Desktop to mobile app asynchronous data exchange via the cloud,

I am looking for a design and software development for a communication solution that securely exchanges data between a desktop app and multiple mobile apps. An important constraint is the mobile devices may only have internet connectivity once a day and the desktop app will not always be online so the data must be stored in the cloud and a copy made on the receiving mobile or desktop device so the apps can operate when offline. The desktop app creates product and price lists and other data that will be uploaded to cloud storage. The mobile apps are used to create customer orders using the product and price data and the orders will be pushed to the cloud for subsequent download by the desktop app. Each customer who purchases this system will have a single desktop app and between 1 and 30 sales staff using the mobile app. The data for each customer is independent of other customers. The solution must include authorisation and authentication. Users will login to the apps and the login credentials used to authenticate with the cloud systems We are targeting a ramp up to 1000 customers in the first year and up to 10,1000 total although these figures are very approximate so we must have scalable running costs. The mobile and desktops apps are under development, this project is only to design and develop the cloud and data exchange code. High Level Features: • The synchronisation of data with both the mobile and desktop apps will be whenever they are have an internet connection and the user requests synchronisation. • Only download new or changed data will be downloaded. • The data will only be removed from the cloud storage by an administration function or by an expiry date. This enables a user to attach a different device or desktop PC to their account and to synchronise with the current data. • The transfer must be 100% reliable and re-tries or fails gracefully when a transfer cannot be completed. • The mobile apps are developed using Qt 5.5 using QtQuick/QML and C++, for Android and iOS. QTQuick on mobile runs JavaScript so this could be used for the communication although to reduce overhead using C++ may be preferable. • The desktop app is written in Qt 5.5 C++ and can run on both Windows and OSX. Again it would be possible to write the communication code in another language that was invoked from the C++ Qt code at the expense of extra complexity. I am aware that Amazon Web Services has a suite of system such as DynamoDB, Cognito S3 and others that could be used to implement this but firstly I am looking for someone with expertise and experience of implementing such systems and secondly proposals for alternate system that may be more suitable Initially I am looking for responses that indicate the ability to develop such a system and I can then enter into more detail on the requirements.