Add pessimistic locking extended example

queries.md

@@ -1191,6 +1191,34 @@ Alternatively, you may use the `lockForUpdate` method. A "for update" lock preve
             ->lockForUpdate()
             ->get();
 
+It is recommended, but not obligatory, to wrap pessimistic locks inside a [transaction](/docs/{{version}}/database#database-transactions), especially when you require the data retrieved to not be altered on the database until the entire transaction is finished. If the transaction fails, the locks will be freed and any changes will be rolled back:
+
+    DB::transaction(function () {
+        $sender = DB::table('users')
+            ->lockForUpdate()
+            ->find(1);
+
+        $receiver = DB::table('users')
+            ->lockForUpdate();
+            ->find(2);
+
+        if ($user->balance < 100) {
+            throw new RuntimeException('Balance too low.');
+        }
+
+        DB::table('users')
+            ->where('id', $sender->id)
+            ->update([
+                'balance' => $sender->balance - 100
+            ]);
+
+        DB::table('users')
+            ->where('id', $receiver->id)
+            ->update([
+                'balance' => $receiver->balance + 100
+            ]);
+    });
+
 <a name="debugging"></a>
 ## Debugging