Databases Reference
In-Depth Information
When you run this code, you might notice that the second user (represented by
context2
)
submits the changes last and the changes made by the first user are lost.
To prevent data loss, database applications have traditionally relied on record locking.
When a user starts editing a particular record, the application would lock it, preventing
others from modifying it at the same time. This approach works better when the number
of users is small and the number of records is large, thus reducing the chance of
contention. However, physical locking can prevent other users not only from changing
but also from reading the locked records. This can lead to significant performance degra-
dation and is generally considered a bad practice.
The Entity Framework has built-in support for an alternative strategy, called
Optimistic
Concurrency Control
. When saving modified entities, the
ObjectContext
generates
UPDATE
statements with a
WHERE
clause that ensures that changes are made only if the entity has
not changed since the last time it was retrieved from the database. In other words, the
framework assumes that chances of contention are low and, optimistically, allows you to
modify the entity without locking the record. If somebody else modified the same record
in the meantime, you simply get an exception when calling the
SaveChanges
method.
To take advantage of the optimistic concurrency support, first you need to modify the
definitions of the properties in the Entity Designer to set their Concurrency Mode values
to
Fixed
, as shown in Figure 2.8. You need to do this for all properties you want the Entity
Framework to check for modifications before saving the entity.
FIGURE 2.8
Setting Property Concurrency Mode.
When the model has been modified, calling the
SaveChanges
for
context1
in the previous
code example generates the following SQL statement:
exec sp_executesql N'update [dbo].[Products]
set [UnitsInStock] = @0
where
(([ProductID] = @1) and ([UnitsInStock] = @2))
',N'@0 smallint,@1 int,@2 smallint',@0=50,
@1=1,@2=20
