Record locking is when you inform Salesforce you want to lock a record for a certain period of time. Your process becomes the only one that can access that record. Any other processes that attempt to access it at the same time receive an error.
Note: The “period of time” usually ends when the current transaction ends. Transactions have different ways of ending (which is another topic entirely).
Record locking is useful when you may have multiple processes accessing the same record and you need to ensure exclusivity. Think of seat reservations for a movie or for an airplane.
To lock a record, you can use apex. We won’t get into the all the technical details, but your usually SOQL statement of
SELECT Seat_Number__c FROM Airplane__c WHERE Id = :requestId;
becomes
SELECT Seat_Number__c FROM Airplane__c WHERE Id = :requestId FOR UPDATE;
This SOQL statement can be executed from flow using an invocable apex class.
Since you’re blocking other processes, you want to release the lock as soon as possible. And you’ll want to gracefully handle the related error message when they are locked out.
The takeaway
While flows are great for most things, sometimes we need to dip into apex for more complex situations. Record locking is one of those situations.