Tutorial: Changes API

Changes API


LokiJS 1.1 introduces a "Changes API" that enables the user to keep track of the changes happened to each collection since a particular point in time, which is usually the start of a work session but it could be a user defined one. This is particularly useful for remote synchronization.

Description of the Changes API

The Changes API is a collection-level feature, hence you can establish which collections may simply contain volatile data and which ones need to keep a record of what has changed.

The Changes API is an optional feature and can be activated/deactivated by either passing the option { disableChangesApi: isDisabled } in the config parameter of a collection constructor, or by calling collection.setChangesApi(isEnabled). Note that LokiJS will always set the fastest performing setting as default on a collection or database, hence the Changes API is disabled by default.

There are three events which will trigger a Changes API operation: inserts, updates and deletes. When either of these events occur, on a collection with Changes API activated, the collection will store a snapshot of the relevant object, associated with the operation and the name of the collection.

From the database object it is then possible to invoke the serializeChanges method which will generate a string representation of the changes occurred to be used for synchronization purposes.


To enable the Changes API make sure to either instantiate a collection using db.addCollection('users', { disableChangesApi: false }), or call users.setChangesApi(true) (given an example users collection).

To generate a string representation of the changes, call db.serializeChanges(). This will generate a representation of all the changes for those collections that have the Changes API enabled. If you are only interested in generating changes for a subset of collections, you can pass an array of names of the collections, i.e. db.serializeChanges(['users']);.

To clear all the changes, call db.clearChanges(). Alternatively you can call flushChanges() on the single collection, normally you would call db.clearChanges() on a callback from a successful synchronization operation.

Each change is an object with three properties: name is the collection name, obj is the string representation of the object and operation is a character representing the operation ("I" for insert, "U" for update, "R" for remove). So for example, inserting user { name: 'joe' } in the users collection would generate a change { name: 'users', obj: { name: 'joe' }, operation: 'I' }. Changes are kept in order of how the happened so a 3rd party application will be able to operate insert updates and deletes in the correct order.