DRF3 Professional needed to design a multiple client data synchronization scheme

Hi, I have a Django Webserver with a DRF3 3.1.1 API. The webserver is the backend for a multi client application. The multiple clients are a website and now an iphone app that we are just starting to develop. Other mobile apps will be added later. The application is a personal expense diary. Users can enter personal expenses and they will be stored in the database on the server. The web client is always online, so it will interact with the API in realtime therefore no further synchronization is required. The mobile apps will have their local databases to replicate the data from the webserver, so the users can use the app on the phone even when they currently don´t have an internet connection. All changes they make on the phone client, like adding new expenses or changing category names etc. are stored in the local database first, and as soon as there is an internet connection available, the mobile client should synchronize changes with the server backend. Synchronization needs some rules and there are several special cases that need to be considered and regulated. For example when a user has 2 mobile clients, and changes the same data on both of these clients offline and starts to synchronize later. Which changes will persist, and which changes will be dropped? I need someone who has implemented multi client synchronization before to help us design and then implement this into our Django Server. The task will be 2 steps. Step one is designing the software that we need (models, code, rules) Step 2 is implementing the task. The 2 steps can be done by different people. If the design is easy to implement, then we could even implement it with our own developer. So you are applying for the design part of the task first.