SQL Server 2012 : ISOLATION LEVELS (part 1) - Serializable

Isolation levels determine how much transactions can see into other transactions, and can range from not-at-all to plenty. Understanding what the isolation levels do so that you can see how they prevent the concurrency side-effects described earlier can help you find an appropriate compromise between locking down too much and providing the necessary protection for your environment.

Many people misunderstand isolation levels. You may have seen large amounts of database code out there that use the NOLOCK hint, for example. To help combat this, you could find ways to educate people about isolation levels. Kendra Little has drawn a poster about them, shown in Figure 1. (You can find all her posters at www.littlekendra.com/sqlserverposters.) It could hang on a wall and serve as a conversation piece — people will ask you about it, providing an opportunity to talk to them about isolation levels.

FIGURE 1

Kendra’s poster highlights the pessimism/optimism balance between the various isolation levels. There are four pessimistic isolation levels, and two optimistic ones. The optimistic levels involve the creation of snapshot data to allow additional concurrency, rather than the pessimistic behavior of blocking.

The following sections first describe the pessimistic isolation levels, followed by the optimistic ones.

Serializable

The serializable isolation level is the most pessimistic isolation level in SQL Server. It simply locks everything up to ensure that no side-effects can take place. It does this by taking out range locks, which appear in the larger of the two lock compatibility matrices. These locks ensure that the whole range of any data that has been queried during the transaction is preserved, including avoiding insertions, to avoid the problem of phantom reads. These range locks typically conflict with each other, much more so than intent locks, thereby keeping the isolation as its utmost level.

Range locks can be seen in the following code (code file Ch6IsolationLevels.sql) and in Figure 2:

FIGURE 2

USE AdventureWorks2012;
GO
 
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
 
BEGIN TRANSACTION; 
 
SELECT  BusinessEntityID
FROM   Person.Person
where BusinessEntityID < 10;
 
SELECT   resource_type
        ,resource_subtype 
        ,resource_description
        ,resource_associated_entity_id
        ,request_mode
        ,request_status
FROM    sys.dm_tran_locks
WHERE   request_session_id= @@SPID;
 
ROLLBACK TRAN;