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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 merge function.

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 string or 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 = require('level-update')
  Update(db, function merge (newValue, oldValue, key) {
    return someSortOfMerge(newValue, oldValue)
  db.put('key', 'VALUE', function (err) {
    //this will be intercepted 

If oldValue is null, the operation is a create.

If newValue is null, the operation is a delete.

Otherwise, the operation is an update.

Use as validation / Authentication.

check that new value is correct, or throw error.

Use for versioning

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!


level-update doesn't support the chained interface for db.batch(), but only the array interface.