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Let's see how to configure Entity Framework to use optimistic data concurrency controls.
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Entities that we want to participate
in our data concurrency control
0:00
need to define a tracking property.
0:04
The tracking property
stores a unique value,
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which is used to determine
the version of an entity.
0:09
For every entity that defines a tracking
property, EF will include its value
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in every update, and delete statement
that's generated for that entity type.
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Later in this video we'll audit an update
statement and see how exactly this works.
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To configure EF to use
optimistic concurrency,
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let's start with adding a tracking
property to the comic book entity class.
0:30
Located in the ComicBooks
shared projects models folder.
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You can name the tracking
column whatever you like.
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But a common name to use is RowVersion.
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Then, we apply the TimeStamp
attribute to this property.
0:55
Which we'll configure EF to use
this property as a tracking column.
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The timestamp attribute gets it's name
from the timestamp database data type,
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that older versions of SQL server
used before it was deprecated and
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replaced with the RowVersion data type.
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You could only apply the timestamp
attribute to one entity property.
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If you apply the timestamp attribute
to more than one property,
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EF will throw an arrow.
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EF enforces this restriction,
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as each database table can only
contain a single RowVersion column.
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The corresponding .NET data type with
a row version database data type is
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a byte array.
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The ComicBooks controllers
edit post action method,
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which handles form posts from
the edit ComicBook page,
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is bound to an instance of the
ComicBooksEditViewModel viewModel class.
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The ComicBooksEditViewModel class inherits
from the ComicBooksBaseViewModel class,
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Which defines a comic
book entity property.
2:04
The comic book entity properties
are populated by MVCs model binding
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using the values posted from the HTML
form on the Edit Comic Book page.
2:12
Now that the ComicBook entity has
a tracking property, we need to update
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the partial view that renders the form
to include the tracking property value.
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If we don't do this,
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the ComicBook entity's tracking
property value would be null.
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When processing a user's form post in
the controllers, edit post action method.
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And no tracking property value would
cause the ComicBook update to fail.
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The tracking property value is a system
concern, meaning that while it's something
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that's required for
our Web app to function properly,
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it's not something that our users either
would care to see or would want to edit.
2:48
Given that, we can render the tracking
property's value to the form using
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the HiddenFor HTML helper method,
which will render a hidden field.
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You can think of the hidden
field as a regular text field,
3:02
that is hidden from the user so
they can't easily edit it's value.
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I say easily because a savvy user
can use the browsers to develop
3:09
a tool to change the value
of the hidden field.
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So it's not a secure way to round trip
sensitive data between the server and
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client.
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That being said, in this specific case
we're not concerned about a user hacking
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the tracking property value.
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So it's a convenient
method that we can use.
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Before we test our changes lets update
the contexts database log property.
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To a lambda that will write
messages to the output window.
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The context is instantiated in the
constructor for the base controller class.
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Which is the base class for
each controller in our web app.
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This will give us a way to audit the SQL
queries in commands that EF generates and
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executes.
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With that last change in place,
let's test our changes.
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EF will detect our model change and
invoke our database initializer,
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which will drop and create the database.
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If it exists, if the database doesn't
exist, then EF will just create it.
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We can use the SQL Server Object Explorer
to verify that the ComicBook table now
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includes a row version column
with a data type of RowVersion.
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And here's our new column.
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Now let's test our data concurrency
control by opening the Edit Comic Book
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page into two tabs for Bone #3.
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Save the first tab.
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Then Save the second tab.
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And we get an exception.
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SystemData.Entity.InfrustructureDbUpdateC-
oncurrencyException.
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Store update, insert, or delete statement,
effected in unexpected number of rows.
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Entities may have been modified or
deleted since entities were loaded.
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In the output window, we can see the SQL
update statement that EF generated and
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executed.
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Notice how EF is including the RowVersion
column in the WHERE clause.
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When EF detects that a tracking
property is present on an entity,
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it'll include its value in every
generated update and delete statement.
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That guarantees that an update or
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delete will only succeed if
the tracking column value matches.
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If an update or
delete doesn't effect any rows,
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meaning that a row wasn't found
that contained the provided ID and
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row version column values, then EF or
throw a DbUpdateConrrencyException.
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The database will update
a rows tracking column value
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every time the row is updated.
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So if some one updates record in between
the time that we retrieve a record and
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attempt an update, all update will fail.
6:14
By not allowing our update EF ensures that
the data stays in a consistent state.
6:17
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