a long time eventually this can cause blocking. Blocking occurs when a transaction is waiting for a lock on a resource usually because a lock is already held by .

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