Why does JPA modifying query require #Transactional annotation? - spring

Question

Given the code below, why is it that when I call PersonService.updateAccountMembership from a controller I need to have the #Transactional annotation on the #Modifying query (AccountRepo.updateMembership)? Isn't having the #Transactional annotation on the service call that calls the modifying query sufficient (AccountService.updateMembership)?
The code below would break if I remove the #Transactional annotation on the modifying query with the following exception:
javax.persistence.TransactionRequiredException: Executing an update/delete query
Service:
#Service
public class PersonService{
#Autowired
PersonRepo personRepo;
#Autowired
AccountService accountService;
public Person updateAccountMembership(Person person, int membership){
person = this.save(person);
accountService.updateMembership(person, membership);
}
#Transactional
public Person save(Person person){
return personRepo.save(person);
}
}
Account Service:
#Service
public class AccountService {
#Autowired
AccountRepo accountRepo;
#Transactional
public void updateMembership(Person person, int membership){
accountRepo.updateMembership(person, membership);
}
}
Account Repo:
public class AccountRepo extends JpaRepository<Account,Integer> {
#Transactional //WHY IS THIS REQUIRED????????
#Modifying
#Query("update .........")
void updateMembership(#Param("person") Person person, #Param("memb") int membership);
}

Answer 1

Sure
You have two kinds of queries in sql. Read-Queries and Write-Queries.
A database can have multiple clients. What if two clients update the gender of a person at the same time? The last one wins!
This is an example using two methods: void buy() and void pay().
Client A read next empty invoice number 0000001
Client B read next empty invoice number 0000001
Client A change invoice0000001-payer to Max
Client A store invoice 0000001
Client B change invoice0000001-payer to Tom
Client B store invoice 0000001 <<--- crash! Already in use!
Problem: Max buyed it, but Tom payed it.
If you use transactions you can bind step 5 with step 6. If Step 6 failed, Step 5 is rolled back.
Your database across-the-board requires transations. You can not modify data without.