I require a scalable chat application (100's - 100,000's of users, if not more) which uses a commercial database back end (e.g. Firebase/Redis/Google Cloud). The chat application should maintain a local store on each device of all messages, and therefore only download the most recent messages, using the data changed approach. Local storage will need to be purged every 30 days. The chat application should have a search facility for current messages. I have an example app to help get things started, but this is not suitable for large scale operation.
If this is an opportunity you wish to take on and help me with, please contact me by Private Message on this community for further details.