How to make a real-time Synchronize between a client database without an independent IP and a CVM database

1. At present, there is such a demand that customers install a financial software on their own computers, and the database is also deployed locally. At present, they want to make a cloud product according to this financial software. Because the client cannot directly connect to the client database without an independent IP, CVM, I want to do Synchronize with the local and server databases. It is possible for both the PS: customer service and the server to add, delete, check and modify.
2. The solution currently in mind is to deploy a service on the client side, sending and requesting data to the server at regular intervals. But it consumes a lot of server resources, and it"s not very stable. Is there a better solution?
Environment: window
Database: sql server

C-sharp windows sqlserver

Nov.23,2021

< H1 > problems and requirements < / H1 >

1. The user is in the private network
2, Customer Server = > Cloud Server changed to Cloud Server = > Customer Server

< H1 > solution < / H1 >

set up VPN on the end of Cloud , and the client actively links VPN, to establish the pipeline and Cloud side on the same virtual intranet.

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e444f8-5902a.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e444f8-5902a.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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