A levelup plugin that can be used implements conditional updates.
The name of this module may change if I think of something better.
level-update intercepts database mutations (
put, del, batch),
retrives the current value for that key, and passed the old value,
and the new value to a user supplied
The merge function can be used a number of ways - if it throws an error, the put/del/batch will callback an error.
If it returns a
Buffer, then that value will be saved instead.
And if it returns something else, then the new value will be saved as normal.
var Update =db
oldValue is null, the operation is a create.
newValue is null, the operation is a delete.
Otherwise, the operation is an update.
check that new value is correct, or throw error.
check that new value comes after old, or merge concurrent updates.
level-update uses lock to preserve
update scemantics, this is necessay to be able so that merged mutations can
behave deterministically -
level-update does a get, and then combines the
current value with the new value. It is essetial that the value has not
changed in between the get and the put!