Concurrent writes to the same record.
WHAT A LOST UPDATE IS
Two transactions reading a value, both modifying it, and one overwriting the other.
WHY IT IS COMMON
Applications read a record, present a form, and save it later.
WHAT THAT PATTERN PRODUCES
Whoever saves last overwrites the other's changes silently.
WHAT PESSIMISTIC LOCKING DOES
Locks the row when read, preventing others until committed.
WHAT IT COSTS
Held locks, and poor behaviour if the user walks away.
WHY THAT MAKES IT UNSUITABLE FOR FORMS
The lock would be held across a human's thinking time.
WHAT OPTIMISTIC LOCKING DOES
Records a version, and checks it has not changed on save.
WHAT HAPPENS ON CONFLICT
The save is rejected, and the user is told.
WHY THAT IS USUALLY CORRECT
It surfaces the conflict rather than silently discarding work.
WHAT TO IMPLEMENT
A version column, incremented on every update An update condition matching the version read Handling of zero rows affected
WHAT ZERO ROWS AFFECTED MEANS
Someone else changed it first.
WHAT TO SHOW THE USER
That the record changed, and what to do.
WHAT TO AVOID
Updating every column from a form.
WHY
It overwrites fields the user never saw.
WHAT TO DO INSTEAD
Update only what changed.
