A regular sync is only possible when both databases where synchronized before and when both have their transaction logs or are able to recreate them. If this is not possible they have to initiate a so called “slow sync”.
During a slow sync the client sends its complete database to the server. The server compares the entries, merges them as necessary and sends back a complete new database to the client. The comparison itself can either be done automatically or with user intervention. Since this particular server implementation should not require any user intervention, some ways for automatic comparison have to be found.