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At the beginning of this chapter, I described two concurrency models that SQL Server can use. Pessimistic concurrency uses locking to guarantee the appropriate transactional behavior and avoid problems such as dirty reads, according to the isolation level you are using. Optimistic concurrency uses a new technology called row versioning to guarantee your transactions. Starting in SQL Server 2005, optimistic concurrency is available after you enable one or both of the database properties called READ_COMMITTED_SNAPSHOT and ALLOW_SNAPSHOT_ ISOLATION. Exclusive locks can be acquired when you use optimistic concurrency, so you still need to be aware of all issues related to lock modes, lock resources, and lock duration, as well as the resources required to keep track of and manage locks. The difference between optimistic and pessimistic concurrency is that with optimistic concurrency, writers and readers do not block each other. Or, using locking terminology, a process requesting an exclusive lock does not block when the requested resource currently has a shared lock. Conversely, a process requesting a shared lock does not block when the requested resource currently has an exclusive lock. It is possible to avoid blocking because as soon as one of the new database options is enabled, SQL Server starts using tempdb to store copies (versions) of all rows that have changed, and it keeps those copies as long as there are any transactions that might need to access them. The space in tempdb used to store previous versions of changed rows is called the version store.
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In earlier versions of SQL Server, the tradeoff in concurrency solutions is that we can avoid having writers block readers if we are willing to risk inconsistent data that is, if we use Read Committed isolation. If our results must always be based on committed data, we need to be willing to wait for changes to be committed. SQL Server 2005 introduced a new isolation level called Snapshot isolation and a new nonblocking avor of Read Committed isolation called Read Committed Snapshot Isolation (RCSI). These row versioning based isolation levels allow a reader to get to a previously committed value of the row without blocking, so concurrency is increased in the system. For this to work, SQL Server must keep old versions of a row when it is updated or deleted. If multiple updates are made to the same row, multiple older versions of the row might need to be maintained. Because of this, row versioning is sometimes called multiversion concurrency control. To support storing multiple older versions of rows, additional disk space is used from the tempdb database. The disk space for the version store must be monitored and managed appropriately, and I point out some of the ways you can do that later in this section. Versioning works by making any transaction that changes data keep the old versions of the data around so that a snapshot of the database (or a part of the database) can be constructed from these old versions.
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When a row in a table or index is updated, the new row is stamped with the transaction sequence number (XSN) of the transaction that is doing the update. The XSN is a monotonically increasing number that is unique within each SQL Server database. The concept of XSN is not the same as Log Sequence Numbers (LSNs), which I discussed in 4, Logging and Recovery. I discuss XSNs in more detail later. When updating a row, the previous version is stored in the version store, and the new row contains a pointer to the old row in the version store. Old rows in the version store might contain pointers to even older versions. All the old versions of a particular row are chained in a linked list, and SQL Server might need to follow several pointers in a list to reach the right version. Version rows must be kept in the version store only as long as there are operations that might require them. In Figure 10-7, the current version of the row is generated by transaction T3, and it is stored in the normal data page. The previous versions of the row, generated by transaction T2 and transaction Tx, are stored in pages in the version store (in tempdb). Row versioning gives SQL Server an optimistic concurrency model to work with when an application requires it or when the concurrency reduction of using the default pessimistic model is unacceptable. Before you switch to the row versioning based isolation levels, you must carefully consider the tradeoffs of using this new concurrency model. In addition to requiring extra management to monitor the increased use of tempdb for the version store, versioning slows the performance of update operations due to the extra work involved in maintaining old versions. Update operations bear this cost, even if there are no current readers of the data. If there are readers using row versioning, they have the extra cost of traversing the link pointers to nd the appropriate version of the requested row.
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