By [Rick Anderson](https://twitter.com/RickAndMSFT), [Tom Dykstra](https://github.com/tdykstra), and [Jon P Smith](https://twitter.com/thereformedprog)
This tutorial shows how to handle conflicts when multiple users update an entity concurrently (at the same time). If you run into problems you can't solve, [download or view the completed app.](https://github.com/aspnet/Docs/tree/master/aspnetcore/data/ef-rp/intro/samples) [Download instructions](xref:index#how-to-download-a-sample).
* The last update wins. That is, the last update values are saved to the DB.
* The first of the current updates are lost.
### Optimistic concurrency
Optimistic concurrency allows concurrency conflicts to happen, and then reacts appropriately when they do. For example, Jane visits the Department edit page and changes the budget for the English department from $350,000.00 to $0.00.
Before Jane clicks **Save**, John visits the same page and changes the Start Date field from 9/1/2007 to 9/1/2013.
Jane clicks **Save** first and sees her change when the browser displays the Index page.
John clicks **Save** on an Edit page that still shows a budget of $350,000.00. What happens next is determined by how you handle concurrency conflicts.
Optimistic concurrency includes the following options:
* You can keep track of which property a user has modified and update only the corresponding columns in the DB.
In the scenario, no data would be lost. Different properties were updated by the two users. The next time someone browses the English department, they will see both Jane's and John's changes. This method of updating can reduce the number of conflicts that could result in data loss. This approach:
* Can't avoid data loss if competing changes are made to the same property.
* Is generally not practical in a web app. It requires maintaining significant state in order to keep track of all fetched values and new values. Maintaining large amounts of state can affect app performance.
* Can increase app complexity compared to concurrency detection on an entity.
The next time someone browses the English department, they will see 9/1/2013 and the fetched $350,000.00 value. This approach is called a *Client Wins* or *Last in Wins* scenario. (All values from the client take precedence over what's in the data store.) If you don't do any coding for concurrency handling, Client Wins happens automatically.
This is called a *Store Wins* scenario. (The data-store values take precedence over the values submitted by the client.) You implement the Store Wins scenario in this tutorial. This method ensures that no changes are overwritten without a user being alerted.
* EF Core verifies that property has not been modified after it was fetched. The check occurs when [SaveChanges](/dotnet/api/microsoft.entityframeworkcore.dbcontext.savechanges?view=efcore-2.0#Microsoft_EntityFrameworkCore_DbContext_SaveChanges) or [SaveChangesAsync](/dotnet/api/microsoft.entityframeworkcore.dbcontext.savechangesasync?view=efcore-2.0#Microsoft_EntityFrameworkCore_DbContext_SaveChangesAsync_System_Threading_CancellationToken_) is called.
* If the property has been changed after it was fetched, a [DbUpdateConcurrencyException](/dotnet/api/microsoft.entityframeworkcore.dbupdateconcurrencyexception?view=efcore-2.0) is thrown.
Concurrency conflicts can be detected at the property level with the [ConcurrencyCheck](/dotnet/api/system.componentmodel.dataannotations.concurrencycheckattribute?view=netcore-2.0) attribute. The attribute can be applied to multiple properties on the model. For more information, see [Data Annotations-ConcurrencyCheck](/ef/core/modeling/concurrency#data-annotations).
* Is SQL Server specific. Other databases may not provide a similar feature.
* Is used to determine that an entity has not been changed since it was fetched from the DB.
The DB generates a sequential `rowversion` number that's incremented each time the row is updated. In an `Update` or `Delete` command, the `Where` clause includes the fetched value of `rowversion`. If the row being updated has changed:
*`rowversion` doesn't match the fetched value.
* The `Update` or `Delete` commands don't find a row because the `Where` clause includes the fetched `rowversion`.
* A `DbUpdateConcurrencyException` is thrown.
In EF Core, when no rows have been updated by an `Update` or `Delete` command, a concurrency exception is thrown.
### Add a tracking property to the Department entity
In *Models/Department.cs*, add a tracking property named RowVersion:
The [Timestamp](/dotnet/api/system.componentmodel.dataannotations.timestampattribute) attribute specifies that this column is included in the `Where` clause of `Update` and `Delete` commands. The attribute is called `Timestamp` because previous versions of SQL Server used a SQL `timestamp` data type before the SQL `rowversion` type replaced it.
The preceding highlighted code shows the `WHERE` clause containing `RowVersion`. If the DB `RowVersion` doesn't equal the `RowVersion` parameter (`@p2`), no rows are updated.
The following highlighted code shows the T-SQL that verifies exactly one row was updated:
[@@ROWCOUNT](/sql/t-sql/functions/rowcount-transact-sql) returns the number of rows affected by the last statement. In no rows are updated, EF Core throws a `DbUpdateConcurrencyException`.
The scaffolding engine created a `RowVersion` column for the Index page, but that field shouldn't be displayed. In this tutorial, the last byte of the `RowVersion` is displayed to help understand concurrency. The last byte isn't guaranteed to be unique. A real app wouldn't display `RowVersion` or the last byte of `RowVersion`.
To detect a concurrency issue, the [OriginalValue](/dotnet/api/microsoft.entityframeworkcore.changetracking.propertyentry.originalvalue?view=efcore-2.0#Microsoft_EntityFrameworkCore_ChangeTracking_PropertyEntry_OriginalValue) is updated with the `rowVersion` value from the entity it was fetched. EF Core generates a SQL UPDATE command with a WHERE clause containing the original `RowVersion` value. If no rows are affected by the UPDATE command (no rows have the original `RowVersion` value), a `DbUpdateConcurrencyException` exception is thrown.
In the preceding code, `Department.RowVersion` is the value when the entity was fetched. `OriginalValue` is the value in the DB when `FirstOrDefaultAsync` was called in this method.
The following code gets the client values (the values posted to this method) and the DB values:
The following highlighted code sets the `RowVersion` value to the new value retrieved from the DB. The next time the user clicks **Save**, only concurrency errors that happen since the last display of the Edit page will be caught.
The `ModelState.Remove` statement is required because `ModelState` has the old `RowVersion` value. In the Razor Page, the `ModelState` value for a field takes precedence over the model property values when both are present.
## Update the Edit page
Update *Pages/Departments/Edit.cshtml* with the following markup:
The browser shows the Index page with the changed value and updated rowVersion indicator. Note the updated rowVersion indicator, it's displayed on the second postback in the other tab.
This browser window didn't intend to change the Name field. Copy and paste the current value (Languages) into the Name field. Tab out. Client-side validation removes the error message.
The Delete page detects concurrency conflicts when the entity has changed after it was fetched. `Department.RowVersion` is the row version when the entity was fetched. When EF Core creates the SQL DELETE command, it includes a WHERE clause with `RowVersion`. If the SQL DELETE command results in zero rows affected:
* The `RowVersion` in the SQL DELETE command doesn't match `RowVersion` in the DB.
* A DbUpdateConcurrencyException exception is thrown.
*`OnGetAsync` is called with the `concurrencyError`.
### Update the Delete page
Update *Pages/Departments/Delete.cshtml* with the following code:
The browser shows the Index page with the changed value and updated rowVersion indicator. Note the updated rowVersion indicator, it's displayed on the second postback in the other tab.
Delete the test department from the second tab. A concurrency error is display with the current values from the DB. Clicking **Delete** deletes the entity, unless `RowVersion` has been updated.department has been deleted.
