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[[persistenceBasics-locking]]
== Locking

=== Optimistic Locking

GORM enables optimistic locking by default via a `version` column. See xref:ormdsl-optimisticLockingAndVersioning[Optimistic Locking and Versioning] for full details.

=== Pessimistic Locking

To acquire a database-level lock on a row, use `lock()`:

[source,groovy]
----
Book.withTransaction {
    def book = Book.lock(1)         // <1>
    book.title = 'Updated Safely'
    book.save()
}                                   // <2>
----
<1> Issues `SELECT ... FOR UPDATE`, preventing concurrent reads/writes to this row.
<2> Lock is released when the transaction commits.

You can also lock an already-loaded instance:

[source,groovy]
----
def book = Book.get(1)
book.lock()                         // upgrades to a pessimistic lock
----

=== Refresh

To reload the current state from the database (discarding in-memory changes):

[source,groovy]
----
def book = Book.get(1)
// ... another thread modifies the book ...
book.refresh()                      // re-reads from the database
----
