How to properly use Locking or Transactions to prevent duplicates using Spring Data -

what best way check if record exists , if doesn't, create (avoiding duplicates)?

keep in mind distributed application running across many application servers.

i'm trying avoid these:

• race conditions

• toctou

a simple example:

person.java

@entity public class person {     @id    @generatedvalue    private long id;     private string firstname;     private string lastname;     //getters , setters omitted  } 

personrepository.java

public interface personrepository extends crudrepository<person, long>{     public person findbyfirstname(string firstname);  } 

some method

public void somemethod() {         person john = new person();         john.setfirstname("john");         john.setlastname("doe");         if(personrepo.findbyfirstname(john.getfirstname()) == null){             personrepo.save(john);         }else{             //don't save person         }    } 

clearly code stands, there chance person inserted in database in between time checked if exists , when insert myself. duplicate created.

how should avoid this?

based on initial research, perhaps combination of

• @transactional

• @lock

but exact configuration i'm unsure of. guidance appreciated. reiterate, application distributed across multiple servers must still work in highly-available, distributed environment.

for inserts: if want prevent same recordsto persisted, may want take precoutions on db side. in example, if firstname should unique, define unique index on column, or agroup of colunsd should unique, , let db handle check, insert & exception if you're inserting record that's inserted.

for updates: use @version (javax.persistence.version) annotation this:

@version private long version; 

define version column in tables, hibernate or other orm automatically populate value & verison clause when entity updated. if try update old entity, prevent this. careful, doesn't throw exception, return update count 0, may want check this.