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OPTIMISTIC LOCKING USING TIMESTAMPS
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When more than one user is working against a database, we might expect some concurrency problems to appear. Let s describe the most common problem. User A reads a record. User B also reads the record and then changes it. Any changes user A now decides to post to the database will overwrite the changes that user B made to it.
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Two standard solutions for this kind of problem are pessimistic locking and optimistic locking. In the pessimistic locking scenario, user A puts a lock on the record so that nobody can change it until he or she is finished with it. (We will not go into the details of how locks are implemented. Locks are covered in detail in other SQL Server books. At this time, it is important to know that it is possible to mark a record so that no,body else can change it.) When user A changes the record, he or she also releases the lock and user B can now access the record and change it. The trouble with this solution is that user A might go out for lunch or on vacation and, if he or she didn t close the application calling SQL Server, the lock will not be removed from the record. This scenario is one of the reasons why this kind of solution is not recommended in a client/server environment. In the optimistic locking scenario, user A locks the record only while he or she is actually performing the change. A mechanism in SQL Server notifies a user if somebody has changed the record in the meantime. The user can then decide either to abandon changes or to overwrite the record. A simple way to find out if somebody has changed the record since user A read it would be to compare all fields. To run this comparison, user A must keep both an original and a changed record, and he or she needs to send them both to the server. Then a process on the server must compare the original record with the record in the table to make sure that it wasn t changed. Only then can the record be updated. This process is obviously slow and it increases network traffic, but there are solutions in the industry that use precisely this method.
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SQL Server has a timestamp datatype. It is used for versioning records in a table. When you insert or update a record in a table with a timestamp field, SQL Server timestamps the change. Figure 10-2 demonstrates such behavior. The table is created with a timestamp field. When a record is inserted, SQL Server automatically sets its value. When the record is updated, SQL Server increases the value of the timestamp.
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Figure 10-2.
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Use of the timestamp datatype
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It is important to realize that timestamp values are not actually a kind of timekeeping. They are just binary values that are increased with every change in the database, and therefore they are unique within the database. You should not make any assumptions about their values and growth. Somebody (or some process) might change something in the database concurrently, and even two changes that you executed consecutively might not have consecutive timestamp values. To make sure that nobody has changed a record in the meantime, you might decide to update it like this:
update #aTable set description = 'test3' where id = 1 and ts = 0x000000000000007A -- not a perfect solution
The record will be updated only if the timestamp is unchanged. The trouble with this solution is that you will not know what
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happens after the statement is executed. Maybe everything is okay and the record has been successfully changed. It is possible that the record was not updated, because the timestamp was changed, but it is also possible that the record is not in the table any more.
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