Revisions and Sync¶
Every entity in the Wunderlist API has a read-only revision property. This property is an integer which is updated in response to changes to that entity or any of its children. When the title of a task is changed, that task’s revision is updated—as well as the revisions of all of the parent items of that task, including list and root entities.
In order to guarantee that updates to Wunderlist entities are correctly executed and kept in sync across clients, any changes to an entity through the API must be accompanied by the revision property. The server uses this property to ensure that the client has the most up-to-date version of the entity. If a client makes a request with an out-of-date revision property, the request will fail, indicating that the client needs to fetch the entity’s current state and try again. If an update request fails, you must fetch the current version of the entity, look for attributes that conflict with your local state e.g. content on a note, and do some sort of local conflict resolution before replaying your changes to the API with the current revision.
You can completely synchronize a local copy of the Wunderlist data model with the Wunderlist API by checking the root revision property, descending if necessary, and repeating the process for each leaf in the tree. When a russian doll sync occurs on a client, the following rules apply: Fetched revision values and data should not be committed to local models and persistence layers unless child resources are successfully fetched. This means you should not update the child-revision of the parent until all child data has been successfully fetched. E.g. you should not apply list data and revision changes unless all tasks were fetched successfully, etc. Deleted items can be found by comparing your local data to the data retrieved during a russian doll sync and comparing for missing ids. However, since tasks may be moved to another list, you should mark a task as missing and only delete it if it is not present in any lists when the russian doll sync has completed successfully. This pattern can be extended to any model type that is “moveable”.