1. Transactions in Entity Framework
SQL
Transactions in Entity Framework
Database
Applications
SoftUni Team
Technical Trainers
Software University
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2. Table of Contents Implicit Transactions in EF Using TransactionScope
Transactional DbContext.SaveChanges()
Optimistic Concurrency Control in EF: Last Wins and First Wins Strategies
Using TransactionScope
Using EF Explicit Transactions

3. Implicit Transactions in Entity Framework

4. Transactions in Entity Framework (EF)
In EF DbContext.SaveChanges() implicitly runs in a transaction
Either all changes are persisted, or none of them
var context = new SoftUniEntities();
var town = new Town { Name = "Developer City " };
context.Towns.Add(town);
var addr = new Address { AddressText = "5, Dev Rd.", Town = town };
context.Addresses.Add(addr);
var ev = new Event { Address = addr, Name = "Party",
Date = new DateTime(2015, 1, 1) };
context.Events.Add(ev);
context.SaveChanges();
Implicit transaction: either town + address + event will be inserted or nothing

5. Transactional EF SaveChanges()
Live Demo

6. Optimistic Concurrency Control in EF
EF runs in optimistic concurrency mode (no locking)
By default the conflict resolution strategy in EF is "last wins"
The last change overwrites all previous concurrent changes
Enabling "first wins" strategy for certain property in EF:
ConcurrencyMode=Fixed (in DB first project)
[ConcurrencyCheck] (in code first projects)

7. Last wins: the second user overwrites the first user's changes
Last Wins – Example
var contextFirst = new SoftUniEntities();
var lastProjectFirstUser = contextFirst.Projects
.OrderByDescending(p => p.ProjectID).First();
lastProjectFirstUser.Name = "Changed by the First User";
// The second user changes the same record
var contextSecondUser = new SoftUniEntities();
var lastProjectSecond = contextSecondUser.Projects
lastProjectSecond.Name = "Changed by the Second User";
// Conflicting changes: last wins
contextFirst.SaveChanges();
contextSecondUser.SaveChanges();
Last wins: the second user overwrites the first user's changes

8. First wins: the second user gets DbUpdateConcurrencyException
First Wins – Example
var contextFirst = new SoftUniEntities();
var lastTownFirstUser = contextFirst
.Towns.OrderByDescending(
t => t.TownID).First();
lastTownFirstUser.Name = "First User";
var contextSecondUser = new SoftUniEntities();
var lastTownSecondUser = contextSecondUser.Towns
.OrderByDescending(t => t.TownID).First();
lastTownSecondUser.Name = "Second User";
contextFirst.SaveChanges();
contextSecondUser.SaveChanges();
First wins: the second user gets DbUpdateConcurrencyException

9. Optimistic Concurrency in EF
Live Demo

10. Managing Transactions in .NET Apps
TransactionScope
Managing Transactions in .NET Apps

11. TransactionScope Class
In .NET environment we can implement implicit transactions with TransactionScope
Each operation in transaction scope joins the existing ambient transaction
Advantages of TransactionScope
Easier to implement, more efficient
Supports distributed transactions (MSDTC)
Disadvantages:
May not be supported in cloud environment

12. Using TransactionScope
When instantiating TransactionScope
You either join the existing (ambient) transaction
Or create a new ambient transaction
Or you don't apply transactional logic at all
The transaction coordinator (MSDTC) determines to which transaction to participate based on
If there is an existing active transaction
The value of the TransactionScopeOption parameter

13. The TransactionScope Class
TransactionScopeOption specifies which transaction to be used (new / existing / none)
TransactionScopeOption
Transaction Already Exists?
Action
Required
(default) No
New transaction is created
Yes
Existing ambient transaction is used
RequiresNew
New transaction is explicitly created
Suppress
No transaction is used

14. TransactionScope – Example
void RootMethod() {
using (TransactionScope tran = new TransactionScope())
/* Perform transactional work here */
SomeMethod();
tran.Complete(); }
void SomeMethod()
using (TransactionScope tran =
new TransactionScope(TransactionScopeOption.Required))
Create a new transaction
Joins the existing transaction

15. Completing a TransactionScope
The method Complete() informs the transaction coordinator that it is acceptable to commit the transaction
If not called, the transaction will be rolled back
Calling it doesn't guarantee that the transaction will be committed
At the end of the using block of the root transaction, it is committed
Only if the root transaction scope and all joined transaction scopes have called Complete()

16. TransactionScope
Live Demo

17. Explicit Transactions in EF
Using Database.BeginTransaction()

18. Explicit Transactions in EF
var context = new SoftUniEntities();
using (var dbContextTransaction = context.Database.BeginTransaction()) {
try
context.Database.ExecuteSqlCommand("UPDATE Employees " +
"SET Salary = Salary * 1.2 WHERE LastName = 'Wilson'");
var empsQuery = context.Employees.Where(
e => e.Town.Projects.Count() >= 5);
foreach (var emp in empsQuery)
emp.JobTitle += "Senior " + emp.JobTitle;
context.SaveChanges();
dbContextTransaction.Commit(); }
catch (Exception)
dbContextTransaction.Rollback();

19. Explicit Transactions in EF
Live Demo

20. Transactions in Entity Framework
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21. License
This course (slides, examples, demos, videos, homework, etc.) is licensed under the "Creative Commons Attribution- NonCommercial-ShareAlike 4.0 International" license
Attribution: this work may contain portions from
"Databases" course by Telerik Academy under CC-BY-NC-SA license
© Software University Foundation –
This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike license.

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