This blog post is part of a series on building an end-to-end mobile application using Couchbase Lite. In this third installment, the author explains how to add syncing functionality to a centralized server. The process involves installing Couchbase Server and Sync Gateway Server on a local machine or cloud environment, creating a user account for the sync gateway, and configuring the replicator configuration with the local database and centralized database details. The author also demonstrates how to enable device-to-server syncing by toggling the sync switch and sending rating requests between users. Finally, the post showcases how to update the main page of the app to display incoming rated requests and change the stars on a request based on user input.